(After VON KENNEL.)
_Al._, alimentary canal; _N._, nerve-cord; _App._, appendage; _I_, _II_,
_III_, the three divisions (lateral, median, and dorso-median) of the
coelom.]
[Illustration: FIG. 158.--SECTION OF PERIPATUS. (After SEDGWICK.)
_Al._, alimentary canal; _N._, nerve-cord; _App._, appendage; _i.v._,
internal, and _e.v._, external vesicles of the segmented excretory tubule
(coxal gland).]
Here, then, in Peripatus, and presumably, therefore, in members of the
Protostraca, we see that the original segmental organs of the annelid have
become a series of nephric organs, which extended into the base of the
appendages, and may therefore be called coxal glands; also it is clear,
from Sedgwick's description, that if the appendages disappeared, the
nephric organs would still remain, not as coxal glands, but as purely
somatic excretory glands. They would still be homologous with the annelid
segmental organs, or with the coxal glands, but would arise _in toto_ from
a part of the ventral coelom or nephrocoele, more dorsal than the former
appendicular part, because the appendages and their enclosed coelom are
always situated ventrally to the body. Again, according to Sedgwick, the
nephric tubules are connected with two coelomic vesicles, the one in the
appendage the internal vesicle, and the other, the so-called bladder, or
the external vesicle, in the body itself, close against the nerve-cord.
Sedgwick appears to consider that either of these vesicles may form the
end-sac of a nephric tubule, for he discusses the question whether the
single vesicle, which in each case gives origin to the nephridia of the
first three legs, corresponds to the internal or external vesicle. He
{400}decides, it is true, in favour of the internal vesicle, and therefore
considers the excretory organ to be appendicular, _i.e._ a coxal gland, in
these segments as well as in those more posterior. Still, the very
discussion shows that in his opinion, at all events, the external vesicle
might represent the end-sac of the tubule, in the absence of the internal
or appendicular vesicle.
Such an arrangement as Sedgwick describes in Peripatus is the very
condition required to give rise to the pronephric and mesonephric tubules,
as deduced by me from the consideration of the vertebrate, and harmonizes
and clears up the controversy about the mesonephros and pronephros in the
most satisfactory manner. Both pronephros and mesonephros are seen to be
derivatives of the original annelid segmental organs, not directly from an
annelid, but by way of an arthropodan ancestor; the difference between the
two is simply that the pronephric organs were coxal glands, and indicate,
therefore, the presence of the original metasomatic appendages, while the
mesonephric organs were homologous organs, formed in segments of later
origin which had lost their appendages. For this reason the pronephros is
said to be formed, in part at least, from a portion of the coelom situated
more ventrally than the purely somatic part which gives rise to the
mesonephros. For this reason Sedgwick, Brauer, etc., can say that the
mesonephros is strictly homodynamous with the pronephros; while equally
Rückert, Semon, and van Wijhe can say it is not homodynamous, in so far
that the two organs are not derived strictly from absolutely homologous
parts of the coelom. For this reason Semon can speak of the mesonephros as
a dorsal derivative of the pronephros, just as Sedgwick says that the
external or somatic vesicle of Peripatus is a derivative of the
appendicular nephric organ. For this reason the pronephros, or rather a
part of it, is always derived from the somatopleuric layer, for, as is
clear from Miss Sheldon's drawing, the part of the coelom in Peripatus
which dips into the appendage is derived from the somatopleuric layer
alone.
Such a coelom as that of Peripatus, Fig. 157, would represent the origin of
the vertebrate coelom, and would therefore represent the procoelom of van
Wijhe. In strict accordance with this, we see that it separates into a
dorsal part, the walls of which give origin to the somatic muscles, or at
all events to the great longitudinal dorsal muscles of the animal, and a
ventral part, which forms a nephrocoele, {401}dips into the appendage, and
gives origin to the muscles of the appendage. In the vertebrate, after the
somatic dorsal part or myocoele has separated off, a ventral part is left,
which forms a nephrocoele in the trunk-region, and gives origin to the
splanchnic striated muscles in the cranial region, _i.e._ to the muscles
which, according to my theory, were once appendicular muscles. This ventral
nephrocoelic part is divisible in the trunk into a segmented part, which
forms the excretory organs proper, and an unsegmented part, the metacoele
or true body-cavity of the vertebrate.
This comparison of the procoelom of the vertebrate and arthropod signifies
that the vertebrate metacoele was directly derived by ventral downgrowth
from the arthropod nephrocoele, so that if, as I suppose, the vertebrate
nervous system represents the conjoined nervous system and alimentary canal
of the arthropod, then the vertebrate metacoele, or body-cavity, must have
been originally confined to the region on each side of the central nervous
system, and from this position have spread ventrally, to enclose ultimately
the new-formed vertebrate gut. This means that the body-cavity (metacoele)
of the vertebrate is not the same as the body-cavity of the annelid, but
corresponds to a ventral extension of the nephrocoele, or ventral part of
such body-cavity.
Such a phylogenetic history is most probable, because it explains most
naturally and simply the facts of the development of the vertebrate
body-cavity; for the mesoblast always originates in the neighbourhood of
the notochord and central nervous system, and the lumen of the body-cavity
always appears first in that region, and then extends laterally and
ventrally on each side until it reaches the most ventral surface of the
embryo, thus forming a ventral mesentery, which ultimately disappears, and
the body-cavity surrounds the gut, except for the dorsal mesentery. Thus
Shipley, in his description of the formation of the mesoblastic plates
which line the body-cavity in Ammocoetes, describes them as commencing in
two bands of mesoblast situated on each side, close against the commencing
nervous system:--
"These two bands are separated dorsally by the juxtaposition of the dorsal
wall of the mesenteron and the epiblast, and ventrally by the hypoblastic
yolk-cells which are in contact with the epiblast over two-thirds of the
embryo. Subsequently, but at a much later date, the mesoblast is completed
ventrally by the downgrowth on {402}each side of these mesoblastic plates.
The subsequent downward growth is brought about by the cells proliferating
along the free ventral edge of the mesoblast, these cells then growing
ventralwards, pushing their way between the yoke-cells and epiblast."
The derivation of the vertebrate pronephric segmental organs from the
metasomatic coxal glands of a primitive arthropod would mean, if the
segmental organs of Peripatus be taken as the type, that such glands opened
to the exterior on every segment, either at the base of the appendage or on
the appendage itself. It is taken for granted by most observers that the
pronephric segmental organs once opened to the exterior on each segment,
and then, from some cause or other, ceased to do so, and the separate
ducts, by a process of fusion, came to form a single segmental duct, which
opened into the cloaca. Many observers have been led to the conclusion that
the pronephric duct is epiblastic in origin, although from its position in
the adult, it appears far removed from all epiblastic formations. However,
at no time in the developmental history is there any clear evidence of
actual fusion of any part of the pronephric organ with the epidermis, and
the latest observer, Brauer, is strongly of opinion that there is never
sufficiently close contact with the epidermis to warrant the statement that
the epiblastic cells take part in the formation of the duct. All that can
be said is, that the formation of the duct takes place at a time when the
pronephric diverticulum is in close propinquity to the epidermis, before
the ventral downgrowth of the myotome has taken place.
The formation of the anterior portion of the pronephric duct is, according
to Maas in Myxine, and Wheeler in Petromyzon, undoubtedly brought about by
the fusion of a number of pronephric tubules, which, according to Maas, are
clearly seen in the youngest specimens as separate segmental tubes; each of
these tubules is supplied by a capillary network from a segmental branch of
the aorta, as in the tubules of Amphioxus according to Boveri, and does not
possess a glomerulus.
The posterior part of the duct into which the mesonephric tubules enter
possesses also a capillary network, which Maas considers to represent the
original capillary network of a series of pronephric tubules, the only
remnant of which is the duct into which the mesonephric tubules open. He
therefore argues that the pronephric duct indicates a series of pronephric
tubules, which originally extended {403}along the whole length of the body,
and were supplanted by the mesonephric tubules, which also belonged to the
same segments.
I also think that the paired appendages which have left the pronephric
tubules as signs of their past existence, existed originally, in the
invertebrate stage, on every segment of the body. But I do not consider
that such a statement is at all equivalent to saying that such pairs of
tubules must have existed upon every one of the segments existing at the
present day; for it seems to me that Rückert is much more likely to be
right when he says that in Selachians the duct clearly does grow back, and
is not formed throughout _in situ_; so that he gives a double explanation
of the formation of the duct--a palingenetic anterior part formed by the
fusion of the extremities of the original excretory tubules, to which a
posterior coenogenetic lengthening has been added.
It does not seem to me at all necessary that the immediate invertebrate
ancestor of the vertebrate should have possessed excretory organs which
opened out separately to the exterior on each segment; a fusion may already
have taken place in the invertebrate stage, and so a single duct have been
acquired for a number of organs. Such a suggestion has been made by
Rückert, because of the fact discovered by Cunningham and E. Meyer, that
the segmental organs of _Lanice conchilega_ are on each side connected
together by a single strong longitudinal canal. I would, however, go
further than this and say, that even although the nephric organs of the
polychæte ancestor opened out on every segment, and although the primitive
arthropodan ancestor derived from such polychæte possessed coxal glands
which opened out either on to or at the base of each appendage, similarly
to those of Peripatus, yet the immediate arthropodan ancestor, with its
palæostracan affinities, may already have possessed metasomatic coxal
glands, all of which opened into a single duct, with a single opening to
the exterior.
Judging from Limulus, such was very probably the case, for Patten and Hazen
have shown (1) that the coxal glands of Limulus are segmental organs
belonging to the prosomatic segments; (2) that the organs belonging to the
cheliceral and ectognathal segments are not developed; (3) that the four
glands belonging to the endognaths become connected together by a _stolon_,
which communicates with a single nephric duct, opening to the exterior on
the basal segment of the 5th prosomatic appendage (the last endognath). At
{404}no time is there any evidence of any separate openings or any fusion
with the ectoderm, such as might indicate separate openings of these
prosomatic coxal segmental organs. Thus we see that in Limulus, which is
presumably much nearer the annelid condition than the vertebrate, all
evidence of separate nephric ducts opening to the exterior on each
prosomatic segment has entirely disappeared, just as is the case in the
metasomatic coxal glands (_i.e._ the pronephros) of the vertebrate. What is
seen in the prosomatic region of Limulus, and doubtless also of the
Eurypterids, may very probably have occurred in the metasomatic region of
the immediate invertebrate ancestors of the vertebrate, and so account for
the single pronephric duct belonging to a number of pronephric organs.
The interpretation of these various embryological investigations may be
summed up as follows:--
1. The ancestor of the vertebrates possessed a pair of appendages on each
segment; into the base of each of these appendages the segmental excretory
organ sent a diverticulum, thus forming a coxal gland.
2. Such coxal glands, even in the invertebrate stage, may have discharged
into a common duct which opened to the exterior most posteriorly.
3. Then, from some cause, the appendages were rendered useless, and
dwindled away, leaving only the pronephric organs to indicate their former
presence. At the end of this stage the animal possessed vertebrate
characteristics.
4. For the purpose of increasing mobility, of forming an efficient
swimming instead of a crawling animal, the body-segments increased in
number, always, as is invariably the case, by the formation of new ones
between those already formed and the cloacal region, and so of necessity
caused an elongation of the pronephric duct. Into this there now opened the
ducts of the segmental organs formed by recapitulation, those, therefore,
belonging to the body-segments--mesonephric--having nothing to do with
appendages, for the latter had already ceased to exist functionally, and
would not, therefore, be repeated with each meristic repetition.
This, so to speak, passive lengthening of the pronephric duct in
consequence of the lengthening of the early vertebrate body by the addition
of metameres, each of which contained only mesonephric and no pronephric
tubules, is, to my mind, an example of a principle {405}which has played an
important part in the formation of the vertebrate, viz. that the meristic
variation by which the spinal region of even the lowest of existing
vertebrates has been formed, has largely taken place in the vertebrate
phylum itself, and that such changes must be eliminated before we can
picture to ourselves the pre-vertebrate condition. As an example, I may
mention the remarkable repetition of similar segments pictured by Bashford
Dean in Bdellostoma. Such repetition leads to passive lengthening of such
parts as are already formed but are not meristically repeated: such are the
notochord, the vertebrate intestine, the canal of the spinal cord, and
possibly the lateral line nerve. The fuller discussion of this point means
the discussion of the formation of the vertebrate alimentary canal; I will
therefore leave it until I come to that part of my subject, and only say
here that the evidence seems to me to point to the conclusion that at the
time when the vertebrate was formed, the respiratory and cloacal regions
were very close together, the whole of the metasoma being represented by
the region of the pronephros alone.
Here, as always, the evidence of Ammocoetes tends to give definiteness to
our conceptions, for Wheeler points out that up to a length of 7 mm. the
pronephros only is formed; there is no sign of the more posteriorly formed
mesonephros. Now we know, as pointed out in Chapter VI., p. 228, this is
the time of Kupffer's larval stage of Ammocoetes. This is the period during
which the invertebrate stage is indicated in the ontogeny, so that, in
accordance with all that has gone before, this means that the metasoma of
the invertebrate ancestor was confined to the region of the pronephros.
