criticises my description of the mode of growth of the
segmental duct. The following is a free translation of what he
says: "In Balfour's, as in other descriptions, an account is
given of a backward growth, which easily leads to the
supposition of a structure formed anteriorly forcing its way
through the tissues behind. This is, however, not the case,
since, to my knowledge, no author has ever detected a sharp
boundary between the growing point of the segmental duct (or
Müllerian duct) and the surrounding tissues." He goes on to say
that "the growth in these cases really takes place by a
differentiation of tissue along a line in the region of the
peritoneal cavity." Although I fully admit that it would be far
easier to homologise the development of the segmental duct in
Amphibia and Elasmobranchii according to this view, I must
nevertheless vindicate the accuracy of my original account. I
have looked over my specimens again, since the appearance of Dr
Fürbringer's paper, and can find no evidence of the end of the
duct becoming continuous with the adjoining mesoblastic
tissues. In the section, before its disappearance, the
segmental duct may, so far as I can make out, be seen as a very
small but distinct rod, which is much more closely connected
with the epiblast than with any other layer. From Gasser's
observations on the Wolffian duct in the bird, I am led to
conclude that it behaves in the same way as the segmental duct
in the Elasmobranchii. I will not deny that it is possible that
the growth of the duct takes place by wandering cells, but on
this point I have no evidence, and must therefore leave the
question an open one.--F. M. B.
The abdominal aperture of the segmental duct in Elasmobranchii, in that it
becomes the permanent abdominal opening of the oviduct, corresponds
physiologically rather with the abdominal opening of the Müllerian duct
than with that of the segmental duct of Amphibia, which, after becoming
divided up to form the pores of the head-kidney, undergoes atrophy.
Morphologically, however, it appears to correspond with the opening of the
segmental duct in Amphibia. We shall allude to this point more than once
again, and give our grounds for the above view on p. 640.
The development of the segmental duct in Elasmobranchii as a solid rod is,
we hope to shew, of special importance for the elucidation of the excretory
system of Aves.
The development of these parts of Petromyzon is not fully known, but from
W. Müller's account (_Jenaische Zeitschrift_, 1875) it would seem that an
anterior invagination of the peritoneal epithelium is continued backwards
as a duct (segmental duct), and that the anterior opening subsequently
becomes divided up into the various apertures of the head-kidney. If this
account is correct, Petromyzon presents a type intermediate between
Amphibia and Elasmobranchii. In certain types, viz. Marsipobranchii and
Teleostei, the segmental duct becomes the duct for the posterior kidney
(segmental tubes), but otherwise undergoes no further differentiation. In
the majority of types, however, the case is different. In Amphibia[440], as
has already been mentioned, a solid rod of cells is split off from its
ventral wall, which afterwards becomes hollow, acquires an opening into the
body-cavity, and forms the Müllerian duct.
Footnote 440: Fürbringer, _loc. cit._
In Elasmobranchii the segmental duct undergoes a more or less similar
division. "It becomes longitudinally split into two complete ducts in the
female, and one complete duct and parts of a second in the male. The
resulting ducts are (1) the Wolffian duct dorsally, which remains
continuous with the excretory tubules of the kidney, and ventrally (2) the
oviduct or Müllerian duct in the female, and the rudiments of this duct in
the male. In the female the formation of these ducts takes place by a
nearly solid rod of cells, being gradually split off from the ventral side
of all but the foremost part of the original segmental duct, with the short
undivided anterior part of which duct it is continuous in front. Into it a
very small portion of the lumen of the original segmental duct is perhaps
continued. The remainder of the segmental duct (after the loss of its
anterior section and the part split off from its ventral side) forms the
Wolffian duct. The process of formation of the ducts in the male chiefly
differs from that in the female, in the fact of the anterior undivided part
of the segmental duct, which forms the front end of the Müllerian duct,
being shorter, and in the column of cells with which it is continuous being
from the first incomplete."
It will be seen from the above that the Müllerian duct consists of two
distinct parts--an anterior part with the abdominal opening, and a
posterior part split off from the segmental duct. This double constitution
of the Müllerian duct is of great importance for a proper understanding of
what takes place in the Bird.
The Müllerian duct appears therefore to develop in nearly the same manner
in the Amphibian and Elasmobranch type, as a solid or nearly solid rod
split off from the ventral wall of the segmental duct. But there is one
important difference concerning the abdominal opening of the duct. In
Amphibia this is a new formation, but in Elasmobranchii it is the original
opening of the segmental duct. Although we admit that in a large number of
points, including the presence of a head-kidney, the urinogenital organs of
Amphibia are formed on a lower type than those of the Elasmobranchii, yet
it appears to us that this does not hold good for the development of the
Müllerian duct.
The above description will, we trust, be sufficient to render clear our
views upon the development of the excretory system in Aves.
In the bird the excretory system consists of the following parts (using the
ordinary nomenclature) which are developed in the order below.
1. Wolffian duct. 2. Wolffian body. 3. Head-kidney. 4. Müllerian duct. 5.
Permanent kidney and ureter.
About 2 and 5 we shall have nothing to say in the sequel.
We have already in the early part of the paper given an account of the
head-kidney and Müllerian duct, but it will be necessary for us to say a
few words about the development of the Wolffian duct (so called). Without
entering into the somewhat extended literature on the subject, we may state
that we consider that the recent paper of Dr Gasser[441] supplies us with
the best extant account of the development of the Wolffian duct.
Footnote 441: _Arch. für Mic. Anat._ Vol. XIV.
The first trace of it, which he finds, is visible in an embryo with eight
protovertebræ as a slight projection from the intermediate cell mass
towards the epiblast in the region of the three hindermost protovertebræ.
In the next stage, with eleven protovertebræ, the solid rudiment of the
duct extends from the fifth to the eleventh protovertebra, from the eighth
to the eleventh protovertebra it lies between the epiblast and mesoblast,
and is quite distinct from both, and Dr Gasser distinctly states that in
its growth backwards from the eighth protovertebra the Wolffian duct never
comes into continuity with the adjacent layers.
In the region of the fifth protovertebra, where the duct was originally
continuous with the mesoblast, it has now become free, but is still
attached in the region of the sixth and to the eighth protovertebra. In an
embryo with fourteen protovertebræ the duct extends from the fourth to the
fourteenth protovertebra, and is now free between epiblast and mesoblast
for its whole extent. It is still for the most part solid though perhaps a
small lumen is present in its middle part. In the succeeding stages the
lumen of the duct gradually extends backwards and forwards, the duct itself
also passes inwards till it acquires its final position close to the
peritoneal epithelium; at the same time its hind end elongates till it
comes into connection with the cloacal section of the hind-gut. It should
be noted that the duct in its backward growth does not appear to come into
continuity with the subjacent mesoblast, but behaves in this respect
exactly as does the segmental duct in Elasmobranchii (vide note on p. 634).
The question which we propose to ourselves is the following:--What are the
homologies of the parts of the Avian urinogenital system above enumerated?
The Wolffian duct appears to us morphologically to correspond _in part_ to
the segmental duct[442], or what Fürbringer would call the duct of the
head-kidney. This may seem a paradox, since in birds it never comes into
relation with the head-kidney. Nevertheless we consider that this homology
is morphologically established, for the following reasons:--
Footnote 442: The views here expressed about the Wolffian duct
are nearly though not exactly those which one of us previously
put forward ("Urinogenital Organs of Vertebrates," &c., pp.
45-46) [This edition, pp. 164, 165], and with which Fürbringer
appears exactly to agree. Possibly Dr Fürbringer would alter
his view on this point were he to accept the facts we believe
ourselves to have discovered. Semper's view also differs from
ours, in that he believes the Wolffian duct to correspond in
its entirety with the segmental duct.
(1) That the Wolffian duct gives rise (vide _supra_, p. 631) to the
Müllerian duct as well as to the duct of the Wolffian body. In this respect
it behaves precisely as does the segmental duct of Elasmobranchii and
Amphibia. That it serves as the duct for the Wolffian body, before the
Müllerian duct originates from it, is also in accordance with what takes
place in other types.
(2) That it develops in a strikingly similar manner to the segmental duct
of Elasmobranchii.
We stated expressly that the Wolffian duct corresponded only in part to the
segmental duct. It does not, in fact, in our opinion, correspond to the
whole segmental duct, but to the segmental duct minus the anterior
abdominal opening in Elasmobranchii, which becomes the head-kidney in other
types. In fact, we suppose that the segmental duct and head-kidney, which
in the Ichthyopsida develop as a single formation, develop in the Bird as
two distinct structures--one of these known as the Wolffian duct, and the
other the head-kidney. If our view about the head-kidney is accepted the
above position will hardly require to be disputed, but we may point out
that the only feature in which the Wolffian duct of the Bird differs in
development from the segmental duct of Elasmobranchii is in the absence of
the knob, which forms the commencement of the segmental duct, and in which
the abdominal opening is formed; so that the comparison of the development
of the duct in the two types confirms the view arrived at from other
considerations.
The head-kidney and Müllerian duct in the Bird must be considered together.
The parts which they eventually give rise to after the atrophy of the
head-kidney have almost universally been regarded as equivalent to the
Müllerian duct of the Ichthyopsida. By Braun[443], however, who from his
researches on the Lizard satisfied himself of the entire independence of
the Müllerian and Wolffian ducts in the Amniota, the Müllerian duct of
these forms is regarded as a completely new structure with no genetic
relations to the Müllerian duct of the Ichthyopsida. Semper[444], on the
other hand, though he accepts the homology of the Müllerian duct in the
Ichthyopsida and Amniota, is of opinion that the anterior part of the
Müllerian duct in the Amniota is really derived from the Wolffian duct,
though he apparently admits the independent growth of the posterior part of
the Müllerian duct. We have been led by our observations, as well as by our
theoretical deductions, to adopt a view exactly the reverse of that of
Professor Semper. We believe that the anterior part of the Müllerian duct
of Aves, which is at first the head-kidney, and subsequently becomes the
abdominal opening of the duct, is developed from the peritoneal epithelium
independently of all other parts of the excretory system; but that the
posterior part of the duct is more or less completely derived from the
walls of the Wolffian duct. This view is clearly in accordance with our
account of the facts of development in Aves, and it fits in very well with
the development of the Müllerian duct in Elasmobranchii. We have already
pointed out that in Elasmobranchii the Müllerian duct is formed of two
factors--(1) of the whole anterior extremity of the segmental duct,
including its abdominal opening; (2) of a rod split off from the ventral
side of the segmental duct. In Birds the anterior part (corresponding to
factor No. 1) of the Müllerian duct has a different origin from the
remainder; so that if the development of the posterior part of the duct
(factor No. 2) were to proceed in the same manner in Birds and
Elasmobranchii, it ought to be formed at the expense of the Wolffian
(_i.e._ segmental) duct, though in connection anteriorly with the
head-kidney. And this is what actually appears to take place.