Again, take Shipley's account of the development of Petromyzon. He says--
"The alimentary canal behind the branchial region may be divided into three
sections. Langerhans has termed these the stomach, midgut, and hindgut, but
as the most anterior of these is the narrowest part of the whole intestine,
it would, perhaps, be better to call it oesophagus. This part of the
alimentary canal lies entirely in front of the yolk, and is, with the
anterior region, which subsequently bears the gills, raised from the rest
of the egg when the head is folded off. It is supported by a dorsal
mesentery, on each side of which lies the head-kidney (pronephros)."
Further on he says--
{406}"The hindgut is smaller than the midgut; its anterior limit is marked
by the termination of the spiral valve, which does not extend into this
region. The two segmental ducts open into it just where it turns ventrally
to open to the exterior by a median ventral anus. Its lumen is from an
early stage lined with cells which have lost their yolk, and it is in wide
communication with the exterior from the first. This condition seems to be,
as Scott suggests, connected with the openings of the ducts of the
pronephros, for this gland is completed and seems capable of functioning
long before any food could find its way through the midgut, or, indeed,
before the stomodæum has opened."
Is there no significance in this statement of Shipley? Even if it be
possible to find some special reason why the branchial and cloacal parts of
the gut are freed from yolk and lined with serviceable epithelium a long
time before the midgut, why should a bit of the midgut, which Shipley calls
the oesophagus, which is connected with the region of the pronephros and
not of the branchiæ, differ so markedly from the rest of the midgut? Surely
the reason is that the branchial region of the gut, the pronephric region
of the gut, and the cloacal region of the gut, belong to a different and
earlier phase in the phylogenetic history of the Ammocoetes than does the
midgut between the pronephric and cloacal regions. This observation of
Shipley fits in with and emphasizes the view that the original animal from
which the vertebrate arose consisted of a cephalic and branchial region,
followed by a pronephric and cloacal region; the whole intermediate part of
the gut, which forms the midgut, with its large lumen and spiral valve, and
belongs to the mesonephric region, being a later formation brought about by
the necessity of increasing the length of the body.
THE ORIGIN OF THE SOMATIC TRUNK-MUSCULATURE AND THE FORMATION OF AN ATRIAL
CAVITY.
Next comes the question, why was the pronephros not repeated in the
meristic repetition that took place during the early vertebrate stage?
What, in fact, caused the disappearance of the metasomatic appendages, and
the formation of the smooth body-surface of the fish?
The embryological evidence given by van Wijhe and others of the manner in
which the original superficially situated pronephros is {407}removed from
the surface and caused to assume the deeper position, as seen in the later
embryo, is perfectly clear and uniform in all the vertebrate groups. The
diagrams at the end of van Wijhe's paper, which I reproduce here,
illustrate the process which takes place. At first the myotome (Fig. 159,
A) is confined to the dorsal region on each side of the spinal cord and
notochord. Then (Fig. 159, B) it separates from the rest of the somite and
commences to extend ventrally, thus covering over the pronephros and its
duct, until finally (Fig. 159, C) it reaches the mid-ventral line on each
side, and the foundations of the great somatic body-muscles are finally
laid.
In order, therefore, to understand how the obliteration of the appendages
took place, we must first find out what is the past history of the
myotomes. Why are they confined at first to the dorsal region of the body,
and extend afterwards to the ventral region, forcing by their growth an
organ that was originally external in situation to become internal?
In the original discussion at Cambridge, I was accused of violating the
important principle that in phylogeny we must look at the most elementary
of the animals whose ancestors we seek, and was told that the lowest
vertebrate was Amphioxus, not Ammocoetes; that therefore any argument as to
the origin of vertebrates must proceed from the consideration of the former
and not the latter animal. My reply was then, and is still, that I was
considering the cranial region in the first place, and that therefore it
was necessary to take the lowest vertebrate which possessed cranial nerves
and sense-organs of a distinctly vertebrate character, a criterion
evidently not possessed by Amphioxus. Such argument does not apply to the
spinal region, so that, now that I have left the cranial region and am
considering the spinal, I entirely agree with my critics that Amphioxus is
likely to afford valuable help, and ought to be taken into consideration as
well as Ammocoetes. The distinction between the value of the spinal
(including respiratory) and cranial regions of Amphioxus for drawing
phylogenetic conclusions is recognized by Boveri, who says that, in his
opinion, "Amphioxus shows simplicity and undifferentiation rather than
degeneration. If truly Amphioxus is somewhat degenerated, then it is so in
its prehensile and masticatory apparatus, its sense organs, and perhaps its
locomotor organs, owing to its method of living."
{408}[Illustration: FIG. 159.--DIAGRAMS TO ILLUSTRATE THE DEVELOPMENT OF
THE VERTEBRATE COELOM. (After VAN WIJHE.)
_N._, central nervous system; _Nc._, notochord; _Ao._, aorta; _Mg._,
midgut. A, _My._, myocoele; _Mes._, mesocoele; _Met._, metacoele; _Hyp._,
hypomere (pronephric). B and C, _My._, myotome; _Mes._, mesonephros;
_S.d._, segmental duct (pronephric); _Met._, body-cavity.]
{409}Hatschek describes in Amphioxus how the coelom splits into a dorsal
segmented portion, the protovertebra, and a ventral unsegmented portion,
the lateral plates. He describes in the dorsal part the formation of
myotome and sclerotome, as in the Craniota. Also, he describes how the
myotome is at first confined to the dorsal region in the neighbourhood of
the spinal cord and notochord, and subsequently extends ventrally, until,
just as in Ammocoetes, the body is enveloped in a sheet of somatic
segmented muscles, the well-known myomeres.
The conclusion to be drawn from this is inevitable. Any explanation of the
origin of the somatic muscles in Ammocoetes must also be an explanation of
the somatic muscles in Amphioxus, and conversely; so that if in this
respect Amphioxus is the more primitive and simpler, then the condition in
Ammocoetes must be looked upon as derived from a more primitive condition,
similar to that found in Amphioxus. Now, it is well known that a most
important distinction exists between Amphioxus and Ammocoetes in the
topographical relation of the ventral portion of this muscle-sheet, for in
the former it is separated from the gut and the body-cavity by the atrial
space, while in the latter there is no such space. Fürbringer therefore
concludes, as I have already mentioned, that this space has become
obliterated in the Craniota, but that it must be taken into consideration
in any attempt at formulating the nature of the ancestors of the
vertebrate.
Kowalewsky described this atrial space as formed by the ventral downgrowth
of pleural folds on each side of the body, which met in the mid-ventral
line and enclosed the branchial portion of the gut. According to this
explanation, the whole ventral portion of the somatic musculature of the
adult Amphioxus belongs to the extension of the pleural folds, the original
body-musculature being confined to the dorsal region. This is expressed
roughly on the external surface of Amphioxus by the direction of the
connective tissue septa between the myotomes (_cf._ Fig. 162, B). These
septa, as is well known, bend at an angle, the apex of which points towards
the head. The part dorsal to the bend represents the part of the muscle
belonging to the original body; the part ventral to the bend is the pleural
part, and represents the extension into the pleural folds.
Lankester and Willey have attempted to give another explanation of the
formation of the atrial cavity; they look upon it as originating from a
ventral groove, which becomes a canal by the meeting of two {410}outgrowths
from the metapleure on each side. This canal then extends dorsalwards on
each side, and so forms the atrial cavity; the metapleure still remains in
the adult; the somatic muscles in the epipleure of the adult are the
original body-muscles, and not extensions into an epipleuric fold, for
there is no such fold.
This explanation is a possible conception for the post-branchial portion of
the atrium, but is impossible for the branchial region; for, as Macbride
points out, as must necessarily be the case, the point of origin of the
atrial wall is, in all stages of development, situated at the end of the
gill-slit. It shifts in position with the position of the gill-slit, but
there can be no backwards extension of the cavity. Macbride therefore
agrees with Kowalewsky that the atrial cavity is formed by the simultaneous
ventral extension of pleural folds, and of the branchial part of the
original pharynx. Thus, in his summing up, he states: "In the larva
practically the whole sides and dorsal portion of the pharynx represent
merely the hyper-pharyngeal groove and the adjacent epithelium of the
pharynx of the adult, the whole of the branchial epithelium of the adult
being represented by a very narrow strip of the ventral wall of the pharynx
of the larva. The subsequent disproportionate growth of this part of the
pharynx of the larva, and of the adjacent portion of the atrial cavity, has
given the impression that the atrial cavity grew upwards and displaced
other structures, which is not the case."
Further, van Wijhe states that the atrium extends beyond the atriopore
right up to the anus, just as must have been the case if the pleural folds
originally existed along the whole length of the body. His words are:
"Allerdings hat sich das Atrium beim _Amphioxus lanceolatus_ eigenthümlich
ausgebildet, indem sich dasselbe durch den ganzen Rumpf bis an den Anus,
d.h. bis an die Wurzel des Schwanzes ausdehnt."
We get, therefore, this conception of the origin of the somatic musculature
of the vertebrate. The invertebrate ancestor possessed on each side, along
the whole length of its body, a lateral fold or pleuron which was segmented
with the body, and capable of movement with the body, because the dorsal
longitudinal somatic muscles extended segmentally into each segment of the
pleuron. By the ventral extension of these pleural folds, not only was the
smooth body-surface of the vertebrate attained, but also the original
appendages obliterated as such, leaving only as signs of their existence
the {411}branchiæ, the pronephric tubules, and the sense-organs of the
lateral line system.
Such an explanation signifies that the somatic trunk-musculature of the
vertebrate was derived from the dorsal longitudinal musculature of the body
of the arthropod, and not from the ventral longitudinal musculature, and
that therefore in the primitive arthropod stage the equivalent of the
myotome of the vertebrate did not give origin to the ventral longitudinal
muscles of the invertebrate ancestor. Now, as I have said, von Kennel
states that in the procoelom of Peripatus a dorsal part (III. in Fig. 157)
is cut off which gives origin to the dorsal body-musculature, while the
ventral part which remains (I. and II. in Fig. 157) gives origin in its
appendicular portion (I.) to the muscles of the appendage, and presumably
in its ventral somatic portion (II.) to the ventral longitudinal muscles of
the body. This dorsal cut-off part might be called the myotome, in the same
sense as the corresponding part of the procoelom in the vertebrate is
called the myotome. In both cases the muscles derived from it form only a
part of the voluntary musculature of the animal, and in both cases the
muscles in question are the dorsal longitudinal muscles of the body, to
which must be added the dorso-ventral body-muscles. Now, the whole of my
theory of the origin of vertebrates arose from the investigation of the
structure of the cranial nerves, which led to the conception that their
grouping is not, like the spinal, a dual grouping of motor and sensory
elements, but a dual grouping to supply two sets of segments, characterized
especially by the different embryological origin of their musculature. The
one set I called the somatic segmentation, because the muscles belonging to
it were the great longitudinal body-muscles; the other I called the
splanchnic segmentation, because its muscles were those connected with the
branchial and visceral arches. According to my theory, this latter
segmentation was due to the segmentation of the appendages in the
invertebrate ancestor; and in previous chapters, dealing as they do with
the cranial region, attention was especially directed to the way in which
the position of the striated splanchnic musculature could be explained by a
transformation of the prosomatic and mesosomatic appendages. Now, I am
dealing with the metasomatic region, in which it is true the appendages
take a very subordinate place, but still something corresponding to the
splanchnic segments of the cranial region might fairly be expected to
exist, and I therefore {412}desire to emphasize what appears to me to be
the fact, that the musculature, which in the region of the trunk would
correspond to that derived from the ventral segmentation of the mesoblast
in the region of the head, may have arisen not only from the musculature of
the appendages, but also from the ventral longitudinal musculature of the
body of the invertebrate ancestor, for it seems probable that this latter
musculature had nothing to do with the origin of the great longitudinal
muscles of the vertebrate body, either dorsal or ventral.
The way in which I imagine the obliteration of the atrial cavity to have
taken place is indicated in Fig. 160, B, which is a modification of a
section across a trilobite-like animal as represented in Fig. 160, A. As is
seen, the pleural folds on each side have nearly met the bulged-out ventral
body-surface. A continuation of the same process would give Fig. 160, C,
which is, to all intents and purposes, the same as Fig. 159, C, taken from
van Wijhe, and shows how the segmental duct is left in the remains of the
atrial cavity. The lining walls of the atrial cavity are represented very
black, in order to indicate the presence of pigment, as indeed is seen in
the corresponding position in Ammocoetes. In these diagrams I have
represented the median ventral surface as a large bulged-out bag, without
indicating any structures in it except the ventral extension of the
procoelom to form the metacoelom. At present I will leave the space between
the central nervous system and the ventral mesentery blank, as in the
diagrams; in my next chapter I will discuss the possible method of
formation within this blank space of the notochord and midgut. Boveri
considers that the obliteration of the atrial cavity in the higher
vertebrates is not complete, but that its presence is still visible in the
shape of the pronephric duct. The evidence of Maas and others that the duct
is formed by the fusion of the pronephric tubules is, it seems to me,
conclusive against Boveri's view; but yet, as may be seen from my
diagrammatic figures, the very place where one would expect to find the
last remnant of the atrial cavity is exactly where the pronephric duct is
situated. For my own part I should expect to find evidence of a former
existence of an atrial cavity rather in the pigment round the pronephros
and its duct than in the duct itself.
{413}[Illustration: FIG. 160.--A, DIAGRAM OF SECTION THROUGH A
TRILOBITE-LIKE ANIMAL; B, DIAGRAM TO ILLUSTRATE SUGGESTED OBLITERATION OF
APPENDAGES AND THE FORMATION OF AN ATRIAL CAVITY BY THE VENTRAL EXTENSION
OF THE PLEURAL FOLDS; C, DIAGRAM TO ILLUSTRATE THE COMPLETION OF THE
VERTEBRATE TYPE BY THE MEETING OF THE PLEURAL FOLDS IN THE MID-VENTRAL LINE
AND THE OBLITERATION OF THE ATRIAL CAVITY.
_Al._, alimentary canal; _N._, nervous system; _My._, myotome; _Pl._,
pleuron; _App._, appendage; _Neph._, nephrocoele; _Met._, metacoele;
_S.d._, segmental duct; _At._, atrial chamber; _V.Mes._, ventral mesentery;
_Mes._, mesonephros. The dotted line represents the splanchnopleuric
mesoblast in all figures.]
{414}The conception that Amphioxus shows us how to account for the great
envelope of somatic muscles which wraps round the vertebrate body, in that
the ancestor of the vertebrate possessed on each side the body a segmented
pleuron, is exactly in accordance with the theory of the origin of
vertebrates deduced from the study of Ammocoetes, as already set forth in
previous chapters. For we see that one of the striking characteristics of
such forms as Bunodes, Hemiaspis, etc., is the presence of segmented
pleural flaps on each side of the main part of the body; and if we pass
further back to the great group of trilobites, we find in the most manifold
form, and in various degrees of extent, the most markedly segmented pleural
folds. In fact, the hypothetical figure (Fig. 160, A) which I have deduced
from the embryological evidence, might very well represent a cross-section
of a trilobite, provided only that each appendage of the trilobite
possessed an excretory coxal gland.
The earliest fishes, then, ought to have possessed segmented pleural folds,
which were moved by somatic muscles, and enveloped the body after the
fashion of Ammocoetes and Amphioxus, and I cannot help thinking that
Cephalaspis shows, in this respect also, its relation to Ammocoetes. It is
well known that some of the fossil representatives of the Cephalaspids show
exceedingly clearly that these animals possessed a very well-segmented
body, and it is equally recognized that this skeleton is a calcareous, not
a bony skeleton, and does not represent vertebræ, etc. It is generally
called an aponeurotic skeleton, meaning thereby that what is preserved
represents not dermal plates alone, or a vertebrate skeleton, but the
calcified septa or aponeuroses between a number of muscle-segments or
myomeres, precisely of the same kind as the septa between the myomeres in
Ammocoetes. The termination of such septa on the surface would give rise to
the appearance of dermal plates or scutes, or the septa may even have been
attached to something of the nature of dermal plates. The same kind of
picture would be represented if these connective tissue dissepiments of
Ammocoetes were calcified, and the animal then fossilized. In agreement
with this interpretation of the spinal skeleton of Cephalaspis, it may be
noted that again and again, in parts of these dissepiments, I have found in
old specimens of Ammocoetes nodules of cartilage formed, and at
transformation it is in this very tissue that the spinal cartilages are
formed.
{415}[Illustration: FIG. 161.--A, FACSIMILE OF WOODWARD'S DRAWING OF A
SPECIMEN OF _Cephalaspis Murchisoni_, AS SEEN FROM THE SIDE. THE CEPHALIC
SHIELD IS ON THE RIGHT AND CAUDAL TO IT THE PLEURAL FRINGES ARE WELL SHOWN;
B, ANOTHER SPECIMEN OF _Cephalaspis Murchisoni_ TAKEN FROM THE SAME BLOCK
OF STONE, SHOWING THE DERMOSEPTAL SKELETON AND IN ONE PLACE THE PLEURAL
FRINGES, _bc._]
Now, the specimens of Cephalaspis all show, as seen in Fig. 161, that the
skeletal septa cover the body regularly, and then along one line are bent
away from the body to form, as it were, a fringe, or rather a free pleuron,
which has been easily pushed at an angle to the body-skeleton in the
process of fossilization. Patten thinks that this fringed appearance is
evidence of a number of segmental appendages which were jointed to the
corresponding body-segments, and in the best specimen at the South
Kensington Natural History Museum he thinks such joints are clearly
visible. He concludes, therefore, that the cephalaspids were arthropods,
and not vertebrates. I have also carefully examined this specimen, and do
not consider that what is seen resembles the joint of an arthropod
appendage; the appearance is rather such as would be produced if the line
of attachment of Patten's appendages to the body were the place where the
pleural body folds became free from the body, and so with any pressure a
{416}bending or fracture of the calcified plates would take place along
this line. There is, undoubtedly, an appearance of finish at the
termination of these skeletal fringes, as though they terminated in a
definitely shaped spear-like point, just as is seen in the trilobite
pleuræ. This, again, to my mind, is rather evidence of pleural fringes than
of true appendages.
[Illustration: FIG. 162.--A, ARRANGEMENT OF SEPTA IN AMMOCOETES (_NC._,
position of notochord); B, ARRANGEMENT OF SEPTA IN AMPHIOXUS.]
As already argued, I look upon Ammocoetes as the only living fish at all
resembling the cephalaspids; it is therefore instructive to compare the
arrangement of this spinal dermo-septal skeleton of Cephalaspis with that
of the septa between the myomeres in the trunk-region of Ammocoetes and
Amphioxus. Such a skeleton in Ammocoetes would be represented by a series
of plates overlapping each other, arranged as in Fig. 162, A, and in
Amphioxus as in Fig. 162, B. I have lettered the corresponding parts of the
two structures by similar letters, _a_, _b_, _c_. Ammocoetes differs in
configuration from Amphioxus in that it possesses an extra dorsal (_a_,
_d_) and an extra ventral bend. Ammocoetes is a much rounder animal than
Amphioxus, and both the dorsal and ventral bends are on the extreme ventral
and dorsal surfaces--surfaces which can hardly be said to exist in
Amphioxus. The part, then, of such an aponeurotic skeleton {417}in
Ammocoetes which I imagine corresponds to _b_, _c_ in Amphioxus, and
therefore would represent the pleural fold, is the part ventral to the bend
at _b_. In both the animals this bend corresponds to the position of the
notochord NC.
The skeleton of Cephalaspis compares more directly with that of Ammocoetes
than that of Amphioxus, for there is the same extra dorsal bend (Fig. 161,
_a_, _d_) as in Ammocoetes; the lateral part of the skeleton again gives an
angle _a_, _b_, _c_; the part from _b_ to _c_ would therefore represent the
pleural fold. I picture to myself the sequence of events somewhat as
follows:--
First, a protostracan ancestor, which, like Peripatus, possessed appendages
on every segment into which coelomic diverticula passed, forming a system
of coxal glands; such glands, being derived from the segmental organs of
the Chætopoda, discharged originally to the exterior by separate openings
on each segment. It is, however, possible, and I think probable, that a
fusion of these separate ducts had already taken place in the protostracan
stage, so that there was only one external opening for the whole of these
metasomatic coxal glands, just as there is only one external opening for
the corresponding prosomatic coxal glands of Limulus. Then, by the ventral
growth of pleural body-folds, such appendages became enclosed and useless,
and the coxal glands of the post-branchial segments, with their segmental
or pronephric duct, were all that remained as evidence of such appendages.
This dwindling of the metasomatic appendages was accompanied by the
getting-rid of free appendages generally, in the manner already set forth,
with the result that a smooth fish-like body-surface was formed; then the
necessity of increasing mobility brought about elongation by the addition
of segments between those last formed and the cloacal region. Each of such
new-formed segments was appendageless, so that its segmental organ was not
a coxal gland, but entirely somatic in position, and formed, therefore, a
mesonephric tubule, not a pronephric one. Such glands could no longer
excrete to the exterior, owing to the enclosing shell of the pleural folds;
but the pronephric duct was there, already formed, and so these nephric
tubules opened into that, instead of, as in the case of the branchial
slits, forcing their way through the pleural walls when the atrium became
closed.
{418}THE MEANING OF THE DUCTLESS GLANDS.
If it is a right conception that the excretory organs of the protostracan
group, which gave origin to the vertebrates as well as to the crustaceans
and arachnids, were of the nature of coxal glands, then it follows that
such coxal glands must have existed originally on every segment, because
they themselves were derived from the segmental organs of the annelids; it
is therefore worth while making an attempt to trace the fate of such
segmental organs in the vertebrate as well as in the crustacean and
arachnid.
Such an attempt is possible, it seems to me, because there exists
throughout the animal kingdom striking evidence that excretory organs which
no longer excrete to the exterior do not disappear, but still perform
excretory functions of a different character. Their cells still take up
effete or injurious substances, and instead of excreting to the exterior,
excrete into the blood, forming either ductless glands of special
character, or glands of the nature of lymphatic glands.
The problem presented to us is as follows:--
The excretory organs of both arthropods and vertebrates arose from those of
annelids, and were therefore originally present in every segment of the
body. In most arthropods and vertebrates they are present only in certain
regions; in the former case, as the coxal glands of the prosomatic or
head-region; in the latter, as the nephric glands of the metasomatic or
trunk-region, and, in the case of Amphioxus, of the mesosomatic or
branchial region.
In the original arthropod, judging from Peripatus, they were present, as in
the annelid, in all the segments of the body, and formed coxal glands.
Therefore, in the ancestors of the living Crustacea and Arachnida, coxal
glands must have existed in all the segments of the body, and we ought to
be able to find the vestiges of them in the mesosomatic or branchial and
metasomatic or abdominal regions of the body.
Similarly, in the vertebrates, derived, as has been shown, not from the
annelids, but from an arthropod stock, evidence of the previous existence
of coxal glands ought to be manifested in the prosomatic or trigeminal
region, in the mesosomatic or branchial region, as well as in the
metasomatic or post-branchial region.
How does an excretory organ change its character when it ceases {419}to
excrete to the exterior? What should we look for in our search after the
lost coxal glands?
The answer to these questions is most plainly given in the case of the
pronephros, especially in Myxine, where Maas has been able to follow out
the whole process of the conversion of nephric tubules into a tissue
resembling that of a lymph-gland.
He states, in the first place, that the pronephros possesses a capillary
network, which extends over the pronephric duct, while the tubules of the
mesonephros possess not only this capillary network, equivalent to the
capillaries over the convoluted tubules in the higher vertebrates, but also
a true glomerulus, in that the nephric segmental arteriole forms a coil
(Knauel), and pushes in the wall of the mesonephric tubule. He describes
the pronephros of large adult individuals as consisting of--
1. Tubules with funnels which open into the pericardial coelom.
2. A large capillary network (the glomus) at the distal end.
3. A peculiar tissue (the 'strittige Gewebe' of the Semon-Spengel
controversy), which Spengel considers to be composed of the altered
epithelium of pronephric tubules, while Semon looks on it as an
amalgamation of glomeruli.
Maas is entirely on the side of Spengel, and shows that this peculiar
tissue is actually formed by modified pronephric tubules, which become more
and more lymphatic in character.
He says: "The pronephros consists of a number of nephric tubules, placed
separately one behind the other, which were originally segmental in
character, each one of which is supplied by a capillary network from a
segmental branch of the aorta. The tubules begin with many mouths
(dorso-lateral and medial-ventral) in the pericardial cavity; on their
other blind end they have lost their original external opening, and there,
in the cranial portion of the head-kidney, before they have joined together
to form a collecting duct, they, together with the vascular network, are
transformed into a peculiar adrenal-like tissue. The most posterior of the
segmental capillary nets retain their original character, and are
concentrated into the separate capillary mass known as the glomus."
Later on he says: "Further, the separate head-kidney is more and more
removed in structure from an excretory organ in the ordinary sense. One
cannot, however, speak of it as an organ becoming rudimentary; this is
proved not only by the progressive transformation {420}of its internal
tissue into a tissue of a very definite character, but also by the cilia in
its canals, and the steady increase in the number of its funnels. It
appears, therefore, to be the conversion of an excretory organ into an
organ for the transference of fluid out of the coelom into a special
tissue, _i.e._ into its blood-sinus; in other words, into an organ which
must be classed as belonging to the lymph-system."
In exact correspondence with this transformation of a nephric tubule into a
ductless gland of the nature of a lymphatic gland, is the formation of the
head-kidney in the Teleostea. Thus, Weldon points out that, though the
observations of Balfour left it highly probable that the "lymphatic" tissue
described by him was really a result of the transformation of part of the
embryonic kidney, he did not investigate the details of its development.