Footnote 443: "Urogenital-System d. Reptilien," _Arb. aus d.
zool.-zoot. Inst. Würzburg_, Vol. IV.
Footnote 444: _Loc. cit._
So far the homologies of the avian excretory system are fairly clear; but
there are still some points which have to be dealt with in connection with
the permanent opening of the Müllerian duct, and the relatively posterior
position of the head-kidney. With reference to the first of these points
the facts of the case are the following:--
In Amphibia the permanent opening of the Müllerian duct is formed as an
independent opening after the atrophy of the head-kidney.
In Elasmobranchii the original opening of the segmental duct forms the
permanent opening of the Müllerian duct and no head-kidney appears to be
formed.
In Birds the anterior of the three openings of the head-kidney remains as
the permanent opening of the Müllerian duct.
With reference to the difficulties involved in there being apparently three
different modes in which the permanent opening of the Müllerian duct is
formed, we would suggest the following considerations:
The history of the development of the excretory system teaches us that
primitively the segmental duct must have served as efferent duct both for
the generative products and kidney secretion (just as the Wolffian duct
still does for the testicular products and secretion of the Wolffian body
in Elasmobranchii and Amphibia); and further, that at first the generative
products entered the segmental duct from the abdominal cavity by one or
more of the abdominal openings of the kidney (almost certainly of the
head-kidney). That the generative products did not enter the segmental duct
at first by an opening specially developed for them appears to us to follow
from Dohrn's principle of the transmutation of function
(_Functionswechsel_). As a consequence (by a process of natural selection)
of the segmental duct having both a generative and a urinary function, a
further differentiation took place, by which that duct became split into
two--a ventral Müllerian duct and dorsal Wolffian duct.
The Müllerian duct without doubt was continuous with the head-kidney, and
so with the abdominal opening or openings of the head-kidney which served
as generative pores. At first the segmental duct was probably split
longitudinally into two equal portions, but the generative function of the
Müllerian duct gradually impressed itself more and more upon the embryonic
development, so that, in the course of time, the Müllerian duct developed
less and less at the expense of the Wolffian duct. This process appears
partly to have taken place in Elasmobranchii, and still more in Amphibia;
the Amphibia offering in this respect a less primitive condition than
Elasmobranchii; while in Aves it has been carried even further. The
abdominal opening no doubt also became specialised. At first it is quite
possible that more than one abdominal pore may have served for the
generative products; one of which, no doubt, eventually came to function
alone. In Amphibia the specialisation of the opening appears to have gone
so far that it no longer has any relation to the head-kidney, and even
develops after the atrophy of the head-kidney. In Elasmobranchii, on the
other hand, the functional opening appears at a period when we should
expect the head-kidney to develop. This state is very possibly the result
of a differentiation (along a different line to that in Amphibia) by which
the head-kidney gradually ceased to become developed, but by which the
primitive opening (which in the development of the head-kidney used to be
divided into several pores leading into the body-cavity) remained undivided
and served as the abdominal aperture of the Müllerian duct. Aves, finally,
appear to have become differentiated along a third line; since in their
ancestors the anterior pore of the head-kidney appears to have become
specialised as the permanent opening of the Müllerian duct.
With reference to the posterior position of the head-kidney in Aves we have
only to remark, that a change in position of the head-kidney might easily
take place after it acquired an independent development. The fact that it
is slightly behind the glomerulus would seem to indicate, on the one hand,
that it has already ceased to be of any functional importance; and, on the
other, that the shifting has been due to its having a connection with the
Müllerian duct.
We have made a few observations on the development of the Müllerian duct in
_Lacerta muralis_, which have unfortunately led us to no decided
conclusions. In a fairly young stage in the development of the Müllerian
duct (the youngest we have met with), no trace of a head-kidney could be
observed, but the character of the abdominal opening of the Müllerian duct
was very similar to that figured by Braun[445]. As to the backward growth
of the Müllerian duct, we can only state that the solid point of the duct
in the young stages is in contact with the wall of the Wolffian duct, and
the relation between the two is rather like that figured by Fürbringer
(Pl. 1, figs. 14-15) in Amphibia.
Footnote 445: _Loc. cit._
DESCRIPTION OF PLATES 27 AND 28.
COMPLETE LIST OF REFERENCE LETTERS.
_ao._ Aorta. _cv._ Cardinal vein. _gl._ Glomerulus. _gr_1. First groove of
head-kidney. _gr_2. Second groove of head-kidney. _gr_3. Third groove of
head-kidney. _ge._ Germinal epithelium. _mrb._ Malpighian body. _me._
Mesentery. _md._ Müllerian duct. _r_1. First ridge of head-kidney. _r_2.
Second ridge of head-kidney. _r_3. Third ridge of head-kidney. _Wd._
Wolffian duct. _x._ Fold in germinal epithelium.
PLATE 27.
SERIES A. Sections through the head-kidney at our second stage. Zeiss 2,
ocul. 3 (reduced one-third). The second and third grooves are represented
with the ridge connecting them, and the rod of cells running backwards for
a short distance.
No. 1. Section through the second groove.
No. 2. Section through the ridge connecting the second and third grooves.
No. 3. Section passing through the same ridge at a point nearer the third
groove.
Nos. 4, 5, 6. Sections through the third groove.
No. 7. Section through the point where the third groove passes into the
solid rod of cells.
No. 8. Section through the rod when quite separated from the germinal
epithelium.
No. 9. Section very near the termination of the rod.
No. 10. Last section in which any trace of the rod is seen.
SERIES B. Sections passing through the head-kidney at our third stage.
Zeiss C, ocul. 2. Our figures are representations of the following sections
of the series, section 1 being the first which passes through the anterior
groove of the head-kidney.
No. 1 SECTION 3. No. 8 SECTION 13.
" 2 " 4. " 9 " 15.
" 3 " 5. " 10 " 16.
" 4 " 6. " 11 " 17.
" 5 " 8. " 12 " 18.
" 6 " 10. " 13 " 19.
" 7 " 11. " 14 " 20.
The Müllerian duct extends through eleven more sections.
The first groove (_gr_1.) extends to No. 3.
The second groove (_gr_2.) extends from No. 4 to No. 7.
The third groove (_gr_3.) extends from No. 11 to No. 13.
The first ridge (_r_1.) extends from No. 2 to No. 5.
The second ridge (_r_2.) extends from No. 8 to No. 11.
The third ridge (_r_3.) extends from No. 13 backwards through twelve
sections, when it terminates by a pointed extremity.
FIG. C. Section through the ridge connecting the second and third grooves
of the head-kidney of an embryo slightly younger than that from which
Series B was taken. Zeiss C, ocul. 3 (reduced one-third).
The fold of the germinal epithelium, which gives rise to a deep groove
(_x._) external to the head-kidney is well marked.
SERIES G. Sections through the rod of cells constituting the termination of
the Müllerian duct at a stage in which the head-kidney is still present.
Zeiss C, ocul. 2.
PLATE 28.
SERIES D. Sections chosen at intervals from a complete series traversing
the peritoneal opening of the Müllerian duct, the remnant of the
head-kidney, and the termination of the Müllerian duct. Zeiss C, ocul. 3
(reduced one-third).
Nos. 1 and 2. Sections through the persistent anterior opening of the
head-kidney (abdominal opening of Müllerian duct). The approach of the
Wolffian duct to the groove may be seen by a comparison of these two
figures. In the sections in front of these (not figured) the two are much
more widely separated than in No. 1.
No. 3. Section through the Müllerian duct, just posterior to the persistent
opening.
Nos. 4 and 5. Remains of the ridges, which at an earlier stage connected
the first and second grooves, are seen passing from the Müllerian duct to
the peritoneal epithelium.
No. 6. Rudiment of the second groove (_gr_2.) of the head-kidney.
Between 6 and 7 is a considerable interval.
No. 7. All traces of this groove (_gr_2.) have vanished, and the Müllerian
duct is quite disconnected from the epithelium.
No. 8. Rudiment of the third groove (_gr_3.).
No. 9. Müllerian duct quite free in the space between the peritoneal
epithelium and the Wolffian duct, in which condition it extends until near
its termination. Between Nos. 9 and 10 is an interval of eight sections.
No. 10. The penultimate section, in which the Müllerian duct is seen. A
lumen cannot be clearly made out.
No. 11. The last section in which any trace of the Müllerian duct is
visible. No line of demarcation can be seen separating the solid end of the
Müllerian duct from the ventral wall of the Wolffian duct.
FIGS. E. and F. Sections through the glomerulus of the head-kidney from an
embryo prior to the appearance of the head-kidney. Zeiss B, ocul. 2. A
comparison of the two figures shows the variation in the thickness of the
stalk of the glomerulus. E. Section anterior to the foremost Malpighian
body. F. Section through both the glomerulus of the head-kidney and that of
a Malpighian body. The two are seen to be connected.
SERIES H. Consecutive sections through the hind end of the Müllerian duct,
from an embryo in which the head-kidney was only represented by a rudiment.
(The embryo was, perhaps, very slightly older than that from which Series D
was taken.) Zeiss C, ocul. 3 (reduced one-third).
No. 1. Müllerian duct is without a lumen, and quite distinct from the
Wolffian wall.
No. 2. The solid end of the Müllerian duct is no longer distinct from the
internal wall of the Wolffian duct.
No. 3. All trace of the Müllerian duct has vanished.
SERIES I. Sections through the hinder end of the Müllerian duct from an
embryo of about the middle of the sixth day. Zeiss C, ocul. 2 (reduced
one-third).
No. 1. The Müllerian duct is distinct and small.
No. 2. Is posterior by twelve sections to No. 1. The Müllerian duct is
dilated, and its cells are vacuolated.