This was afterwards done by Emery, with the following results: "In those
Teleostea which he has studied, Professor Emery finds that at an early
stage the kidney consists entirely of a single pronephric funnel, opening
into the pericardium, and connected with the segmental duct, which already
opens to the exterior. Behind this funnel, the segmental duct is surrounded
by a blastema, derived from the intermediate cell-mass, which afterwards
arranges itself more or less completely into a series of solid cords,
attaching themselves to the duct. These develop a lumen, and become normal
segmental tubules, but it is, if I may be allowed the expression, a matter
of chance how much of the blastema becomes so transformed into kidney
tubules, and how much is left as the 'lymphatic' tissue of Balfour, this
'lymphatic' tissue remaining either in the pronephros only, or in both pro-
and meso-nephros."
If we turn now to the invertebrates, we see also how close a connection
exists between lymphatic and phagocytic organs and excretory organs. The
chief merit for this discovery is due to Kowalewsky, who, taking a hint
from Heidenhain's work on the kidney, in which he showed how easy it was to
find out the nature of different parts of the mammalian excretory organ by
the injection of different substances, such as a solution of ammoniated
carmine, or of indigo-carmine, has injected into a large number of
different invertebrates various colouring matters, or litmus, or bacilli,
and thus shown the existence, not only of known excretory organs, but also
of others, lymphatic or lymphoid in nature, not hitherto suspected.
In all cases he finds that a phagocytic action with respect to solid
{421}bodies is a property of the leucocytes, and that these leucocytes
which are found in the coelomic spaces of the Annelida, etc., are
apparently derived from the epithelium of such spaces. Also by the
proliferation of such epithelium in places, _e.g._ the septal glands of the
terrestrial Oligochæta, segmental glandular masses of such tissue are
formed which take up the colouring matter, or the bacilli. In the
limicolous Oligochæta such septal glands are not found, but at the
commencement of the nephridial organ, immediately following upon the
funnel, a remarkable modification of the nephridial wall takes place to
form a large cellular cavernous mass, the so-called filter, which in Euaxes
is full of leucocytes; the cells are only definable by their nuclei, and
look like and act in the same way as the free leucocytes outside this
nephridial appendage. As G. Schneider points out, the whole arrangement is
very like that described by Kowalewsky in the leeches Clepsine and
Nephelis, where, also immediately succeeding the funnel of the nephridial
organ, a large accessory organ is found, which is part of the nephridium,
and is called the nephridial capsule. This is the organ _par excellence_
which takes up the solid carmine-grains and bacilli, and apparently, from
Kowalewsky's description, contains leucocytes in large numbers. We see,
then, that in such invertebrates, just as in the vertebrate, modifications
of the true excretory organ may give rise to phagocytic glands of the
nature of lymphatic glands. Further, these researches of Kowalewsky suggest
in the very strongest manner that whenever by such means new, hitherto
unsuspected glands are discovered, such glands must belong to the excretory
system, _i.e._ must be derived from coelomic epithelium, even when all
evidence of any coelom has disappeared. Kowalewsky himself was evidently so
impressed with the same feeling that he heads one of his papers "The
Excretory Organs of the Pantopoda," although the organs in question had
been discovered by him by this method, and appeared as ductless glands with
no external opening.
To my mind these observations of Kowalewsky are of exceeding interest, for
it is immediately clear that if the segmental organs of the annelids, which
must have existed on all the segments of the forefathers of the Crustacea
and Arachnida (the Protostraca), have left any sign of their existence in
living crustaceans and arachnids, then such indication would most likely
take the form of lymphatic glands in the places where the excretory organs
ought to have been.
Now, as already pointed out in Peripatus, such segmental organs {422}were
formed by the ventral part of the coelom, and dipped originally into each
appendage. We know also that each segment of an arachnid embryo possesses a
coelomic cavity in its ventral part which extends into the appendage on
each side; this cavity afterwards disappears, and is said to leave no trace
in the adult of any excretory coxal gland derived from its walls. If,
however, it is found that in the very position where such organ ought to
have been formed a segmentally arranged ductless gland is situated, the
existence of which is shown by its taking up carmine, etc., then it seems
to me that in all probability such gland is the modification of the
original coxal gland.
This is what Kowalewsky has done. Thus he states that Metschnikoff had fed
Mysis with carmine-grains, and found tubules at the base of the thoracic
feet coloured red with carmine. He himself used an allied species,
_Parapodopsis cornutum_, and found here also that the carmine was taken up
by tubules situated in the basal segments of the feet. In Nebalia, feeding
experiments with alizarin blue and carmine stained the antennal glands, and
showed the existence of glands at the base of the eight thoracic feet.
These glands resemble the foot-glands of Mysis, Parapodopsis, and Palæmon,
and lie in the space through which the blood passes from the thoracic feet,
_i.e._ from the gills, to the heart. In Squilla also, in addition to the
shell-glands, special glands were discovered on the branchial feet on the
path of the blood to the heart. These glands form continuous masses of
cells which constitute large compact glands at the base of the branchial
feet. Single cells of the same sort are found along the whole course of the
branchial venous canal, right up to the pericardium.
These observations show that the Crustacea possess not only true excretory
organs in the shape of coxal glands, _i.e._ antennary glands, shell-glands,
etc., in the cephalic region, but also a series of segmental glands
situated at the base of the appendages, especially of the respiratory
appendages: a system, that is to say, of coxal glands which have lost their
excretory function, through having lost their external opening, but have
not in consequence disappeared, but still remain _in situ_, and still
retain an important excretory function, having become lymphatic glands
containing leucocytes. Such glands are especially found in the branchial
appendages, and are called branchial glands by Cuénot, who describes them
for all Decapoda.
Further, it is significant that the same method reveals the {423}existence
in Pantopoda of a double set of glands of similar character, one set in the
basal segments of the appendage, and the other in the adjacent part of the
body.
In scorpions also, Kowalewsky has shown that the remarkable lymphatic organ
situated along the whole length of the nerve-cord in the abdominal region
takes up carmine grains and bacilli; an organ which in Androctonus does not
form one continuous gland, but a number of separate, apparently irregularly
grouped, glandular bodies.
In addition to this median lymphatic gland, Kowalewsky has discovered in
the scorpion a pair of lateral glands, to which he gives the name of
lymphoid glands, which communicate with the thoracic body-cavity (_i.e._
the pseudocoele), are phagocytic, and, according to him, give origin to
leucocytes by the proliferation of their lining cells, thus, as he remarks,
reminding us of the nephridial capsules of Clepsine. These glands are so
closely related in position to the coxal glands on each side that he has
often thought that the lumen of the gland communicated with that of the
coxal gland; he, however, has persuaded himself that there is no true
communication between the two glands. Neither of these organs appears to be
segmental, and until we know how they are developed it is not possible to
say whether they represent fused segmental organs or not.
The evidence, then, is very strong that in the Crustacea and Arachnida the
original segmental excretory organs do not disappear, but remain as
ductless glands, of the nature of lymphatic glands, which supply leucocytes
to the system.
Further, the evidence shows that the nephric organs, or parts of the coelom
in close connection with these organs, may be transformed into ductless
glands, which do not necessarily contain free leucocytes as do
lymph-glands, but yet are of such great importance as excretory organs that
their removal profoundly modifies the condition of the animal. Such a gland
is the so-called adrenal or suprarenal body, disease of which is a feature
of Addison's disease; a gland which forms and presumably passes into the
blood a substance of remarkable power in causing contraction of
blood-vessels, a substance which has lately been prepared in crystalline
form by Jokichi Takamine, and called by him "adrenalin"; a gland,
therefore, of very distinctly peculiar properties, which cannot be regarded
as rudimentary, but is of vital importance for the due maintenance of the
healthy state.
In the Elasmobranchs two separate glandular organs have been {424}called
suprarenal; a segmental series of paired organs, each of which possesses a
branch from the aorta and a sympathetic ganglion, and an unpaired series in
close connection with the kidneys, to which Balfour gave the name of
interrenal glands. Of these two sets of glands, Swale Vincent has shown
that the extract of the interrenals has no marked physiological effect, in
this respect resembling the extract of the cortical part of the mammalian
gland, while the extract of the paired segmental organs of the Elasmobranch
produces the same remarkable rise of blood-pressure as the extract of the
medullary portion of the mammalian gland.
The development also of these two sets of glands is asserted to be
different. Balfour considered that the suprarenals were derived from
sympathetic ganglion-cells, but left the origin of the interrenals
doubtful. Weldon showed that the cortical part of the suprarenals in the
lizard was derived from the wall of the glomerulus of a number of
mesonephric tubules. In Pristiurus, he stated that the mesoblastic rudiment
described by Balfour as giving origin to the interrenals is derived from a
diverticulum of each segmental tubule, close to the narrowing of its
funnel-shaped opening into the body-cavity. With respect to the paired
suprarenals he was unable to speak positively, but doubted whether they
were derived entirely from sympathetic ganglia.
Weldon sums up the results of his observations by saying: "That all
vertebrates except Amphioxus have a portion of the kidney modified for some
unknown purpose not connected with excretion; that in Cyclostomes the
pronephros alone is so modified, in Teleostei the pro- and part of the
meso-nephros; while in the Elasmobranchs and the higher vertebrates the
mesonephros alone gives rise to this organ, which has also in these forms
acquired a secondary connection with certain of the sympathetic ganglia."
Since Weldon's paper, a large amount of literature on the origin of the
adrenals has appeared, a summary of which, up to 1891, is given by Hans
Rabl in his paper, and a further summary by Aichel in his paper published
in 1900. The result of the investigations up to this latter paper may be
summed up by saying that the adrenals, using this term to include all these
organs of whatever kind, are in all cases, partly at all events, derived
from some part of the walls of either the mesonephric or pronephric
excretory organs, but that in addition a separate origin from the
sympathetic nervous system must {425}be ascribed to the medullary part of
the organ and to the separate paired organs in the Elasmobranchs, which are
equivalent to the medullary part in other cases.
The evidence, then, of the transformation of the known vertebrate excretory
organs--the pronephros and the mesonephros--leads to the conclusion that in
our search for the missing coxal glands of the meso- and pro-somatic
regions, we must look for either lymphatic glands, or ductless glands of
distinct importance to the body. I have already considered the question in
the prosomatic region, and have given my reasons why the pituitary gland
must be looked upon as the descendant of the arthropod coxal gland. In this
case also the resulting ductless gland is still of functional importance,
for disease of it is associated with acromegaly. If, as is possible, it is
homologous with the Ascidian hypophysial gland, then it is confirmatory
evidence that this latter is said by Julin to be an altered nephridial
organ.
Finally, I come to the mesosomatic or branchial region; and here,
strikingly enough, we find a perfectly segmental glandular organ of
mysterious origin--the thymus gland--segmental with the branchiæ, not
necessarily with the myotomes, belonging, therefore, to the appendicular
system; and since the branchiæ represent, according to my theory, the basal
part of the appendage, such segmental glands would be in the position of
coxal glands. Here, then, in the thymus may be the missing mesosomatic
coxal glands.
What, then, is the thymus?
The answer to this question has been given recently by Beard, who strongly
confirms Kölliker's original view that the thymus is a gland for the
manufacture of leucocytes, and that such leucocytes are directly derived
from the epithelial cells of the thymus. Kölliker also further pointed out
that the blood of the embryo is for a certain period destitute of
leucocytes. Beard confirms this last statement, and says that up to a
certain stage (varying from 10 to 16 mm. in length of the embryo) the
embryos of _Raja batis_ have no leucocytes in the blood or elsewhere. Up to
this period the thymus-placode is well formed, and the first leucocytes can
be seen to be formed in it from its epithelial cells; then such formation
takes place with great rapidity, and soon an enormous discharge of
leucocytes occurs from the thymus into the tissue-spaces and blood. He
therefore concludes that all lymphoid tissues in the body arise originally
from the thymus gland, _i.e._ from leucocytes discharged from the thymus.
{426}The segmental branchial glands, known by the name of thymus, are,
according to this view, the original lymphatic glands of the vertebrate;
and it is to be noted that, in fishes and in Amphibia, lymphatic glands,
such as we know them in the higher mammals, do not exist; they are
characteristic of the higher stages of vertebrate evolution. In the lower
vertebrates, the only glandular masses apart from the cell-lining of the
body-cavity itself, which give rise to leucocyte-forming tissue, are these
segmental branchial glands, or possibly also the modified post-branchial
segmental glands, known as the head-kidney in Teleostea, etc.
The importance ascribed by Beard to the thymus in the formation of
leucocytes in the lowest vertebrates would be considerably reduced in value
if the branchial region of Ammocoetes possessed neither thymus glands nor
anything equivalent to them. Such, however, is not the case. Schaffer has
shown that in the young Ammocoetes masses of lymphatic glandular tissue are
found segmentally arranged in the neighbourhood of each gill-slit--tissue
which soon becomes converted into a swarming mass of leucocytes, and shows
by its staining, etc., how different it is from a blood-space. The presence
of this thymus leucocyte-forming tissue, as described by Schaffer, is
confirmed by Beard, and I myself have seen the same thing in my youngest
specimen of Ammocoetes.