No. 3. Penultimate section, in which the Müllerian duct is visible; it is
separated by three sections from No. 2.
No. 4. Last section in which any trace of the Müllerian duct is visible;
the lumen, which was visible in the previous section, is now absent.
No. 5. No trace of Müllerian duct. Nos. 3, 4, and 5 are consecutive
sections.
FIG. K. Section through the hind end of the abdominal opening of the
Müllerian duct of a chick of 123 hours. Zeiss C, ocul. 2 (reduced
one-third). It illustrates the peculiar cord connecting the Müllerian and
Wolffian ducts.
XIV. ON THE EARLY DEVELOPMENT OF THE LACERTILIA, TOGETHER WITH SOME
OBSERVATIONS ON THE NATURE AND RELATIONS OF THE PRIMITIVE STREAK[446].
Footnote 446: From the _Quarterly Journal of Microscopical
Science_, Vol. XIX. 1879.
(With Plate 29.)
Till quite recently no observations were recorded on the early
developmental changes of the reptilian ovum. Not long ago Professors
Kupffer and Benecke published a preliminary note on the early development
of _Lacerta agilis_ and _Emys Europea_[447]. I have myself also been able
to make some observations on the embryo of _Lacerta muralis_. The number of
my embryos has been somewhat limited, and most of those which I have had
have been preserved in bichromate of potash, which has turned out a far
from satisfactory hardening reagent. In spite of these difficulties I have
been led on some points to very different results from those of the German
investigators, and to results which are more in accordance with what we
know of other Sauropsidan types. I commence with a short account of the
results of Kupffer and Benecke.
Footnote 447: _Die Erste Entwicklungsvorgänge am Ei der
Reptilien_, Königsberg, 1878.
Segmentation takes place exactly as in birds, and the resulting blastoderm,
which is thickened at its edge, spreads rapidly over the yolk. Shortly
before the yolk is half enclosed a small embryonic shield (area pellucida)
makes its appearance in the centre of the blastoderm, which has, in the
meantime, become divided into two layers. The upper of these is the
epiblast, and the lower the hypoblast. The embryonic shield is mainly
distinguished from the remainder of the blastoderm by the more columnar
character of its constituent epiblast cells. It is somewhat pyriform in
shape, the narrower end corresponding with the future posterior end of the
embryo. At the narrow end an invagination takes place, which gives rise to
an open sac, the blind end of which is directed forwards. The opening of
this sac is regarded by the authors as the blastopore. A linear thickening
of epiblast arises in front of the blastopore, along the median line of
which the medullary groove soon appears. In the caudal region the medullary
folds spread out and enclose between them the blastopore, behind which they
soon meet again. On the conversion of the medullary groove into a closed
canal the blastopore becomes obliterated. The mesoblast grows out from the
lip of the blastopore as four masses. Two of these are lateral: a third is
anterior and median, and, although at first independent of the epiblast,
soon attaches itself to it, and forms with it a kind of axis-cord. A fourth
mass applied itself to the walls of the sac formed by invagination.
With reference to the very first developmental phenomena my observations
are confined to two stages during the segmentation[448]. In the earliest of
these the segmentation was about half completed, in the later one it was
nearly over. My observations on these stages bear out generally the
statements of Kupffer and Benecke. In the second of them the blastoderm was
already imperfectly divided into two layers--a superficial epiblastic layer
formed of a single row of cells, and a layer below this several rows deep.
Below this layer fresh segments were obviously being added to the
blastoderm from the subjacent yolk.
Footnote 448: For these two specimens, which were hardened in
picric acid, I am indebted to Dr Kleinenberg.
Between the second of these blastoderms and my next stage there is a
considerable gap. The medullary plate is just established, and is marked by
a shallow groove which becomes deeper in front. A section through the
embryo is represented in Pl. 29, Series A, fig. 1. In this figure there may
be seen the thickened medullary plate with a shallow medullary groove,
below which are two independent plates of mesoblast (_me.p._), one on each
side of the middle line, very imperfectly divided into somatopleuric and
splanchnopleuric layers. Below the mesoblast is a continuous layer of
hypoblast (_hy._), which develops a rod-like thickening along the axial
line (_ch._). This rod becomes in the next stage the notochord. Although
this embryo is not well preserved I feel very confident in asserting the
continuity of the notochord with the hypoblast at this stage.
At the hind end of the embryo is placed a thickened ridge of tissue which
continues the embryonic axis. In this ridge all the layers coalesce, _and I
therefore take it to be equivalent to the primitive streak of the avian
blastoderm_. It is somewhat triangular in shape, with the apex directed
backward, the broad base placed in front.
At the junction between the primitive streak and the blastoderm is situated
a passage, open at both extremities, leading from the upper surface of the
blastoderm obliquely forwards to the lower.
The dorsal and anterior wall of this passage is formed of a distinct
epithelial layer, continuous at its upper extremity with the epiblast, and
at its lower with the notochordal plate, so that it forms a layer of cells
connecting together the epiblast and hypoblast. The hinder and lower wall
of the passage is formed by the cells of the primitive streak, which only
assume a columnar form near the dorsal opening of the passage (vide
fig. 4). This passage is clearly the blind sac of Kupffer and Benecke, who,
if I am not mistaken, have overlooked its lower opening. As I hope to show
in the sequel, it is also the equivalent of the neurenteric passage, which
connects the neural and alimentary canals in the Ichthyopsida, and
therefore represents the blastopore of Amphioxus, Amphibians, &c.
Series A, figs. 2, 3, 4, 5, illustrate the features of the passage and its
relation to the embryo.
Fig. 2 passes through the ventral opening of the passage. The notochordal
plate (_ch´._) is vaulted over the opening, and on the left side is
continuous with the mesoblast as well as the hypoblast. Figs. 3 and 4 are
taken through the middle part of the passage (_ne._), which is bounded
above by a continuation of the notochordal plate, and below by the tissue
of the primitive streak. The hypoblast (_hy._), in the middle line, is
imperfectly fused with the mesoblast of the primitive streak, which is now
continuous across the middle line. The medullary groove has disappeared,
but the medullary plate (_mp._) is quite distinct.
In fig. 5 is seen the dorsal opening of the passage (_ne._). If a section
behind this had been figured, as is done for the next series (B), it would
have passed through the primitive streak, and, as in the chick, all the
layers would have been fused together. The epiblast in the primitive streak
completely coalesces with the mesoblast; but the hypoblast, though attached
to the other layers in the middle line, can always be traced as a distinct
stratum.
Fig. B is a surface view of my next oldest embryo. The medullary groove has
become much deeper, especially in front. Behind it widens out to form a
space equivalent to the sinus rhomboidalis of the embryo bird. The amnion
forms a small fold covering over the cephalic extremity of the embryo,
which is deeply embedded in the yolk. Some somites (protovertebræ) were
probably present, but this could not be made out in the opaque embryo.
[Illustration: FIG. 1. Diagrammatic longitudinal section of an embryo of
Lacerta. _pp._ body-cavity. _am._ Amnion. _ne._ Neurenteric canal. _ch._
Notochord. _hy._ Hypoblast. _ep._ Epiblast. _pr._ Primitive streak.]
The woodcut (fig. 1) represents a diagrammatic longitudinal section through
this embryo, and the sections belonging to Series B illustrate the features
of the hind end of the embryo and of the primitive streak.
As is shown in fig. 1, the notochord (_ch._) has now throughout the region
of the embryo become separated from the subjacent hypoblast, and the
lateral plates of mesoblast are distinctly divided into somatic and
splanchnic layers. The medullary groove is continued as a deepish groove up
to the opening of the neurenteric passage, which thus forms a perforation
in the floor of the hinder end of the medullary groove (vide Series B,
figs. 2, 3, and 4).
The passage itself is somewhat shorter than in the previous stage, and the
whole of it is shown in a single section (fig. 4). This section must either
have been taken somewhat obliquely, or else the passage have been
exceptionally short in this embryo, since in an older embryo it could not
all be seen in one section.
The front wall of the passage is continuous with the notochord, which for
two sections or so in front remains attached to the hypoblast (figs. 2 and
3). Behind the perforation in the floor of the medullary groove is placed
the primitive streak (fig. 5), where all the layers become fused together,
as in the earlier stage. Into this part a narrow diverticulum from the end
of the medullary groove is continued for a very short distance (vide
fig. 5, _mc._).
The general features of the stage will best be understood by an examination
of the diagrammatic longitudinal section, represented in woodcut, fig. 1.
In front is shown the amnion (_am._), growing over the head of the embryo.
The notochord (_ch._) is seen as an independent cord for the greater part
of the length of the embryo, but falls into the hypoblast shortly in front
of the neurenteric passage. The neurenteric passage is shown at _ne._, and
behind it is shown the primitive streak.
In a still older stage, represented in surface view on Pl. 29, fig. C, the
medullary folds have nearly met above, but have not yet united. The
features of the passage from the neural groove to the hypoblast are
precisely the same in the embryo just described, although the lumen of the
passage has become somewhat narrower. There is still a short primitive
streak behind the embryo.
The neurenteric passage persists but a very short time after the complete
closure of the medullary canal. It is in no way connected with the
allantois, as conjectured by Kupffer and Benecke, but the allantois is
formed, as I have satisfied myself by longitudinal sections of a later
stage, in the manner already described by Dobrynin, Gasser, and Kölliker
for the bird and mammal.
The general results of Kupffer's and Benecke's observations, with the
modifications introduced by my own observations, are as follows:--After the
segmentation and the formation of the embryonic shield (area pellucida) the
blastoderm becomes distinctly divided into epiblast and hypoblast[449]. At
the hind end of the shield a somewhat triangular primitive streak is formed
by the fusion of the epiblast and hypoblast with a number of cells between
them, which are probably derived from the lower rows of the segmentation
cells. At the front end of the streak a passage arises, open at both
extremities, leading obliquely forwards through the epiblast to the space
below the hypoblast. The walls of the passage are formed of a layer of
columnar cells continuous both with epiblast and hypoblast. In front of the
primitive streak the body of the embryo becomes first differentiated by the
formation of a medullary plate, and at the same time there grows out from
the primitive streak a layer of mesoblast, which spreads out in all
directions between the epiblast and hypoblast. In the axis of the embryo
the mesoblast plate is stated by Kupffer and Benecke to be continuous
across the middle line, but this appears very improbable. In a slightly
later stage the medullary plate becomes marked by a shallow groove, and the
mesoblast of the embryo is then undoubtedly constituted of two lateral
plates, one on each side of the median line. In the median line the
notochord arises as a ridge-like thickening of the hypoblast, which becomes
very soon quite separated from the hypoblast, except at the hind end, where
it is continued into the front wall of the neurenteric passage. It is
interesting to notice the remarkable relation of the notochord to the walls
of the neurenteric passage. More or less similar relations are also well
marked in the case of the goose and the fowl (Gasser)[450], and support the
conclusion deducible from the lower forms of vertebrata, that the notochord
is essentially hypoblastic.
Footnote 449: This appears to me to take place before the
formation of the embryonic shield.
Footnote 450: Gasser, _Der Primitivstreifen bei
Vogelembryonen_, Marburg, 1878.
The passage at the front end of the primitive streak forms the posterior
boundary of the medullary plate, though the medullary groove is not at
first continued back to it. The anterior wall of this passage connects
together the medullary plate and the notochordal ridge of the hypoblast. In
the succeeding stages the medullary groove becomes continued back to the
opening of the passage, which then becomes enclosed in the medullary folds,
and forms a true neurenteric passage. It becomes narrowed as the medullary
folds finally unite to form the medullary canal, and eventually disappears.
I conclude this paper with a concise statement of what appears to me the
probable nature of the much-disputed organ, the primitive streak, and of
the arguments in support of my view.
In a paper on the primitive streak in the _Quart. Journ. of Mic. Sci._, in
1873 (p. 280) [This edition, p. 45], I made the following statement with
reference to this subject:--"It is clear, therefore, that the primitive
groove must be the rudiment of some ancestral feature.... It is just
possible that it is the last trace of that involution of the epiblast by
which the hypoblast is formed in most of the lower animals."
At a later period, in July, 1876, after studying the development of
Elasmobranch fishes, I enlarged the hypothesis in a review of the first
part of Prof. Kölliker's _Entwicklungsgeschichte_. The following is the
passage in which I speak of it[451]:
Footnote 451: _Journal of Anat. and Phys._, Vol. X. pp. 790
and 791. Compare also my _Monograph on Elasmobranch Fishes_,
note on p. 68 [This edition, p. 281].
"In treating of the exact relation of the primitive groove to the
formation of the embryo, Professor Kölliker gives it as his view that
though the head of the embryo is formed independently of the primitive
groove, and only secondarily unites with this, yet that the remainder
of the body is without doubt derived from the primitive groove. With
this conclusion we cannot agree, and the very descriptions of
Professor Kölliker appear to us to demonstrate the untenable nature of
his results. We believe that the front end of the primitive groove at
first occupies the position eventually filled by about the third pair
of protovertebræ, but that as the protovertebræ are successively
formed, and the body of the embryo grows in length, the primitive
groove is carried further and further back, so as always to be
situated immediately behind the embryo. As Professor Kölliker himself
has shewn it may still be seen in this position even later than the
fortieth hour of incubation.
"Throughout the whole period of its existence it retains a character
which at once distinguishes it in sections from the medullary groove.
"Beneath it the epiblast and mesoblast are _always fused_, though they
are always separate elsewhere; this fact, which was originally shewn
by ourselves, has been very clearly brought out by Professor
Kölliker's observations.
"The features of the primitive groove which throw special light on its
meaning are the following:--
"(1) It does not enter directly into the formation of the embryo.
"(2) The epiblast and mesoblast always become fused beneath it.
"(3) It is situated immediately behind the embryo.
"Professor Kölliker does not enter into any speculations as to the
meaning of the primitive groove, but the above-mentioned facts appear
to us clearly to prove that the primitive groove is a rudimentary
structure, the origin of which can only be completely elucidated by a
knowledge of the development of the Avian ancestors.
"In comparing the blastoderm of a bird with that of any anamniotic
vertebrate, we are met at the threshold of our investigations by a
remarkable difference between the two. Whereas in all the lower
vertebrates the embryo is situated at the _edge_ of the blastoderm, it
is in birds and mammals situated in the centre. This difference of
position at once suggests the view that the primitive groove may be in
some way connected with the change of position in the blastoderm which
the ancestors of birds must have undergone. If we carry our
investigations amongst the lower vertebrates a little further, we find
that the Elasmobranch embryo occupies at first the normal position at
the edge of the blastoderm, but that in the course of development the
blastoderm grows round the yolk far more slowly in the region of the
embryo than elsewhere. Owing to this, the embryo becomes left in a
bay, the two sides of which eventually meet and coalesce in a linear
fashion immediately behind the embryo, thus removing the embryo from
the edge of the blastoderm and forming behind it a linear streak not
unlike the primitive streak. We would suggest the hypothesis that the
primitive groove is a rudiment which gives the last indication of a
change made by the Avian ancestors in their position in the
blastoderm, like that made by Elasmobranch embryos when removed from
the edge of the blastoderm and placed in a central situation similar
to that of the embryo bird. On this hypothesis the situation of the
primitive groove immediately behind the embryo, as well as the fact of
its not becoming converted into any embryonic organ would be
explained. The central groove might probably also be viewed as the
groove naturally left between the coalescing edges of the blastoderm.
"Would the fusion of epiblast and mesoblast also receive its
explanation on this hypothesis? We are of opinion that it would. At
the edge of the blastoderm which represents the blastopore mouth of
Amphioxus all the layers become fused together in the anamniotic
vertebrates. So that if the primitive groove is in reality a rudiment
of the coalesced edges of the blastoderm, we might naturally expect
the layers to be fused there, and the difficulty presented by the
present condition of the primitive groove would rather be that the
hypoblast is not fused with the other layers than that the mesoblast
is indissolubly united with the epiblast. The fact that the hypoblast
is not fused with the other layers does not appear to us to be fatal
to our hypothesis, and in Mammalia, where the primitive and medullary
grooves present precisely the same relations as in birds, all three
layers are, according to Hensen's account, fused together. This,
however, is denied by Kölliker, who states that in Mammals, as in
Birds, only the epiblast and mesoblast fuse together. Our hypothesis
as to the origin of the primitive groove appears to explain in a
fairly satisfactory manner all the peculiarities of this very
enigmatical organ; it also relieves us from the necessity of accepting
Professor Kölliker's explanation of the development of the mesoblast,
though it does not, of course, render that explanation in any way
untenable."
At a somewhat later period Rauber arrived at a more or less similar
conclusion, which, however, he mixes up with a number of opinions from
which I am compelled altogether to dissent[452].
Footnote 452: "Primitivrinne u. Urmund," _Morphologisches
Jahrbuch_, Band II. p. 551.
The general correctness of my view, as explained in my second quotation,
appears to me completely established by Gasser's beautiful researches on
the early development of the chick and goose[453], and by my own
observations just recorded on the lizard. While at the same time the
parallel between the blastopore of Elasmobranchii and of the Sauropsida, is
rendered more complete by the discovery of the neurenteric passage in the
latter group, which was first of all made by Gasser.
Footnote 453: Gasser, _Der Primitivstreifen bei
Vogelembryonen_, Marburg, 1878.
The following paragraphs contain a detailed attempt to establish the above
view by a careful comparison of the primitive streak and its adjuncts in
the amniotic vertebrates with the blastopore in Elasmobranchii.
In Elasmobranchii the blastopore consists of the following parts:--(1), a
section at the end of the medullary plate, which becomes converted into the
neurenteric canal[454]; (2), a section forming what may be called the yolk
blastopore, which eventually constitutes a linear streak connecting the
embryo with the edge of the blastoderm (vide monograph on Elasmobranch
fishes, pp. 281 and 296). In order to establish my hypothesis on the nature
of the primitive streak, it is necessary to find the representatives of
both these parts in the primitive streak of the amniotic vertebrates. The
first section ought to appear as a passage from the neural to the enteric
side of the blastoderm at the posterior end of the medullary plate. At its
front edge the epiblast and hypoblast should be continuous, as they are at
the hind end of the embryo in Elasmobranchii, and, finally, the passage
should, on the closure of the medullary groove, become converted into the
_neurenteric canal_. All these conditions are exactly fulfilled by the
opening at the front end of the primitive streak of the lizard (vide
woodcut, fig. 1, p. 647). In the chick there is at first no such opening,
but, as I hope to shew in a future paper, it is replaced by the epiblast
and hypoblast falling into one another at the front end of the primitive
streak. At a later period, as has been shewn by Gasser[455], there is a
distinct rudiment of the neurenteric canal in the chick, and a complete
canal in the goose. Finally, in mammals, as has been shewn by Schäffer[456]
for the guinea-pig, there is at the front end of the primitive streak a
complete continuity between epiblast and hypoblast. The continuity of the
epiblast and hypoblast at the hind end of the embryo in the bird and the
mammal is a rudiment of the continuity of these layers at the dorsal lip of
the blastopore in Elasmobranchii, Amphibia, &c. The second section of the
blastopore in Elasmobranchii or yolk blastopore is, I believe, partly
represented by the primitive streak. The yolk blastopore in Elasmobranchii
is the part of the blastopore belonging to the yolk sac as opposed to that
belonging to the embryo, and it is clear that the primitive streak cannot
correspond to the whole of this, since the primitive streak is far removed
from the edge of the blastoderm long before the yolk is completely
enclosed. Leaving this out of consideration the primitive streak, in order
that the above comparison may hold good, should satisfy the following
conditions:
Footnote 454: I use this term for the canal connecting the
neural and alimentary tract, which was first discovered by
Kowalevsky.
Footnote 455: _Loc. cit._
Footnote 456: "A contribution to the history of the
development in the Guinea-pig," _Journal of Anat. and Phys._
Vol. XI. pp. 332-336.
1. It should connect the embryo with the edge of the blastoderm.
2. It should be constituted as if formed of the fused edges of the
blastoderm.
3. The epiblast of it should eventually not form part of the medullary
plate of the embryo, but be folded over on to the ventral side.
The first of these conditions is only partially fulfilled, but, considering
the rudimentary condition of the whole structure, no great stress can, it
seems to me, be laid on this fact.