Further, the very methods by which Kowalewsky has brought to light the
segmental lymph-glands of the branchial region of the Crustacea, etc., are
the same as those by which Weiss discovered the branchial nephric glands in
Amphioxus--excretory organs which Boveri considers to represent the
pronephros of the Craniota. In this supposition Boveri is right, in so far
that both pronephros and the tubules in Amphioxus belong to the same system
of excretory organs; but I entirely agree with van Wijhe that the region in
Amphioxus is wrong. The tubules in Amphioxus ought to be represented in the
branchial region of the Craniota, not in the post-branchial region; van
Wijhe therefore suggests that further researches may homologize them with
the thymus gland in the Craniota, not with the pronephros. This suggestion
of van Wijhe appears to me a remarkably good one, especially in view of the
position of the thymus glands in Ammocoetes and the nephric branchial
glands in Amphioxus. If, as I have pointed out, the atrial cavity of
Amphioxus has been closed in Ammocoetes by the apposition of {427}the
pleural fold with the branchial body-surface, then the remains of the
position of the atrial chamber must exist in Ammocoetes as that
extraordinary space between the somatic muscles and the branchial
basket-work filled with blood-spaces and modified muco-cartilage. It is in
this very space, close against the gill-slits, that the thymus glands of
Ammocoetes are found, in the very place where the nephric tubules of
Amphioxus would be found if its atrial cavity were closed completely.
Instead, therefore, of considering with Boveri that the branchial nephric
tubules of Amphioxus still exist in the Craniota as the pronephros, and
that the atrial chamber has narrowed down to the pronephric duct, I would
agree with van Wijhe that the pronephros is post-branchial, and suggest
that by the complete closure of the atrial space in the branchial region
the branchial nephric tubules have lost all external opening, and
consequently, as in all other cases, have changed into lymphatic tissue and
become the segmental thymus glands.
As van Wijhe himself remarks, the time is hardly ripe for making any
positive statement about the relationship between the thymus gland and
branchial excretory organs. There is at present not sufficient consensus of
opinion to enable us to speak with any certainty on the subject, yet there
is so much suggestiveness in the various statements of different authors as
to make it worth while to consider the question briefly.
On the one hand, thymus, tonsils, parathyroids, epithelial cell-nests, and
parathymus, are all stated to be derivatives of the epithelium lining the
gill-slits, and Maurer would draw a distinction between the organs derived
from the dorsal side of the gill-cleft and those derived from the ventral
side--the former being thymus, the latter forming the epithelial
cell-nests, _i.e._ parathyroids. The thymus in Ammocoetes, according to
Schaffer, lies both ventral and dorsal to the gill-cleft; Maurer thinks
that only the dorsal part corresponds to the thymus, the ventral part
corresponding to the parathyroids, etc. Structurally, the thymus,
parathyroids, and the epithelial cell-nests are remarkably similar, so that
the evidence appears to point to the conclusion that, in the neighbourhood
of the gill-slits, segmentally arranged organs of a lymphatic character are
situated, which give origin to the thymus, parathyroids, tonsils, etc. Now,
among these organs, _i.e._ among those ventrally situated, Maurer places
the carotid gland, so that, if he is right, the origin of the carotid gland
{428}might be expected to help in the elucidation of the origin of the
thymus.
The origin of the carotid gland has been investigated recently by Kohn, who
finds that it is associated with the sympathetic nervous system in the same
way as the suprarenals. He desires, in fact, to make a separate category
for such nerve-glands, or paraganglia, as he calls them, and considers them
all to be derivatives of the sympathetic nervous system, and to have
nothing to do with excretory organs. The carotid gland is, according to
him, the foremost of the suprarenal masses in the Elasmobranchs, viz. the
so-called axillary heart.
In my opinion, nests of sympathetic ganglion-cells necessarily mean the
supply of efferent fibres to some organ, for all such ganglia are efferent,
and also, if they are found in the organ, would have been brought into it
by way of the blood-vessels supplying the organ, so that Aichel's statement
of the origin of the suprarenals in the Elasmobranchs seems to me much more
probable than a derivation from nerve-cells. If, then, it prove that Aichel
is right as to the origin of the suprarenals, and Kohn is right in
classifying the carotid gland with the suprarenals, then Maurer's
statements would bring the parathyroids, thymus, etc., into line with the
adrenals, and suggest that they represent the segmented glandular excretory
organs of the branchial region, into which, just as in the interrenals of
Elasmobranchs, or the cortical part of the adrenals of the higher
vertebrates, there has been no invasion of sympathetic ganglion-cells.
Wheeler makes a most suggestive remark in his paper on Petromyzon: he
thinks he has obtained evidence of serial homologues of the pronephric
tubules in the branchial region of Ammocoetes, but has not been able up to
the present to follow them out. If what he thinks to be serial homologues
of the pronephric tubules in the branchial region should prove to be the
origin of the thymus glands of Schaffer, then van Wijhe's suggestion that
the thymus represents the excretory organs of the branchial region would
gain enormously in probability. Until some such further investigation has
been undertaken, I can only say that it seems to me most likely that the
thymus, etc., represent the lymphatic branchial glands of the Crustacea,
and therefore represent the missing coxal glands of the branchial region.
This, however, is not all, for the appendages of the mesosomatic region, as
I have shown, do not all bear branchiæ; the foremost or {429}opercular
appendage carries the thyroid gland. Again, the basal part of the appendage
is all that is left; the thyroid gland is in position a coxal gland. It
ought, therefore, to represent the coxal gland of this appendage, just as
the thymus, tonsils, etc., represent the coxal glands of the rest of the
mesosomatic appendages. In the thyroid gland we again see a ductless gland
of immense importance to the economy, not a useless organ, but one, like
the other modified coxal glands, whose removal involves far-reaching vital
consequences. Such a gland, on my theory, was in the arthropod a part of
the external genital ducts which opened on the basal joint of the
operculum. What, then, is the opinion of morphologists as to the meaning of
these external genital ducts?
In a note to Gulland's paper on the coxal glands of Limulus, Lankester
states that the conversion of an externally-opening tubular gland (coxal
gland) into a ductless gland is the same kind of thing as the history of
the development of the suprarenal from a modified portion of mesonephros,
as given by Weldon. Further, that in other arthropods with glands of a
tubular character opening to the exterior at the base of the appendages, we
also have coxal nephridia, such as the shell-glands of the Entomostraca,
green glands of Crustacea (antennary coxal gland); and further on he
writes: "When once the notion is admitted that ducts opening at the base of
limbs in the Arthropoda are possibly and even probably modified nephridia,
we immediately conceive the hypothesis that the genital ducts of the
Arthropoda are modified nephridia."
So, also, Korschelt and Heider, in their general summing up on the
Arthropoda, say: "In Peripatus, where the nephridia appear, as in the
Annelida, in all the trunk-segments, a considerable portion of the
primitive segments is directly utilized for the formation of the nephridia.
In the other groups, the whole question of the rise of the organs known as
nephridia is still undecided, but it may be mentioned as very probable that
the salivary and anal glands of Peripatus, the antennal and shell-glands of
the Crustacea, the coxal glands of Limulus and the Arachnida, as well as
the efferent genital ducts, are derived from nephridia, and in any case are
mesodermal in origin."
The necessary corollary to this exceedingly probable argument is that
glandular structures such as the uterine glands of the scorpion already
described, which are found in connection with these terminal {430}genital
ducts, may be classed as modified nephridial glands, and that therefore the
thyroid gland of Ammocoetes, which, on the theory of this book, arose in
connection with the opercular genital ducts of the palæostracan ancestor,
represents the coxal glands of this fused pair of appendages. Such a gland,
although its function in connection with the genital organs had long
disappeared, still, in virtue of its original excretory function,
persisted, and even in the higher vertebrates, after it had lost all
semblance of its former structure and become a ductless gland of an
apparently rudimentary nature, still, by its excretory function,
demonstrates its vital importance even to the highest vertebrate.
By this simple explanation we see how these hitherto mysterious ductless
glands, pituitary, thymus, tonsils, thyroid, are all accounted for, are all
members of a common stock--coxal glands--which originally, as in Peripatus,
excreted at the base of the prosomatic and mesosomatic appendages, and are
still retained because of the importance of their excretory function,
although ductless owing to the modification of their original appendages.
Finally, there is yet another organ in the vertebrate which follows the
same law of the conversion of an excretory organ into a lymphatic organ
when its connection with the exterior is obliterated, and that is the
vertebrate body-cavity itself. According to the scheme here put forth, the
body-cavity of the vertebrate arose by the fusion of a ventral prolongation
of the original nephrocoele on each side; prolongations which accompanied
the formation of the new ventral midgut, and by their fusion formed
originally a pair of cavities along the whole length of the abdomen, being
separated from each other by the ventral mesentery of the gut.
Subsequently, by the ventral fusion of these two cavities, the body-cavity
of the adult vertebrate was formed.
This is simply a statement of the known method of formation of the
body-cavity in the embryo, and its phylogenetic explanation is that the
body-cavity of the vertebrate must be looked upon as a ventral prolongation
of the original ancestral body-cavity. Embryology clearly teaches that the
original body-cavity or somite was confined to the region of the notochord
and central nervous system, and there, just as in Peripatus, was divisible
into a dorsal part, giving origin to the myocoele, and a ventral part,
forming the nephrocoele. From this original nephrocoele are formed the
pronephric excretory organs, the mesonephric excretory organs, and the
body-cavity.
{431}That the vertebrate body-cavity was originally a nephrocoele is
generally accepted, and its excretory function is shown by the fact that it
communicates with the exterior in all the lower vertebrates, either through
abdominal pores or by way of nephridial funnels. Bles has shown how largely
these two methods of communicating with the exterior mutually exclude each
other. In the higher vertebrates both channels become closed, except in the
case of the Fallopian tubes, and thus, so to speak, the body-cavity becomes
a ductless gland, still, however, with an excretory function, but now, as
in all other cases, forming a part of the lymphatic rather than of the true
excretory system.
SUMMARY.
The consideration of the formation of the vertebrate cranial region, as
set forth in previous chapters, indicates that the ancestor of the
vertebrates was not an arachnid purely or a crustacean purely, but
possessed partly crustacean and partly arachnid characters. In order to
express this conclusion, I have used the term Protostraca, invented by
Korschelt and Heider, to indicate a primitive arthropod group, from which
both arachnids and crustaceans may be supposed to have arisen, and have
therefore stated that the vertebrate did not arise directly from the
annelids, but from the Protostraca. Such an origin signifies that the
origin of the excretory organs of the vertebrate must not be looked for
in the segmental organs of the annelid, but rather in such modified
annelid organs as would naturally exist in a primitive arthropod group.
The nature of such organs may be inferred, owing to the fortunate
circumstance that so primitive an arthropod as Peripatus still exists,
and we may conclude that the protostracan ancestor possessed in every
segment a pair of appendages and a pair of coelomic cavities, which
extended into the base of these appendages. The ventral portion of each
of these coelomic cavities separated off from the dorsal and formed a
nephrocoele, giving origin to a segmental excretory organ, which, seeing
that its end-vesicle was in the base of the appendage, and seeing also
the nature of the known arachnid and crustacean excretory organs, may
fitly be termed a coxal gland. This, then, is the working hypothesis to
explain the difficulties connected with the origin of the pronephros and
mesonephros--that the original segmental organs were coxal glands, and
therefore indicated the presence of appendages. This hypothesis leads to
the following conclusions:--
1. The coxal glands belonging to the post-branchial appendages of the
invertebrate ancestor are represented by the pronephric tubules, and
existed over the whole metasomatic region.
2. Such glands discharged into a common duct--the pronephric duct--which
opened into the cloacal region, either in the protostracan stage, when
the metasomatic appendages were still in existence, just as the coxal
glands of the prosomatic region in Limulus discharge into a common duct,
or else the pronephric duct was formed when the appendages were
obliterated.
{432}3. The metasomatic appendages disappeared owing to their enclosure
by pleural folds, which, meeting in the mid-ventral line, not only caused
the obliteration of the appendages, and gave a smooth fish-like
body-surface to the animal, but also caused the formation of an atrial
cavity.
4. Into these pleural folds the dorsal longitudinal muscles of the body
extended, and ultimately reached to the ventral surface, thus forming the
somatic muscles of the vertebrate body.
5. When the pleural folds had met in the mid-ventral line the animal had
become a vertebrate, and was dependent for its locomotion on the
movements of these somatic muscles, and not on the movements of
appendages. Consequently, elongation of the trunk-region took place, for
the purpose of increasing mobility, by the formation of new metameres.
6. Each of such metameres possessed its own segmental excretory organ,
formed in the same way as the previous pronephric organs, but, as there
were no appendages in these new-formed segments, the excretory organs
took on the characters of a mesonephros, not a pronephros, and opened
into the pronephric duct, because the direct way to the exterior was
blocked by the enveloping pleural folds.
7. The group of annelids from which the protostracan ancestor of the
vertebrates arose was the highest annelidan group, viz. the Polychæta, as
shown by the nature of the excretory organs in Amphioxus.
8. The coxal glands of the protostracan ancestor existed on all the
segments, and were, therefore, divisible into three groups, prosomatic,
mesosomatic, and metasomatic; these three groups of coxal glands still
exist in the vertebrate as ductless glands.
9. The prosomatic coxal glands form the pituitary body.
10. The mesosomatic coxal glands form the thymus, thyroid, parathyroids,
tonsils, etc.
11. The metasomatic coxal glands form the adrenals.
12. The procoelom of the vertebrate is the procoelom of the protostracan
ancestor, which splits into a dorsal part, the myocoele, and a ventral
part, the nephrocoele. This latter part not only forms the pronephros and
mesonephros, but also by a ventral extension gives origin to the walls of
the vertebrate body-cavity or metacoele.
13. This ventral extension of the original nephrocoele at first excreted
to the exterior, through abdominal pores, or through peritoneal funnels.
When such paths to the exterior became closed, it also became a ductless
gland, belonging to the lymphatic system.
{433}CHAPTER XIII
_THE NOTOCHORD AND ALIMENTARY CANAL_
Relationship between notochord and gut.--Position of unsegmented tube of
notochord.--Origin of notochord from a median groove.--Its function as an
accessory digestive tube.--Formation of notochordal tissue in
invertebrates from closed portions of the digestive tube.--Digestive
power of the skin of Ammocoetes.--Formation of new gut in Ammocoetes at
transformation.--Innervation of the vertebrate gut.--The three outflows
of efferent nerves belonging to the organic system.--The original close
contiguity of the respiratory chamber to the cloaca.--The elongation of
the gut.--Conclusion.
In the previous chapters all the important organs of the arthropod have
been found in the vertebrate in their appropriate place, of similar
structure, and innervated from corresponding parts of the central nervous
system. Such comparison is possible only as long as the ventral and dorsal
surfaces of the vertebrate correspond with the respective surfaces of the
arthropod, and no reversal is assumed. This method of comparative anatomy
is the surest and most certain guide to the relationship between two
animals, and when the facts obtained by the anatomical method are so
strikingly confirmatory of the palæontological evidence, the combined
evidence becomes so strong as to amount almost to a certainty that
vertebrates did arise from arthropods in the manner mapped out in previous
chapters, and not from a hypothetical group of animals, such as is
postulated in the theory of their origin from forms like Balanoglossus.
The latter theory derives the alimentary canal of the vertebrate from that
of the invertebrate, and finds in the latter the commencement of the
notochord. In the comparison which I have made the alimentary canal of the
invertebrate ancestor has become the tube of the central nervous system of
the vertebrate, and there is no sign of a notochord whatever. All the
organs of the arthropod have already been allocated; where the notochord is
situated in the {434}vertebrate there is nothing but a gap in the
invertebrate, but the position of that gap can be settled with great
accuracy from the previous comparison of organs in the two groups. So,
also, the alimentary canal of the vertebrate is from the very nature of the
case a new organ, yet, as has been shown in Chapter V., the comparison of
the respiratory organs in the two groups gives a strong suggestion of the
manner in which such a canal was formed.
THE ORIGIN OF THE NOTOCHORD.
The time has now come to endeavour to frame a plausible theory of the
method of formation of the notochord and the new alimentary canal, and thus
to complete the diagram on p. 413. The comparative method is no longer
available, for these structures are both unrepresented as such in the
arthropod; any suggested explanation, therefore, must be more tentative,
and cannot give the same feeling of certainty as is the case with all the
organs already considered. Our only chance of finding out the past history
of the notochord lies in the embryological method, in the hope that,
according to the 'law of recapitulation,' the ancestral history may be
repeated in the ontogeny with sufficient clearness to enable some
conclusion to be drawn.
At the outset, one point comes out clearly--the close relationship between
the notochord and the vertebrate gut; they are both derived from the same
layer, both parts of the same structure. On this point all embryologists
are agreed; it is expressed in such statements as, "the notochord, as well
as the alimentary canal, is formed from hypoblast"; "the notochord arises
as a thickening in the dorsal wall of the alimentary canal." The two
structures are so closely connected together that they must be considered
together. If we can conjecture the origin of the one, we may be sure that
we have the clue to the origin of the other. The two together form the one
new organ which distinguishes the vertebrate from the arthropod, the only
thing left which requires explanation for the completion of this strange
history.
What, then, is the notochord? What are its characteristics? In the highest
vertebrates it is conspicuous only in the embryo; with the development of
the axial skeleton it is more and more squeezed out of existence, until in
the adult it is no longer visible. By the 'law of recapitulation' this
developmental history implies that, as we descend the vertebrate phylum,
the notochord ought to be more and {435}more conspicuous, more and more
permanent during the life of the animal. Such is, indeed, found to be the
case, until at last, in the lowest vertebrates, such as the lamprey, and in
forms like Amphioxus, the notochord persists throughout the life of the
animal as a large important axial supporting rod.
This rod has a number of striking characteristics which distinguish it from
all other structures, and are the only means of guessing its probable
origin. Its position in the body is always the same in all vertebrates and
is very significant, for it lies just ventrally to the central nervous
system, along nearly the whole length of the animal, not quite the whole
length, for it invariably terminates close to the place where the
infundibulum comes to the surface of the brain; it is, in fact, always
confined to the infra-infundibular and spinal cord part of the central
nervous system. Interpreting this into the language of the arthropod, it
means that a rod was formed just ventrally to the nervous system, which
extended the whole length of the infraoesophageal and ventral chain of
ganglia, and terminated at the orifice of the mouth. Moreover, this rod was
unsegmented, for the notochord is devoid of segmentation.
At the anterior end the rod tapers to a point, as in Fig. 166. In its
middle part it is very large and conspicuous, cylindrical in shape; its
interior is filled with a peculiar vacuolated tissue, different to any
other known vertebrate tissue, which has therefore received the name of
notochordal tissue. Outside this is a thick sheath formed of many layers,
of which the external one gives the staining reactions of elastin, and is
called the external elastic layer. Between this sheath and the notochordal
tissue a thin layer of lining cells, of normal appearance, is conspicuous
in Ammocoetes. These cells secrete the layers of the sheath, and have
originally, by proliferation, given rise to the notochordal tissue. In the
notochord of Ammocoetes there is no sign of either nerves, blood-vessels,
or muscles.
The centre of the notochord presents the appearance of a slight slit, as
though it had originated from a tube, and that is the opinion now generally
held, for its mode of formation in the embryo is as that of a tube formed
from an open groove, as will be explained immediately.
We may, then, conceive of the notochord as originally a tube lying in the
mid-line just ventrally to the central nervous system, and extending from
the original mouth to the end of the body. Translate this into the language
of the arthropod and it denotes a tube on the {436}mid-ventral surface of
the body, which extended from mouth to anus. Such a tube might be formed
from the mid-ventral surface as follows:--
In Fig. 163, A, the lining of the ventral surface between two appendages is
represented flat, in B is shown how the formation of a solid rod may arise
from the bulging of that ventral surface, and in C how a groove on that
surface may lead to the formation of a tube between the two appendages. The
difference between a notochordal rod formed as in B from that in C would be
shown in the sheath, for in B the sheath would be formed from the cuticle
of the lining cells, and in C from the basement membrane. The structure of
the sheath is in accordance with the embryological evidence that the
notochord is formed as a tube from a groove, as in C, and not as a solid
rod as in B, for it possesses a well-marked elastin layer, and elastin has
never yet been found as a constituent of any cuticular secretion, but
invariably in connection with basement-membranes.
[Illustration: FIG. 163.--DIAGRAM OF TWO POSSIBLE METHODS OF THE FORMATION
OF A NOTOCHORD.]
The position, then, of the notochord and its method of formation suggests
that the mid-ventral surface of the arthropod ancestor of the vertebrate
formed a deep groove between the bases of all the prosomatic, mesosomatic,
and metasomatic appendages, which was subsequently converted into a tube
extending along the whole of the body between mouth and anus, and finally,
by the proliferation of its lining cells and their conversion into
notochordal tissue, became the notochordal rod of the vertebrate.
As already frequently stated, Apus and Branchipus are the two living
arthropods which most nearly resemble the extinct trilobites. The beautiful
specimens of Triarthrus (Fig. 165) found by Beecher give an idea of the
under surface of the trilobite such as has never been obtained before, and
demonstrate how closely the condition of things found in Apus (Fig. 164)
was similar to that occurring in the trilobites. In both cases the
mid-ventral surface of the animal formed a deep groove which extended the
whole length of the {437}animal; on each side of this groove in Apus are
closely set the gnatho-bases of the appendages, in such a manner that the
groove can be easily converted into a canal by the movements of these
bases--a canal which, owing to the great number of the appendages and their
closeness to each other, can be completely and efficiently closed.
[Illustration: FIG. 164.--UNDER-SURFACE OF APUS. (After BRONN.)]
[Illustration: FIG. 165.--UNDER-SURFACE OF A TRILOBITE (_Triarthrus_).
(From BEECHER.)]
All those who have seen Apus in the living state assert that this canal so
formed is actually used by the animal for feeding purposes. By the
movements of the gnatho-bases food is passed up from the hind end of the
animal along the whole length of this ventral canal to the mouth, where it
is taken in and swallowed. In this way Apus has been seen to swallow its
own eggs.
In the trilobites there is a similar deep channel formed by the mid-ventral
surface, similar gnatho-bases, and closely set appendages, and the membrane
of this ventral groove was extremely thin.
Here, then, in the very group of animals which were the progenitors of the
presumed palæostracan ancestor of the vertebrate--a group which is
characterized by its extensive prevalence and its {438}enormous variety of
form during the great trilobite era--the formation of a mid-ventral canal
out of this deep ventral groove is seen to be not only easy to imagine, but
most probable, provided that a necessity arose for such a conversion.
For what purpose might such a tube have been formed? I would suggest that
it might have acted as an accessory food-channel, which was of sufficient
value at the time to give some advantage in the struggle for existence to
those members of the group who were thus able to supplement their intake of
food, but at the same time was so inefficient that it was quickly
superseded by the new alimentary canal, and thus losing its temporary
function, became solid, and was utilized to form an axial supporting rod.
There is a very considerable amount of evidence in favour of the view that
the notochord was originally a digestive tube; in fact, as far as I know,
this conclusion is universally accepted. The evidence is based essentially
upon its development and upon its structure. It is formed in the vertebrate
from the same layer as the alimentary canal, _i.e._ the hypoblast, and in
Amphioxus it commences as a groove in the dorsal wall of the future
alimentary canal; this groove then closes to form the tube of the
notochord, and separates from the alimentary canal. Embryologically, then,
the notochord is looked upon as a tube formed directly from the alimentary
canal.
As regards its structure, its tissue is, as already stated, something _sui
generis_. Notochordal tissue has no resemblance to bone or cartilage, or
any of the usual supporting tissues. Such a tissue is not, however,
entirely confined to the notochord of the vertebrates, but tissue closely
resembling it has been found not only in Amphioxus and the Tunicata, but in
certain other invertebrates, in the Enteropneusta (Balanoglossus, etc.), in
Cephalodiscus, and in Actinotrocha. In all these latter cases, such a
tissue is invariably found in disused portions of the alimentary canal; a
diverticulum of the alimentary canal becomes closed, vacuolation of its
lining cells takes place, and a tissue resembling notochordal tissue is
formed.
Owing to the notochord being invariably so striking and mysterious a
feature of the lowest vertebrates, the term vertebrate, which is
inappropriate in the members of the group which do not yet possess
vertebræ, has been largely superseded by the term chordate, with the result
of attributing an undue preponderance to this tissue in any system of
classification. Hence, wherever any animal has been found {439}with a
tissue resembling that of the notochord, enthusiasts have immediately
jumped to the conclusion that a relationship must exist between it and the
chordate animals; and, accordingly, they have classified such animals as
follows: Amphioxus belongs to the group _Cephalochorda_ because the
notochord projects beyond the central nervous system; the Tunicata are
called _Urochorda_ because it is confined to the tail; the Enteropneusta,
_Hemichorda_, because this tissue is confined to a small diverticulum of
the gut, and, finally, _Diplochorda_ has been suggested for Actinotrocha
and Phoronis because two separate portions of the gut are transformed in
this way.
This exaggerated importance given to any tissue resembling in structure
that of the notochord is believed in by many of those who profess to be our
teachers on this subject, the very men who can deliberately shut their eyes
to the plain reading of the story of the pineal eyes, and say, "In our
opinion this pineal organ was not an eye at all."