The second condition seems to me very completely satisfied. Where the two
edges of the blastoderm become united we should expect to find a complete
fusion of the layers such as takes place in the primitive streak; and the
fact that in the primitive streak the hypoblast does not so distinctly
coalesce with the mesoblast as the mesoblast with the epiblast cannot be
urged as a serious argument against me.
The growth outwards of the mesoblast from the axis of the primitive streak
is probably a remnant of the invagination of the hypoblast and mesoblast
from the lip of the blastopore in Amphibia, &c.
The groove in the primitive streak may with great plausibility be regarded
as the indication of a depression which would naturally be found along the
line where the thickened edges of the blastoderm became united.
With reference to the third condition, I will make the following
observations. The neurenteric canal, as it is placed at the extreme end of
the embryo, must necessarily, with reference to the embryo, be the
hindermost section of the blastopore, and therefore the part of the
blastopore apparently behind this can only be so owing to the embryo not
being folded off from the yolk sac; and as the yolk sac is in reality a
specialised part of the ventral wall of the body, the yolk blastopore must
also be situated on the ventral side of the embryo.
Kölliker and other distinguished embryologists have believed that the
epiblast of the whole of the primitive streak became part of the neural
plate. If this view were correct, which is accepted even by Rauber, the
hypothesis I am attempting to establish would fall to the ground. I have,
however, no doubt that these embryologists are mistaken. The very careful
observations of Gasser shew that the part of the primitive streak adjoining
the embryo becomes converted into the tail-swelling, and that the posterior
part is folded in on the ventral side of the embryo, and, losing its
characteristic structure, forms part of the ventral wall of the body. On
this point my own observations confirm those of Gasser. In the lizard the
early appearance of the neurenteric canal at the front end of the primitive
streak clearly shews that here also the primitive streak can take no share
in forming the neural plate.
The above considerations appear to me sufficient to establish my hypothesis
with reference to the nature of the primitive streak, which has the merit
of explaining, not only the structural peculiarities of the primitive
streak, but also the otherwise inexplicable position of the embryo of the
amniotic vertebrates in the centre of the blastoderm.
DESCRIPTION OF PLATE 29.
COMPLETE LIST OF REFERENCE LETTERS.
_am._ Amnion. _ch._ Notochord. _ch´._ Notochordal thickening of hypoblast.
_ep._ Epiblast. _hy._ Hypoblast. _m.g._ Medullary groove. _me.p._
Mesoblastic plate. _ne._ Neurenteric canal (blastopore). _pr._ Primitive
streak.
SERIES A. Sections through an embryo shortly after the formation of the
medullary groove. x 120[457].
Footnote 457: The spaces between the layers in these sections
are due to the action of the hardening reagent.
Fig. 1. Section through the trunk of the embryo.
Figs. 2-5. Sections through the neurenteric canal.
Fig. B. Surface view of a somewhat older embryo than that from which Series
A is taken. x 30.
SERIES B. Sections through the embryo represented in Fig. B. x 120.
Fig. 1. Section through the trunk of the embryo.
Figs. 2, 3. Sections through the hind end of the medullary groove.
Fig. 4. Section through the neurenteric canal.
Fig. 5. Section through the primitive streak.
Fig. C. Surface view of a somewhat older embryo than that represented in
Fig. B. x 30.
XV. ON CERTAIN POINTS IN THE ANATOMY OF PERIPATUS CAPENSIS[458].
Footnote 458: From the _Proceedings of the Cambridge
Philosophical Society_, Vol. III. 1879.
The discovery by Mr Moseley[459] of a tracheal system in Peripatus must be
reckoned as one of the most interesting results obtained by the naturalists
of the "Challenger." The discovery clearly proves that the genus Peripatus,
which is widely distributed over the globe, is the persisting remnant of
what was probably a large group of forms, from which the present tracheate
Arthropoda are descended.
Footnote 459: "On the Structure and Development of _Peripatus
Capensis_," _Phil. Trans._, Vol. CLXIV. 1874.
The affinities of Peripatus render any further light on its anatomy a
matter of some interest; and through the kindness of Mr Moseley I have had
an opportunity of making investigations on some well preserved examples of
_Peripatus capensis_, a few of the results of which I propose to lay before
the Society.
I shall confine my observations to three organs. (1) The segmental organs,
(2) the nervous system, (3) the so-called fat bodies of Mr Moseley.
In all the segments of the body, with the exception of the first two or
three postoral ones, there are present glandular bodies, apparently
equivalent to the segmental organs of Annelids.
These organs have not completely escaped the attention of previous
observers. The anterior of them were noticed by Grube[460], but their
relations were not made out. By Saenger[461], as I gather from Leuckart's
_Bericht_ for the years 1868-9, these structures were also noticed, and
they were interpreted as segmental organs. Their external openings were
correctly identified. They are not mentioned by Moseley, and no notice of
them is to be found in the text-books. The observations of Grube and
Saenger seem, in fact, to have been completely forgotten.
Footnote 460: "Bau von _Perip. Edwardsii_," _Archiv f. Anat.
u. Phys._ 1853.
Footnote 461: _Moskauer Naturforscher Sammlung_, Abth. Zool.
1869.
The organs are placed at the bases of the feet in two lateral divisions of
the body-cavity shut off from the main central median division of the
body-cavity by longitudinal septa of transverse muscles.
Each fully developed organ consists of three parts:
(1) A dilated vesicle opening externally at the base of a foot.
(2) A coiled glandular tube connected with this and subdivided again into
several minor divisions.
(3) A short terminal portion opening at one extremity into the coiled tube
(2) and at the other, as I believe, into the body-cavity. This section
becomes very conspicuous in stained preparations by the intensity with
which the nuclei of its walls absorb the colouring matter.
The segmental organs of Peripatus, though formed on a type of their own,
more nearly resemble those of the Leech than of any other form with which I
am acquainted. The annelidan affinities shewn by their presence are of some
interest. Around the segmental organs in the feet are peculiar cells richly
supplied with tracheæ, which appear to me to be similar to the fat bodies
in insects. There are two glandular bodies in the feet in addition to the
segmental organs.
The more obvious features of the nervous system have been fully made out by
previous observers, who have shewn that it consists of large paired
supra-oesophageal ganglia connected with two widely separated ventral
cords--stated by them not to be ganglionated. Grube describes the two cords
as falling into one another behind the anus--a feature the presence of
which is erroneously denied by Saenger. The lateral cords are united by
numerous (5 or 6 for each segment) transverse cords.
The nervous system would appear at first sight to be very lowly organised,
but the new points I believe myself to have made out, as well as certain
previously known features in it appear to me to shew that this is not the
case.
The following is a summary of the fresh points I have observed in the
nervous system:
(1) Immediately underneath the oesophagus the oesophageal commissures
dilate and form a pair of ganglia equivalent to the annelidan and
arthropodan sub-oesophageal ganglia. These ganglia are closely approximated
and united by 5 or 6 commissures. They give off large nerves to the oral
papillæ.
(2) The ventral nerve cords are covered on their ventral side by a thick
ganglionic layer[462], and at each pair of feet they dilate into a small
but distinct _ganglionic swelling_. From each ganglionic swelling are given
off a pair of large nerves[463] to the feet; and the ganglionic swellings
of the two cords are connected together by _a pair of commissures
containing ganglion cells_[464]. The other commissures connecting the two
cords together do not contain ganglion cells.
Footnote 462: This was known to Grube, _loc. cit._
Footnote 463: These nerves were noticed by Milne-Edwards, but
Grube failed to observe that they were much larger than the
nerves given off between the feet.
Footnote 464: These commissures were perhaps observed by
Saenger, _loc. cit._
The chief feature in which Peripatus was supposed to differ from normal
Arthropoda and Annelida, viz. the absence of ganglia on the ventral cords,
does not really exist. In other particulars, as in the amount of nerve
cells in the ventral cords and the completeness of the commissural
connections between the two cords, &c., the organisation of the nervous
system of Peripatus ranks distinctly high. The nervous system lies within
the circular and longitudinal muscles, and is thus not in proximity with
the skin. In this respect also Peripatus shews no signs of a primitive
condition of the nervous system.
A median nerve is given off from the posterior border of the
supra-oesophageal ganglion to the oesophagus, which probably forms a
rudimentary sympathetic system. I believe also that I have found traces of
a paired sympathetic system.
The organ doubtfully spoken of by Mr Moseley as a fat body, and by Grube as
a lateral canal, is in reality a glandular tube, lined by beautiful
columnar cells containing secretion globules, which opens by means of a
non-glandular duct into the mouth. It lies close above the ventral nerve
cords in a lateral compartment of the body-cavity, and extends backwards
for a varying distance.
This organ may perhaps be best compared with the simple salivary gland of
Julus. It is not to be confused with the slime glands of Mr Moseley, which
have their opening in the oral papillæ. If I am correct in regarding it as
homologous with the salivary glands so widely distributed amongst the
Tracheata, its presence indicates a hitherto unnoticed arthropodan affinity
in Peripatus.
XVI. ON THE MORPHOLOGY AND SYSTEMATIC POSITION OF THE SPONGIDA[465].
Footnote 465: From the _Quarterly Journ. of Microscopical
Science_, Vol. XIX. 1879.
Professor Schulze's[466] last memoir on the development of Calcareous
Sponges, confirms and enlarges Metschnikoff's[467] earlier observations,
and gives us at last a fairly complete history of the development of one
form of Calcareous Sponge. The facts which have been thus established have
suggested to me a view of the morphology and systematic position of the
Spongida, somewhat different to that now usually entertained. In bringing
forward this view, I would have it understood that it does not claim to be
more than a mere suggestion, which if it serves no other function may,
perhaps, be of use in stimulating research.
Footnote 466: "Untersuchungen über d. Bau u. d. Entwicklung
der Spongien," _Zeit. f. wiss. Zool._ Bd. XXXI. 1878.
Footnote 467: "Zur Entwicklungsgeschichte der Kalkschwämme,"
_Zeit. f. wiss. Zool._ Bd. XXIV. 1874.