The only legitimate inference to be drawn from the similarity of structure
between the notochord and these degenerated gut-diverticula, is that the
structure of the notochord may have arisen in the same way, and that
therefore the notochord may once have functioned as a gut. With cessation
of its function its cells became vacuolated, as in these other cases, and
its lumen became filled with notochordal tissue. This evidence strongly
confirms the suggestion that the notochord was once a digestive tube, but
by no means signifies that such tissue, wherever found, indicates the
presence of a notochord.
In order to resemble a notochord, this tissue must possess not only a
definite structure but a definite position, and this position is a
remarkably striking and suggestive one. The notochordal tube is
unsegmented, although the vertebrate is markedly segmented. But in all
segmented animals the only unsegmented tube which extends the whole length
of the body, from mouth to anus, is invariably the gut. In the vertebrate
there are three such tubes: (1) the gut itself, (2) the central canal of
the nervous system, and (3) the notochordal tube.
The first is the present gut, the second the gut of the invertebrate
ancestor, and the third the tube in question.
These three unsegmented tubes, extending along the whole length {440}of the
segmented animal, constitute the great peculiarity of the vertebrate group;
it is not the unsegmented notochord alone which requires explanation, but
the presence of three such tubes in the same animal. Any one of them might
be the unsegmented gut of the segmented animal. The most ventral tube is
the actual gut of the present vertebrate; the most dorsal--the neural
canal--was, according to my view, the original gut of the invertebrate
ancestor; the middle one--the notochordal tube--was, in all probability,
also once a gut, formed at the time when the exigencies of the situation
made it difficult for food to pass along the original gut.
[Illustration: FIG. 166.--DIAGRAM TO SHOW THE MEETING OF THE FOUR TUBES IN
SUCH A VERTEBRATE AS THE LAMPREY.
_Nc._, neural canal with its infundibular termination; _Nch._, notochord;
_Al._, alimentary canal with its anterior diverticulum; _Hy._, hypophysial
or nasal tube; _Or._, oral chamber closed by septum.]
Yet another circumstance in favour of this suggestion is the very striking
position of the anterior termination of the notochord. It terminates at the
point of convergence of three structures:--
(1) The tube of the hypophysis or nasal tube.
(2) The infundibulum or old mouth-termination.
(3) The notochordal tube.
To these may be added, according to Kupffer, in the embryonic stage, the
anterior diverticulum of the gut (Fig. 166).
This is a very significant point. Here originally, in the invertebrate
stage, the olfactory passage opened into the old mouth and oesophagus.
Here, finally, in the completed vertebrate the same olfactory passage opens
into the new pharynx. In the stage between the two it may well have opened
into an intermediate gut, the notochordal tube, its separation from which
would leave the end of the {441}notochord blind, just as it had already
left the end of the infundibulum blind.
The whole evidence points to the derivation of the notochord from a ventral
groove on the surface of the animal, which closed to form a tube capable of
acting as an accessory gut at the critical period before the new gut was
fully formed. The essentials of a gut tube are absorption and digestion of
food; is it likely that a tube formed as I have suggested would be
efficient for such purposes?
As far as absorption is concerned, no difficulty would arise. The gut of
the arthropod is lined with a thin layer of chitin, which is traversed,
like all other chitinous surfaces, by fine canaliculi. Through these
canaliculi, absorption of fluid material takes place, from the gut to the
body. Similar canaliculi occur in the chitin covering the animal
externally, so that, if such external surface formed a tube, and food in
the right condition for absorption passed along it, absorption could easily
take place through the chitinous surface. The evidence of Apus proves that
food does pass along such a tube in the open condition, and in the
trilobites the chitinous surface lining a similar groove was apparently
very thin, a condition still more favourable to such an absorption process.
At first sight the second essential of a gut-tube--the power of
digestion--appears to present an insuperable difficulty to this method of
forming an accessory gut-tube, for it necessitates the formation of a
secretion capable of digesting proteid material by the external cells of
the body, whereas until recently it was supposed that such a function was
confined to cells belonging to the so-called hypoblastic layer. Experiments
were made now years ago of turning a Hydra inside out so that its internal
layer should become external, and _vice versâ_, and they were said to have
been successful. Such an animal could go on living and absorbing and
digesting food, although its epiblastic surface was now its digestive
internal surface. More recent observations have shown that these
experiments were fallacious. At night-time, when the observer was not
looking, the hydra reinverted itself, so that again its original digestive
surface was inside and it lived and prospered as before.
Another piece of evidence of somewhat similar kind, which has not as yet
been discredited, is seen in the Tunicata. In many of these, new
individuals are formed from the parent by a process of budding, and it has
been proved that frequently the gut of the new {442}individual thus budded
off arises not from the gut or hypoblastic layer of the parent, but from
the surface or epiblastic layer. Such gut so formed possesses as efficient
digestive powers as the gut of the parent.
The most remarkable evidence of all has been afforded by Miss Alcock's
experiments. She examined the different tissues of Ammocoetes for the
express purpose of finding out their power of digesting fibrin, with the
result that the most active cells were those of the liver. Next in activity
came the extract of the lining cells of the respiratory chamber and of the
skin. The intestine itself when freed from the liver-secretion had very
little digestive power; extracts of muscle, nervous system, and thyroid
gland had no power whatever, but the extract of the skin-cells possessed a
powerful digesting action.
Furthermore, it is not necessary to make an extract of the skin in order to
obtain this digestive fluid, for under the influence of chloroform the skin
of Ammocoetes secretes copiously, and this fluid thus secreted was found to
possess strong digestive powers. So, also, Miss Alcock has demonstrated the
power of digesting fibrin in a similar secretion of the epithelial cells
lining the carapace of the crayfish. In both cases a very plausible reason
for the presence of a digestive ferment in a skin-secretion is found in the
necessity of preventing the growth of parasites, fungoid, or otherwise,
especially in those parts where the animal cannot keep itself clean by
'preening.' Thus in a crayfish, in which the oesophageal commissures had
been cut, fungus was found to grow on the ventral side, but not on the
dorsal carapace. The animal was accustomed to keep its ventral surface
clean by preening; owing to the paralysis it could not do so, and
consequently the fungus grew there. In the lamprey I found that wherever
there was a removal of the surface-epithelium, from whatever cause, that
spot was immediately covered with a fungoid growth, although in the intact
lamprey the skin was invariably smooth and clean.
I imagine, then, that this digestive power of the skin arose as a
protective mechanism against parasitic attacks; it is self-evident how a
tube formed of such material must _ab initio_ act as a digestive tube.
In yet another respect this skin secretion of Ammocoetes is most
instructive. The surface of Ammocoetes is absolutely smooth, no scales
{443}of any kind exist; this smoothness is due to the presence of a very
well-defined cuticular layer secreted by the underlying epithelial cells.
This cuticle is very much thicker than is usually found in vertebrates,
and, strangely enough, has been thought to contain chitin. Whether it
really contains chitin or not I am unable to say, but it certainly
resembles a chitinous layer in one respect; it is perforated by innumerable
very fine tubes or canaliculi, along which, by appropriate staining, it is
easy to see the secretion of the underlying cell pass to the exterior (Fig.
140). This marked digestive power of the skin of Ammocoetes, together with
the easy passage of the secretion through the thin cuticular layer, renders
it almost certain that a tube formed from the deep ventral groove of the
trilobite would, from the very first, act as a digestive as well as an
absorbent tube; in other words, the notochord as soon as formed was able to
act as an accessory digestive tube.
This suggested origin of the notochord from a groove along the mid-ventral
surface of the body not only indicates a starting-point from a markedly
segmented portion of the body, but also points to its formation at a stage
previous to the formation of the operculum by the fusion of the two
foremost mesosomatic appendages--indicates therefore its formation at a
stage more nearly allied to the trilobite than to the sea-scorpion. The
chance of ever finding any direct evidence of such a chordate trilobite
stage appears to me exceedingly improbable, and I greatly fear that this
conception of the mode of formation of the notochord can never be put to
direct proof, but must always remain guesswork.
On the other hand, evidence of a kind in favour of its origin from a
segmented part of the body does exist, and that evidence has this special
value, that it is found only in that most primitive animal, Amphioxus.
This evidence is as follows:--
At fairly regular intervals, the sheath of the notochord is interrupted on
each side of the mid-dorsal line by a series of holes, which penetrate the
whole thickness of the sheath. This dorsal part is pressed closely against
the spinal cord, and through these holes fibres appear to pass from the
spinal cord to the interior of the notochord. So greatly do these fibres
present the appearance of ventral roots to the notochord, that Miss Platt
looks upon them as paired motor roots to the notochord, or at all events as
once having been such motor {444}roots. Lwoff and Rolph both describe a
direct communication between the spinal cord and the notochord by means of
fibres passing through these holes, without however looking upon this
connection as a nervous one. Joseph alone asserts that no absolute
connection exists, for the internal elastic layer of the notochord,
according to him, is not interrupted at these holes, and forms, therefore,
a barrier between the fibres from the spinal cord and those from the
interior of the notochord. Still, whatever is the ultimate verdict as to
these fibres, the suggestive fact remains of the spaces in the notochordal
sheath and of the corresponding projecting root-like fibres from the spinal
cord. The whole appearance gives the impression of some former connection,
or rather series of connections, between the spinal cord and the notochord,
such as would have occurred if nerves had once passed into the notochord.
On the other hand, such nerves were not arranged segmentally with the
myotomes, for, according to Joseph, in the middle of the animal ten to
twelve such holes occur in one body-segment. In Apus the appendages are
more numerous than the body-segments, so that it is not necessary for a
segmental arrangement to coincide with that of the body-segments.
THE ORIGIN OF THE ALIMENTARY CANAL.
In close connection with the notochord is the alimentary canal. Any
explanation of the one must be of assistance in explaining the other.
According to the prevalent embryological teaching, the body is formed of
three layers, epiblast, hypoblast, and mesoblast, and the gastræa theory of
the origin of all Metazoa implies of necessity that the formation of every
individual commences with the formation of the gut. For this reason the
alimentary canal must in every case be regarded as the earliest formed
organ, however late in the development it may attain its finished
appearance. Hence the notochord is spoken of as developed from the
mid-dorsal wall of the alimentary canal. It is possible to look at the
question the other way round, and suppose that the organ whose development
is finished first is older than the one still in process of making. In this
case it would be more right to say a ventral extension of the tissue, which
gives rise to the notochord, takes place and forms the alimentary canal. It
is, to my mind, perfectly possible, and indeed probable, that {445}the
formation of the vertebrate alimentary canal was a repetition of the same
process which had already led to the formation of the notochordal tube. The
formation of the anterior part of the alimentary canal in Ammocoetes at the
time of transformation strongly suggests the marked similarity of the two
processes.
Of all the startling surprises which occur at transformation, this
formation of a new anterior gut is the most startling. From the oral
chamber of Petromyzon two tubes start: the one leads into the
gill-chambers, is known as the bronchus, and is entirely concerned with
respiration; the other leads without a break from the mouth to the anus,
has no connection with respiration, and is the alimentary canal of the
animal. Any one looking at Petromyzon would say that its alimentary canal
was absolutely non-respiratory in character. Before transformation, this
kind of alimentary canal commences at the end of the respiratory chamber;
from here to the anus it is of the same character as in Petromyzon, but in
Ammocoetes the non-respiratory anterior part simply does not exist: the
whole anterior chamber is both respiratory and affords passage to food.
This part of the alimentary canal of the adult is formed anew. We see,
then, here the formation of a part of the alimentary canal taking place,
not in an embryo full of yolk, but in a free-living, independent, grown-up
larval form in which all yolk has long since disappeared: a condition
absolutely unique in the vertebrate kingdom, but one which more than any
other may be expected to give a clue to the method of formation of a
vertebrate gut.
The formation of this new gut can be easily followed at transformation, and
was originally described by Schneider. His statement has been confirmed by
Nestler, and its absolute truth has been demonstrated to me again and again
by Miss Alcock, in her specimens illustrative of the transformation
process. First, in the mid-dorsal line of the respiratory chamber a
distinct groove is formed, the edges of which come together and form a
solid rod. This solid rod blocks the opening of the respiratory chamber
into the mid-gut, so that during this period of the transformation no food
can pass out of the pharyngeal chamber. A lumen then begins to appear in
this solid rod at the posterior end, which steadily advances mouthwards
until it opens into the oral chamber and thus forms an open tube connecting
the mouth with the gut.
Here, then, is the foundation of a new gut on very similar lines {446}to
that of the notochord, by the conversion of a groove into a tube. Still
more suggestive is it to find that the tube so formed has no appearance
whatever of segmentation; it is as unsegmented as the rest of the gut,
although, as is seen in Fig. 62, the dorsal wall of the respiratory chamber
from which it arose is as markedly segmented as any part of the animal.
Here under our very eyes, in the course of a few days or weeks, an
object-lesson in the process of the manufacture of an alimentary canal is
carried out and completed, and the teaching of that lesson is that a
gut-tube may be formed in the same way as the notochordal tube, by the
conversion of a grooved surface into a canal, and that gut-tube so formed,
like the notochord, loses all sign of segmentation, even although the
original grooved surface was markedly segmented.