To render clear what I have to say, I commence with a very brief statement
of the facts which may be considered as established with reference to the
development of _Sycandra raphanus_, the form which was studied by both
Metschnikoff and Schulze. The segmentation of the ovum, though in many ways
remarkable, is of no importance for my present purpose, and I take up the
development at the close of the segmentation, while the embryo is still
encapsuled in the parental tissues. It is at this stage lens-shaped, with a
central segmentation cavity. An equatorial plane divides it into two parts,
which have equal shares in bounding the segmentation cavity. One of these
halves is formed of about thirty-two large, round, granular cells, the
other of a larger number of ciliated clear columnar cells. While the embryo
is still encapsuled a partial invagination of the granular cells takes
place, reducing the segmentation cavity to a mere slit; this invagination
is, however, quite temporary and unimportant, and on the embryo becoming
free, which shortly takes place, no trace of it is visible; but, on the
contrary, the segmentation cavity becomes larger, and the granular cells
project very much more prominently than in the encapsuled state.
[Illustration: FIG. 1.
Two free stages in the development of _Sycandra raphanus_ (copied from
Schulze).
A. Amphiblastula stage; B. a later stage after the ciliated cells have
commenced to become invaginated; _cs._ segmentation cavity; _ec._ granular
cells which will form the ectoderm; _en._ ciliated cells which become
invaginated to form the entoderm.]
The larva, after it has left the parental tissues, has an oval form and is
transversely divided into two areas (fig. 1, A). One of these areas is
formed of the elongated, clear, ciliated cells, with a small amount of
pigment near the inner ends (_en_), and the other and larger area of the
thirty-two granular cells already mentioned (_ec_). Fifteen or sixteen of
these are arranged as a special ring on the border of the clear cells. In
the centre of the embryo is a segmentation cavity (_cs_) which lies between
the granular and the clear cells, but is mainly bounded by the vaulted
inner surface of the latter. This stage is known as the amphiblastula
stage. After the larva has for some time enjoyed a free existence, a
remarkable series of changes takes place, which result in the invagination
of the half of it formed of the clear cells, and form a prelude to the
permanent attachment of the larva. The entire process of invagination is
completed in about half an hour. The whole embryo first becomes flattened,
but especially the ciliated half which gradually becomes less prominent
(fig. 1, B), and still later the cells composing it undergo a true process
of invagination. As a result of this invagination the segmentation cavity
is obliterated and the larva assumes a compressed plano-convex form with a
central gastrula cavity, and a blastopore in the middle of the flattened
surface. The two layers of the gastrula may now be spoken of as ectoderm
and entoderm. The blastopore becomes gradually narrowed by the growth over
it of the outer row of granular cells. When it has become very small the
attachment of the larva takes place by the flat surface where the
blastopore is situated. It is effected by protoplasmic processes of the
outer ring of ectoderm cells, which, together with the other ectoderm
cells, now become amoeboid. At the same time they become clearer and permit
a view of the interior of the gastrula. Between the ectoderm cells and the
entoderm cells which line the gastrula cavity there arises a hyaline
structureless layer, which is more closely attached to the ectoderm than to
the entoderm, and is probably derived from the former. A view of the
gastrula stage after the larva has become fixed is given in fig. 2.
[Illustration: FIG. 2.
Fixed Gastrula stage of _Sycandra raphanus_ (copied from Schulze).
The figure shews the amoeboid ectoderm cells (_ec_) derived from the
granular cells of the earlier stage, and the columnar entoderm cells,
lining the gastrula cavity, derived from the ciliated cells of the earlier
stage. The larva is fixed by the amoeboid cells on the side on which the
blastopore is situated.]
[Illustration: FIG. 3.
The young of _Sycandra raphanus_ shortly after the development of the
_spicula_ (copied from Schulze).
A. View from the side; B. view from the free extremity; _os._ osculum;
_ec._ ectoderm; _en._ entoderm composed of collared ciliated cells. The
terminal osculum and lateral pores are represented as oval white spaces.]
After invagination the cilia of the entoderm cells can no longer be seen,
and are probably absorbed, and their disappearance is nearly coincident
with the complete obliteration of the blastopore, an event which takes
place shortly after the attachment of the larva. After the formation of the
structureless layer between the ectoderm and entoderm, calcareous spicules
make their appearance in it as delicate unbranched rods pointed at both
extremities. The larva when once fixed rapidly grows in length and assumes
a cylindrical form (fig. 3, A). The sides of the cylinder are beset with
calcareous spicules which project beyond the surface, and in addition to
the unbranched forms, spicules are developed with three and four rays as
well as some with a blunt extremity and serrated edge. The extremity of the
cylinder opposite the attached surface is flattened, and though surrounded
by a ring of four-rayed spicules is itself free from them. At this
extremity a small perforation is formed leading into the gastric cavity
which rapidly increases in size and forms an exhalent osculum (_os_). A
series of inhalent apertures are also formed at the sides of the cylinder.
The relative times of appearance of the single osculum and smaller
apertures is not constant for the different larvæ. On the central gastrula
cavity of the sponge becoming placed in communication with the external
water, the entoderm cells lining it become ciliated afresh (fig. 3, B,
_en_) and develop the peculiar collar characteristic of the entoderm cells
of the Spongida. When this stage of development is reached we have a fully
developed sponge of the type made known by Haeckel as Olynthus.
Till the complete development of other forms of Spongida has been worked
out it is not possible to feel sure how far the phenomena observable in
Sycandra hold good in all cases. Quite recently the Russian embryologist,
M. Ganin[468], has given an account, without illustrations, of the
development of _Spongilla fluviatilis_, which does not appear reconcileable
with that of Sycandra. Considering the difficulties of observation it
appears better to assume for this and some other descriptions that the
observations are in error rather than that there is a fundamental want of
uniformity in development amongst the Spongida.
Footnote 468: "Zur Entwicklung d. Spongilla fluviatilis,"
_Zoologischer Anzeiger_, Vol. I. No. 9, 1878.
The first point in the development of Sycandra which deserves notice is the
character of the free swimming larva. The peculiar larval form, with one
half of the body composed of amoeboid granular cells and the other of clear
ciliated cells is nearly constant amongst the Calcispongiæ, and widely
distributed in a somewhat modified condition amongst the Fibrospongiæ and
Myxospongiæ. Does this larva retain the characters of an ancestral type of
the Spongida, and if so what does its form mean? It is, of course, possible
that it has no ancestral meaning but has been secondarily acquired; I
prefer myself to think that this is not the case, more especially as it
appears to me that the characters of the larva may be plausibly explained
by regarding it as a transitional form between the Protozoa and Metazoa.
According to this view the larva is to be considered as a colony of
Protozoa, one half of the individuals of which have become differentiated
into nutritive forms, and the other half into locomotor and respiratory
forms. The granular amoeboid cells represent the nutritive forms, and the
ciliated cells represent the locomotor and respiratory forms. That the
passage from the Protozoa to the Metazoa may have been effected by such a
differentiation is not improbable on _à priori_ grounds, and fits in very
well with the condition of the free swimming larva of Spongida, though
another and perhaps equally plausible suggestion as to this passage has
been put forward by my friend Professor Lankester[469].
Footnote 469: "Notes on Embryology and Classification."
_Quarterly Journal of Microscopical Science_, Vol. XVII. 1877.
It seems not impossible, if the speculations in this paper have
any foundation that while the views here put forward as to the
passage from the Protozoon to the Metazoon condition may hold
true for the Spongida, some other mode of passage may have
taken place in the case of the other Metazoa.
While the above view seems fairly satisfactory for the free swimming stage
of the larval Sponge there arises in the subsequent development a
difficulty which appears at first sight fatal to it. This difficulty is the
invagination of the ciliated cells instead of the granular ones. If the
granular cells represent the nutritive individuals of the colony, they and
not the ciliated cells ought most certainly to give rise to the lining of
the gastrula cavity, according to the generally accepted views of the
morphology of the Spongida. The suggestion which I would venture to put
forward in explanation of this paradox involves a completely new view of
the nature and functions of the germinal layers of adult Sponges.
It is as follows:--When the free swimming ancestor of the Spongida became
fixed, the ciliated cells by which its movements used to be effected must
have to a great extent become functionless. At the same time the amoeboid
nutritive cells would need to expose as large a surface as possible. In
these two considerations there may, perhaps, be found a sufficient
explanation of the invagination of the ciliated cells, and the growth of
the amoeboid cells over them. Though respiration was, no doubt, mainly
effected by the ciliated cells, it is improbable that it was completely
localised in them, but the continuation of their function was provided for
by the formation of an osculum and pores. The ciliated collared cells which
line the ciliated chambers, or in some cases the radial tubes, are
undoubtedly derived from the invaginated cells, and if there is any truth
in the above suggestion, the collared cells in the adult Sponge must be
mainly respiratory and not digestive in function, while the normal
epithelial cells which cover the surface of the sponge, and in most cases
line the greater part of the passages through its substance, must carry on
the digestion[470]. If the reverse is the case the whole theory falls to
the ground. It has not, so far as I know, been definitely made out where
the digestion is carried on. Lieberkühn would appear to hold the view that
the amoeboid lining cells of the passages are mainly concerned with
digestion, while Carter holds that digestion is carried on by the collared
cells of the ciliated chambers.
Footnote 470: That the flat cells which line the greater part
of the passages of most Sponges are really derived from
ectodermic invaginations appears to me clearly proved by
Schulze's and Barrois' observations on the young fixed stages
of Halisarea. Ganin appears, however, to maintain a contrary
view for Spongilla.
If it is eventually proved by actual experiments on the nutrition of
Sponges, that digestion is carried on by the general cells lining the
passages, and not by the ciliated cells, it is clear that neither the
ectoderm nor entoderm of Sponges will correspond with the similarly named
layers in the Coelenterata and the Metazoa. The invaginated entoderm will
be the respiratory layer and the ectoderm the digestive and sensory layer;
the sensory function being probably mainly localised in the epithelium on
the surface, and the digestive one in the epithelium lining the passages.
Such a fundamental difference in the germinal layers between the Spongida
and the other Metazoa, would necessarily involve the creation of a special
division of the Metazoa for the reception of the former group.
XVII. NOTES ON THE DEVELOPMENT OF THE ARANEINA[471].
Footnote 471: From the _Quarterly Journ. of Microscopical
Science_, Vol. XX. 1880.
(With Plates 30, 31, 32.)
The following observations do not profess to contain a complete history of
the development even of a single species of spider. They are the result of
investigations carried on at intervals during rather more than two years,
on the ova of _Agelena labyrinthica_; and I should not have published them
now, if I had any hope of being able to complete them before the appearance
of the work I am in the course of publishing on Comparative Embryology. It
appeared to me, however, desirable to publish in full such parts of my
observations as are completed before the appearance of my treatise, since
the account of the development of the Araneina is mainly founded upon them.
My investigations on the germinal layers and organs have been chiefly
conducted by means of sections. To prepare the embryos for sections, I
employed the valuable method first made known by Bobretzky. I hardened the
embryos in bichromate of potash, after placing them for a short time in
nearly boiling water. They were stained as a whole with hæmatoxylin after
the removal of the membranes, and embedded for cutting in coagulated
albumen.
The number of investigators who have studied the development of spiders is
inconsiderable. A list of them is given at the end of the paper.
The earliest writer on the subject is Herold (No. 4); he was followed after
a very considerable interval of time by Claparède (No. 3), whose memoir is
illustrated by a series of beautiful plates, and contains a very
satisfactory account of the external features of development.
Balbiani (No. 1) has gone with some detail into the history of the early
stages; and Ludwig (No. 5) has published some very important observations
on the development of the blastoderm. Finally, Barrois (No. 2) has quite
recently taken up the study of the group, and has added some valuable
observations on the development of the germinal layers.
In addition to these papers on the true spiders, important investigations
have been published by Metschnikoff on other groups of the Arachnida,
notably the scorpion. Metschnikoff's observations on the formation of the
germinal layers and organs accord in most points with my own.
The development of the Araneina may be divided into four periods: (1) the
segmentation; (2) the period from the close of the segmentation up to the
period when the segments commence to be formed; (3) the period from the
commencing formation of the segments to the development of the full number
of limbs; (4) the subsequent stages up to the attainment of the adult form.
In my earliest stage the segmentation was already completed, and the embryo
was formed of a single layer of large flattened cells enveloping a central
mass of polygonal yolk-segments.
Each yolk-segment is formed of a number of large clear somewhat oval
yolk-spherules. In hardened specimens the yolk-spherules become polygonal,
and in ova treated with hot water prior to preservation are not
unfrequently broken up. Amongst the yolk-segments are placed a fair number
of nucleated bodies of a very characteristic appearance. Each of them is
formed of (1) a large, often angular, nucleus, filled with deeply staining
bodies (nucleoli?); (2) a layer of protoplasm surrounding the nucleus,
prolonged into a protoplasmic reticulum. The exact relation of these
nucleated bodies to the yolk-segments is not very easy to make out, but the
general tendency of my observations is to shew (1) that each nucleated body
belongs to a yolk-sphere, and (2) that it is generally placed not at the
centre, but to one side of a yolk-sphere. If the above conclusions are
correct each complete yolk-segment is a cell, and each such cell consists
of a normal nucleus, protoplasm, and yolk-spherules. There is a special
layer of protoplasm surrounding the nucleus, while the remainder of the
protoplasm consists of a reticulum holding together the yolk-spherules.
Yolk-cells of this character are seen in Pls. 31 and 32, figs. 10-21.
The nuclei of the yolk-cells are probably derived by division from the
nuclei of the segmentation rosettes (vide Ludwig, No. 5), and it is
probable that they take their origin at the time when the superficial layer
of protoplasm separates from the yolk-columns below to form the blastoderm.
The protoplasm of the yolk-cells undergoes rapid division, as is shewn by
the fact that there are often two nucleated bodies close together, and
sometimes two nuclei in a single mass of protoplasm (fig. 10). It is
probable that in some cases the yolk-spheres divide at the same time as the
protoplasm belonging to them; the division of the nucleated bodies is,
however, in the main destined to give rise to fresh cells which enter the
blastoderm.
I have not elucidated to my complete satisfaction the next stage or two in
the development of the embryo; and have not succeeded in completely
reconciling the results of my own observations with those of Claparède and
Balbiani. In order to shew exactly where my difficulties lie it is
necessary briefly to state the results arrived at by the above authors.
According to Claparède the first differentiation in Pholcus consists in the
accumulation of the cells over a small area to form a protuberance, which
he calls the _primitive cumulus_. Owing to its smaller specific gravity the
part of the ovum with the cumulus always turns upwards, like the
blastodermic pole of a fowl's egg.
After a short time the cumulus elongates itself on one side, and becomes
connected by a streak with a white patch, which appears on the surface of
the egg, about 90° from the cumulus. This patch gradually enlarges, and
soon covers the whole surface of the ovum except the region where the
cumulus is placed. It becomes the ventral plate or germinal streak of the
embryo, its extremity adjoining the cumulus is the anal extremity, and its
opposite extremity the cephalic one. The cumulus itself is placed in a
depression on the dorsal surface of the ovum. Claparède compares the
cumulus to the dorsal organ of many Crustacea.
Balbiani (No. 1) describes the primitive cumulus in _Tegenaria domestica_,
_Epeira diadema_, and _Agelena labyrinthica_, as originating as a
protuberance at the centre of the ventral surface, surrounded by a
specialised portion of the blastoderm (p. 57), which I will call the
ventral plate. In _Tegenaria domestica_ he finds that it encloses the
so-called yolk-nucleus, p. 62. By an unequal growth of the ventral plate
the primitive cumulus comes to be placed at the cephalic pole of the
ventral plate. The cumulus now becomes less prominent, and in a few cases
disappears. In the next stage the central part of the ventral plate becomes
very prominent and forms the procephalic lobe, close to the anterior border
of which is usually placed the primitive cumulus (p. 67). The space between
the cumulus and the procephalic lobe grows larger, so that the latter
gradually travels towards the dorsal surface and finally vanishes. Behind
the procephalic lobe the first traces of the segments make their
appearance, as three transverse bands, before a distinct anal lobe becomes
apparent.
The points which require to be cleared up are, (1) what is the nature of
the primitive cumulus? (2) where is it situated in relation to the embryo?
Before attempting to answer these questions I will shortly describe the
development, so far as I have made it out, for the stages during which the
cumulus is visible.
The first change that I find in the embryo (when examined after it has been
hardened)[472] is the appearance of a small, whitish spot, which is at
first very indistinct. A section through such an ovum (Pl. 31, fig. 10)
shews that the cells of about one half of the ovum have become more
columnar than those of the other half, and that there is a point (_pr.c._)
near one end of the thickened half where the cells are more columnar, and
about two layers or so deep. It appears to me probable that this point is
the whitish spot visible in the hardened ovum. In a somewhat later stage
(Pl. 30, fig. 1) the whitish spot becomes more conspicuous (_pc._), and
appears as a distinct prominence, which is, without doubt, the primitive
cumulus, and from it there proceeds on one side a whitish streak. The
prominence, as noticed by Claparède and Balbiani, is situated on the
flatter side of the ovum. Sections at this stage shew the same features as
the previous stage, except that (1) the cells throughout are smaller, (2)
those of the thickened hemisphere of the ovum more columnar, and (3) the
cumulus is formed of several rows of cells, though not divided into
distinct layers. In the next stage the appearances from the surface are
rather more obscure, and in some of my best specimens a coagulum, derived
from the fluid surrounding the ovum, covers the most important part of the
blastoderm. In Pl. 30, fig. 2, I have attempted to represent, as truly as I
could, the appearances presented by the ovum. There is a well-marked
whitish side of the ovum, near one end of which is a prominence (_pc._),
which must, no doubt, be identified with the cumulus of the earlier stages.
Towards the opposite end, or perhaps rather nearer the centre of the white
side of the ovum, is an imperfectly marked triangular white area. There can
be no doubt that the line connecting the cumulus with the triangular area
is the future long axis of the embryo, and the white area is, without
doubt, the procephalic lobe of Balbiani.
Footnote 472: I was unfortunately too much engaged, at the
time when the eggs were collected, to study them in the fresh
condition; a fact which has added not a little to my
difficulties in elucidating the obscure points in the early
stages.
A section of the ovum at this stage is represented in Pl. 31, fig. 11. It
is not quite certain in what direction the section is taken, but I think it
probable it is somewhat oblique to the long axis. However this may be, the
section shews that the whitish hemisphere of the blastoderm is formed of
columnar cells, for the most part two or so layers deep, but that there is,
not very far from the middle line, a wedge-shaped internal thickening of
the blastoderm where the cells are several rows deep. With what part
visible in surface view this thickened portion corresponds is not clear. To
my mind it most probably corresponds to the larger white patch, in which
case I have not got a section through the terminal prominence. In the other
sections of the same embryo the wedge-shaped thickening was not so marked,
but it, nevertheless, extended through all the sections. It appears to me
probable that it constitutes a longitudinal thickened ridge of the
blastoderm. In any case, it is clear that the white hemisphere of the
blastoderm is a thickened portion of the blastoderm, and that the
thickening is in part due to the cells being more columnar, and, in part,
to their being more than one row deep, _though they have not become divided
into two distinct germinal layers_. It is further clear that the increase
in the number of cells in the thickened part of the blastoderm is, _in the
main, a result of the multiplication of the original single row of cells_,
while a careful examination of my sections proves that it is also partly
due to cells, derived from the yolk, having been added to the blastoderm.
In the following stage which I have obtained (which cannot be very much
older than the previous stage, because my specimens of it come from the
same batch of eggs), a distinct and fairly circumscribed thickening forming
the ventral surface of the embryo has become established. Though its
component parts are somewhat indistinct, it appears to consist of a
procephalic lobe, a less prominent caudal lobe, and an intermediate portion
divided into about three segments; but its constituents cannot be clearly
identified with the structures visible in the previous stage. I am
inclined, however, to identify the anterior thickened area of the previous
stage with the procephalic lobe, and a slight protuberance of the caudal
portion (visible from the surface) with the primitive cumulus. I have,
however, failed to meet with any trace of the cumulus in my sections.
To this stage, which forms the first of the second period of the larval
history, I shall return, but it is necessary now to go back to the
observations of Claparède and Balbiani.
There can, in the first place, be but little doubt that what I have called
the primitive cumulus in my description is the structure so named by
Claparède and Balbiani.
It is clear that Balbiani and Claparède have both failed to appreciate the
importance of the organ, which my observations shew to be the part of the
ventral thickening of the blastoderm where two rows of cells are first
established, and therefore the point where the first traces of the future
mesoblast becomes visible.