The suggestion then is, that the new gut may have been formed by a
repetition of the same process which had already given origin to the
notochord.
Such a method of formation is not, in my opinion, opposed to the evidence
given by embryology, but in accordance with it; the discussion of this
point will come best in the next chapter, which treats of the embryological
evidence as a whole, and will therefore be left till then.
THE EVIDENCE GIVEN BY THE INNERVATION OF THE VERTEBRATE ALIMENTARY CANAL.
Throughout this investigation the one fixed landmark to which all other
comparisons must be referred, is the central nervous system, and the
innervation of every organ has given the clue to the meaning of that organ.
So also it must be with the new alimentary canal; by its innervation we
ought to obtain some insight into the manner of its origination. In any
organ the nerves which are specially of value in determining its
innervation, are of necessity the efferent or motor nerves, for the limits
of their distribution in the organ are much more easily determined than
those of the afferent or sensory nerves. The question therefore of primary
importance in endeavouring to determine the nature of the origin of the
alimentary canal from its innervation is the determination of the efferent
supply to the musculature of its walls.
Already in previous chapters a commencement has been made in {447}this
direction; thus the musculature of the oral chamber has been derived
directly from the musculature of the prosomatic appendages; the muscles
which move the eyes from the prosomatic and mesosomatic dorso-ventral
somatic muscles; the longitudinal body-muscles from the dorsal longitudinal
somatic muscles of the arthropod; the muscles of respiration from the
dorso-ventral muscles of the mesosomatic appendages.
In all these cases we have been dealing with striated musculature and
consequently with only the motor nerves of the muscle; but the gut
posterior to the pharyngeal or respiratory chamber contains unstriped
instead of striped muscle, and is innervated by two sets of nerves, those
which cause contraction and are motor, and those which cause relaxation and
are inhibitory. It is by no means certain that these two sets of nerves
possess equal value from a morphological point of view. The meaning of an
inhibitory nerve is at present difficult to understand, and in this
instance, is rendered still more doubtful owing to the presence of
Auerbach's plexus along the whole length of the intestine--an elaborate
system of nerve-cells and nerve-fibres situated between the layers of
longitudinal and circular muscles surrounding the gut-walls, which has been
shown by the recent experiments of Magnus, to constitute a special enteric
nervous system.
One of the strangest facts known about the system of inhibitory nerves is
their marked tendency to leave the central nervous system at a different
level to the corresponding motor nerves, as is well known in the case of
the heart, where the inhibitory nerve--the vagus--arises from the medulla
oblongata, while the motor nerve--the augmentor or accelerator--leaves the
spinal cord in the upper thoracic region. It is very difficult to obtain
any idea of the origin of such a peculiarity; I know of only one suggestive
fact, which concerns the innervation of the muscles which open and close
the chela of the crayfish, lobster, etc. These muscles are antagonistic to
each other, and both possess inhibitory as well as motor nerves. The
central nervous system arrangements are of such a character that the
contraction of the one muscle is accompanied by the inhibition of its
opposer, and the nerves which inhibit the contraction of the one, leave the
central nervous system with the nerves which cause the other to contract.
Thus the inhibitory and motor nerves of either the abductor (opener) or
adductor (closer) muscles of the crayfish claw do not leave the central
nervous system together, but in separate nerves.
{448}If now for some cause the one set of muscles either disappeared, or
were so altered as no longer to present any appearance of antagonism, then
there would be left a single set of muscles, the inhibitory and motor
nerves of which would leave the central nervous system at different levels,
and the older such systems might be, the greater would be the modification
in the shape and arrangements of parts in the animal, so that the two sets
of fibres might ultimately arise from very different levels.
As mentioned in the introductory chapter, the whole of this investigation
into the origin of vertebrates arose from my work on the system of efferent
nerves which innervate the vascular and visceral systems. One of the main
points of that investigation was the proof that such nerves did not leave
the central nervous system uniformly along the whole length of it, but in
three great outflows, cranial, thoracico-lumbar, and sacral; there being
two marked gaps separating the three outflows, caused by the interpolation
of the plexuses for the innervation of the anterior and posterior limbs
respectively. All these nerves are characterized by the presence of
ganglion-cells in their course to the periphery, they are, therefore,
distinguished from ordinary motor nerves to striated muscle in that their
impulses pass through a ganglion-cell before they reach the muscle.
The ganglia of the large middle thoracico-lumbar outflow constitute the
ganglia of the sympathetic system.
The functions of the nerves constituting these three outflows are very
different, as I pointed out in my original papers. Since then a large
amount of further information has been obtained by various observers,
especially Langley and Anderson, which enable the following statements to
be made:--
All the nerves which cause contraction of the unstriped muscles of the
skin, whether pilomotor or not, all the nerves which cause secretion of
sweat glands wherever situated, all the nerves which cause contraction or
augmentation of the action of muscles belonging to the vascular system, all
the nerves which are motor to the muscles belonging to all organs derived
from the Wolffian and Müllerian ducts, _e.g._ the uterus, ureters, urethra,
arise from the thoracico-lumbar outflow, never from the cranial or sacral
outflows. It is essentially an efferent skin-system.
On the other hand, the latter two sets of nerves are concerned {449}with
the supply of motor nerves to the alimentary canal; they form essentially
an efferent gut-system in contradistinction to the sympathetic or
skin-system.
A marked distinction exists between these cranial and sacral nerves. The
vagus never supplies the large intestine, the sacral nerves never supply
the small intestine. Associated with the large intestine is the bladder,
the whole system arising from the original cloacal region; the vagus never
supplies the bladder, its motor nerves belong to the sacral outflow. The
motor nerves to the ureters, to the urethra, and to the trigonal portion of
the bladder between the ureters and the urethra, do not arise from the
sacral outflow, but from the thoracico-lumbar. These muscles belong really
to the muscles in connection with the Müllerian and Wolffian ducts and
skin, not to the cloacal region.
The motor innervation then of the alimentary canal reveals this striking
and suggestive state of affairs. The motor innervation of the whole of the
small intestine arises from the cranial region, and is immediately followed
by an innervation from the sacral region for the whole of the muscles of
the cloaca. It thus indicates a head-region and a tail-region in close
contiguity, the whole of the spinal cord region between these two extremes
being apparently unrepresented. Not, however, quite unrepresented, for
Elliott has shown recently that the ileo-colic valve at the junction of the
small and large intestine is in reality an ileo-colic sphincter muscle, and
that this muscle receives its motor nerves neither from the vagus nor from
the sacral nerves, but from the thoracico-lumbar outflow or sympathetic
system. This may mean one of two things, either that a band of fibres
belonging to the skin-system has been added to the gut-musculature, for the
purpose of forming a sphincter at this spot, or that the region between the
vagus territory and the cloaca is represented by this small band of muscle.
The second explanation seems to me the more probable of the two. Between
the mesosomatic region represented by the vagus, and the cloacal region,
there existed a small metasomatic region, represented by the pronephros,
with its segmental duct, as already discussed in Chapter XII. That part of
the new alimentary canal which belonged to this region is the short piece
indicated by the ileo-colic sphincter, and innervated, therefore, from the
same region as the organs derived from the segmental duct.
Such innervation seems to me to suggest that originally the {450}vertebrate
consisted, as far as its gut was concerned, of a prosomatic and mesosomatic
(branchial) region, close behind which came the cloaca and anus. Between
the two there was a short metasomatic region (possibly pronephric), so that
the respiratory chamber did not open directly into the cloaca.
Such an interpretation is, I think, borne out by the study of the most
ancient forms of fish. In Bothriolepis, according to Patten, and in
Drepanaspis, according to Traquair, the cloacal region and anus follow
immediately upon the posterior end of the head-shield, _i.e._ immediately
after that region which presumably contained the branchiæ. Similarly, on
the invertebrate side, all those forms which resembled Limulus must have
possessed a very short region between the branchial and cloacal parts of
the body. The original cloacal part of the vertebrate gut may well have
been the original cloaca of the arthropod, into which its intestine emptied
itself, especially when we see the tendency of the scorpion group of
animals to form an accessory cloacal pouch known as the stercoral pouch or
pocket.
Again, it is striking to see how, in certain of the scorpion group, _e.g._
Thelyphonus and Phrynus, there is a caudal massing of the central
nerve-cells as well as a cephalic massing, so that their central nervous
system is composed of a cephalic and caudal brain. These two brains are
connected together by commissures extending the whole length of the body,
in which I have been unable to find any sign of ganglion-cells. What this
caudal brain innervates I do not know; it is, I think, a matter worth
further investigation, especially as there are many indications in the
vertebrate that the lumbo-sacral region of the cord possesses higher
functions than the thoracic region.
The method of formation of the alimentary canal as indicated by its
innervation is as follows:--
In front an oral chamber, formed, as already pointed out, by the
modification of the prosomatic appendages, followed by a respiratory
chamber, the muscles and branchiæ of which were the muscles and branchiæ of
the mesosomatic appendages. This mesosomatic, or branchial, part was in
close contiguity to the cloaca and anus, being separated from it only by a
short tube formed in the metasomatic or pronephric region.
I imagine that this connection was originally in the form of an {451}open
groove, as already explained for both notochord and the anterior part of
the gut itself in Ammocoetes; an open groove formed from the mid-ventral
surface of the body, on each side of which were the remnants of the
pronephric appendages. By the closure of this groove ventrally, and the
growing round of the pleural folds, as already suggested, the remains of
the pronephric appendages are indicated by the segmental duct and the form
of the vertebrate body is attained.
Even in the branchial region the same kind of thing must, I think, have
occurred. The grooved ventral surface became a tube, on each side of which
were lying in regular order the in-sunk branchial appendages, the whole
being subsequently covered by the pleural folds to form an atrial chamber.
A tube thus formed from the grooved ventral surface would carry with it to
the new ventral surface the longitudinal venous sinuses, and thus form, in
the way already suggested, the heart and ventral aorta. Posterior to the
heart in the pronephric region, the same process would give rise to the
sub-intestinal vein.
The evidence of comparative anatomy bears out most conclusively the
suggestion that in the original vertebrate the gut was mainly a respiratory
chamber. In man and all mammals the oral chamber opens into a small
pharynx, followed by the oesophagus, stomach and small intestine. Of this
whole length, a very small part is taken up by the pharynx, in which, in
the embryo, the branchial arches are found, showing that this represents
the original respiratory part of the gut. In the ordinary fish this
branchial part is much more conspicuous, occupies a large proportion of the
gut, and in the lowest fishes, such as Ammocoetes and Amphioxus, the
branchial region extends over a large portion of the animal, while the
intestine proper is a straight tube, the length of which is insignificant
in comparison with its length in the higher vertebrates.
Such a tube was able to act as a digestive tube, owing, as already pointed
out, to the digestive powers of the skin-epithelium, and I imagine at first
the respiratory chamber, seeing that it composed very nearly the whole of
the gut, was at the same time the main digestive chamber; even in
Ammocoetes its digestive power is superior to that of the intestine itself.
Just posterior to the branchial part a diverticulum of the gut was formed
at an early stage, as seen in Amphioxus, and provided the {452}commencement
of the liver. This simple liver-diverticulum became the tubular liver of
Ammocoetes, and formed, curiously enough, not a glandular organ of the same
character as the liver of the higher vertebrates, but a hepato-pancreas,
like the so-called liver of the arthropods, which also is a special
diverticulum of the gut, or rather the main true gut of the animal. In both
cases the liver is the chief agent in digestion, for in Ammocoetes the
liver-extract is very much more powerful in the digestion of proteids than
the extract of any other organ tried by Miss Alcock. Subsequently in the
vertebrate the gastric and pancreatic glands arise and relieve the liver of
the burden of proteid digestion.
It is, to my mind, somewhat significant that the liver on its first
formation in the vertebrate should have arisen as a digestive organ of the
same character as the so-called liver in the arthropods; whether it
originally belonged to any separate segment is in our present state of
knowledge difficult to say.
CONCLUSION.
In conclusion, I will endeavour to illustrate crudely the way in which, on
my theory, the notochord and vertebrate gut may have been formed, the
agencies at work being in the main two, viz. the dwindling of appendages as
mere organs of locomotion, and the conversion of a ventral groove into a
tube.
I imagine that, among the Protostraca, forms were found somewhat resembling
trilobites with markedly polychætan affinities; which, like Apus, possessed
a deep ventral groove from one end of the body to the other, and also
pleural fringes, as in many trilobites. This might be called the Trilobite
stage (Fig. 167, A).
This groove became converted into a tube and so gave rise to the notochord,
while the appendages were still free and the pleuræ had not met to form a
new ventral surface. This might be called the Chordate Trilobite stage
(Fig. 167, B).
Then, passing from the protostracan to the palæostracan stage, the oral and
respiratory chambers were formed, not communicating with each other, in the
manner described in previous chapters, a ventral groove in the metasomatic
region being the only connection between respiratory chamber and cloaca.
This might be called the Chordate Palæostracan stage (Fig. 167, C).