Though Claparède and Balbiani differ somewhat as to the position of the
organ, they both make it last longer than I do: I feel certainly inclined
to doubt whether Claparède is right in considering a body he figures after
six segments are present, to be the same as the dorsal organ of the embryo
before the formation of any segments, especially as all the stages between
the two appear to have escaped him. In Agelena there is undoubtedly no
organ in the position he gives when six segments are found.
Balbiani's observations accord fairly with my own up to the stage
represented in fig. 2. Beyond this stage my own observations are not
satisfactory, but I must state that I feel doubtful whether Balbiani is
correct in his description of the gradual separation of the procephalic
lobe and the cumulus, and the passage of the latter to the dorsal surface,
and think it possible that he may have made a mistake as to which side of
the procephalic lobe, in relation to the parts of the embryo, the cumulus
is placed.
Although there appear to be grounds for doubting whether either Balbiani
and Claparède are correct in the position they assign to the cumulus, my
observations scarcely warrant me in being very definite in my statements on
this head, but, as already mentioned, I am inclined to place the organ near
the posterior end (and therefore, as will be afterwards shewn, in a
somewhat dorsal situation) of the ventral embryonic thickening.
In my earliest stage of the third period there is present, as has already
been stated, a procephalic lobe, and an indistinct and not very prominent
caudal portion, and about three segments between the two. The definition of
the parts of the blastoderm at this stage is still very imperfect, but from
subsequent stages it appears to me probable that the first of the three
segments is that of the first pair of ambulatory limbs, and that the
segments of the cheliceræ and pedipalpi are formed later than those of the
first three ambulatory appendages.
Balbiani believes that the segment of the cheliceræ is formed later than
that of the six succeeding segments. He further concludes, from the fact
that this segment is cut off from the procephalic portion in front, that it
is really part of the procephalic lobe. I cannot accept the validity of
this argument; though I am glad to find myself in, at any rate, partial
harmony with the distinguished French embryologist as to the facts.
Balbiani denies for this stage the existence of a caudal lobe. There is
certainly, as is very well shewn in my longitudinal sections, a thickening
of the blastoderm in the caudal region, though it is not so prominent in
surface views as the procephalic lobe.
A transverse section through an embryo at this stage (Pl. 31, fig. 12)
shews that there is a ventral plate of somewhat columnar cells more than
one row deep, and a dorsal portion of the blastoderm formed of a single row
of flattened cells. Every section at this stage shews that the inner layer
of cells of the ventral plate is receiving accessions of cells from the
yolk, which has not to any appreciable extent altered its constitution. A
large cell, passing from the yolk to the blastoderm, is shewn in fig. 12 at
_y.c_.
_The cells of the ventral plate are now divided into two distinct layers._
The outer of these is the _epiblast_, the inner the _mesoblast_. The cells
of both layers are quite continuous across the median line, and exhibit no
trace of a bilateral arrangement.
This stage is an interesting one on account of the striking similarity
which (apart from the amnion) exists between a section through the
blastoderm of a spider and that of an insect immediately after the
formation of the mesoblast. The reader should compare Kowalevsky's (_Mém.
Acad. Pétersbourg_, Vol. XVI. 1871) fig. 26, Pl. IX. with my fig. 12. The
existence of a continuous ventral plate of mesoblast has been noticed by
Barrois (p. 532), who states that the two mesoblastic bands originate from
the longitudinal division of a primitive single band.
In a slightly later stage (Pl. 30, fig. 3_a_ and 3_b_) six distinct
segments are interpolated between the procephalic and the caudal lobes. The
two foremost, _ch_ and _pd_ (especially the first), of these are far less
distinct than the remainder, and the first segment is very indistinctly
separated from the procephalic lobe. From the indistinctness of the first
two somites, I conclude that they are later formations than the four
succeeding ones. The caudal and procephalic lobes are very similar in
appearance, but the procephalic lobe is slightly the wider of the two.
There is a slight protuberance on the caudal lobe, which is possibly the
remnant of the cumulus. The superficial appearance of segmentation is
produced by a series of transverse valleys, separating raised intermediate
portions which form the segments. The ventral thickening of the embryo now
occupies rather more than half the circumference of the ovum.
Transverse sections shew that considerable changes have been effected in
the constitution of the blastoderm. In the previous stage, the ventral
plate was formed of an uniform external layer of epiblast, and a continuous
internal layer of mesoblast. The mesoblast has now become divided along the
whole length of the embryo, except, perhaps, the procephalic lobes, into
two lateral bands which are not continuous across the middle line (Pl. 31,
fig. 13, _me_). It has, moreover, become a much more definite layer,
closely attached to the epiblast. Between each mesoblastic band and the
adjoining yolk there are placed a few scattered cells, which in a somewhat
later stage become the splanchnic mesoblast. These cells are derived from
the yolk-cells; and almost every section contains examples of such cells in
the act of joining the mesoblast.
The epiblast of the ventral plate has not, to any great extent, altered in
constitution. It is, perhaps, a shade thinner in the median line than it is
laterally. The division of the mesoblast plate into two bands, together,
perhaps, with the slight reduction of the epiblast in the median ventral
line, gives rise at this stage to an imperfectly marked median groove.
The dorsal epiblast is still formed of a single layer of flat cells. In the
neighbourhood of this layer the yolk nuclei are especially concentrated.
The yolk itself remains as before.
The segments continue to increase regularly, each fresh segment being added
in the usual way between the last formed segment and the unsegmented caudal
lobe. At the stage when about nine or ten segments have become established,
the first rudiments of appendages become visible. At this period (Pl. 30,
fig. 4) there is a distinct median ventral groove, extending through the
whole length of the embryo, which becomes, however, considerably shallower
behind. The procephalic region is distinctly bilobed. The first segment
(that of the cheliceræ) is better marked off from it than in the previous
stage, but is without a trace of an appendage, and exhibits therefore, in
respect to the development of its appendages, the same retardation that
characterised its first appearance. The next five segments, viz. those of
the pedipalpi and four ambulatory appendages, present a very well-marked
swelling at each extremity. These swellings are the earliest traces of the
appendages. Of the three succeeding segments, only the first is well
differentiated. The caudal lobe, though less broad than the procephalic
lobe, is still a widish structure. The most important internal changes
concern the mesoblast, which is now imperfectly though distinctly divided
into somites, corresponding with segments visible externally. Each
mesoblastic somite is formed of a distinct somatic layer closely attached
to the epiblast, and a thinner and less well-marked splanchnic layer. In
the appendage-bearing segments the somatic layer is continued up into the
appendages.
The epiblast is distinctly thinner in the median line than at the two
sides.
The next stage figured (Pl. 30, figs. 5 and 6) is an important one, as it
is characterized by the establishment of the full number of appendages. The
whole length of the ventral plate has greatly increased, so that it
embraces nearly the circumference of the ovum, and there is left uncovered
but a very small arc between the two extremities of the plate (Pl. 30,
fig. 6; Pl. 31, fig. 15). This arc is the future dorsal portion of the
embryo, which lags in its development immensely behind the ventral portion.
There is a very distinctly bilobed procephalic region (_pr.l_) well
separated from the segment with the cheliceræ (_ch_). It is marked by a
shallow groove opening behind into a circular depression (_st._)--the
earliest rudiment of the stomodæum. The six segments behind the procephalic
lobes are the six largest, and each of them bears two prominent appendages.
They constitute the six appendage-bearing segments of the adult. The four
future ambulatory appendages are equal in size: they are slightly larger
than the pedipalpi, and these again than the cheliceræ. Behind the six
somites with prominent appendages there are four well-marked somites, each
with a small protuberance. These four protuberances are provisional
appendages. They have been found in many other genera of Araneina
(Claparède, Barrois). The segments behind these are rudimentary and
difficult to count, but there are, at any rate, five, and at a slightly
later stage probably six, including the anal lobe. These fresh segments
have been formed by the continued segmentation of the anal lobe, which has
greatly altered its shape in the process. The ventral groove of the earlier
stage is still continued along the whole length of the ventral plate.
By the close of this stage the full number of post-cephalic segments has
become established. They are best seen in the longitudinal section (Pl. 31,
fig. 15). There are six anterior appendage-bearing segments, followed by
four with rudimentary appendages (not seen in this figure), and six without
appendages behind. There are, therefore, sixteen in all. This number
accords with the result arrived at by Barrois, but is higher by two than
that given by Claparède.
The germinal layers (vide Pl. 31, fig. 14) have by this stage undergone a
further development. The mesoblastic somites are more fully developed. The
general relations of these somites is shewn in longitudinal section in
Pl. 31, fig. 15, and in transverse section in Pl. 31, fig. 14. In the tail,
where they are simplest (shewn on the upper side in fig. 14), each
mesoblastic somite is formed of a somatic layer of more or less cubical
cells attached to the epiblast, and a splanchnic layer of flattened cells.
Between the two is placed a completely circumscribed cavity, which
constitutes part of the embryonic body-cavity. Between the yolk and the
splanchnic layer are placed a few scattered cells, which form the latest
derivatives of the yolk-cells, and are to be reckoned as part of the
splanchnic mesoblast. The mesoblastic somites do not extend outwards beyond
the edge of the ventral plate, and the corresponding mesoblastic somites of
the two sides do not nearly meet in the middle line. In the limb-bearing
somites the mesoblast has the same general characters as in the posterior
somites, but the _somatic_ layer is prolonged as a hollow papilliform
process into the limb, so that each limb has an axial cavity continuous
with the section of the body-cavity of its somite. The description given by
Metschnikoff of the formation of the mesoblastic somites in the scorpion,
and their continuation into the limbs, closely corresponds with the history
of these parts in spiders. In the region of each procephalic lobe the
mesoblast is present as a continuous layer underneath the epiblast, but in
the earlier part of the stage, at any rate, is not formed of two distinct
layers with a cavity between them.
The epiblast at this stage has also undergone important changes. Along the
median ventral groove it has become very thin. On each side of this groove
it exhibits in each appendage-bearing somite a well-marked thickening,
which gives in surface views the appearance of a slightly raised area
(Pl. 30, fig. 5), between each appendage and the median line. These
thickenings are the first rudiments of the ventral nerve ganglia. The
ventral nerve cord at this stage is formed of two ridge-like thickenings of
the epiblast, widely separated in the median line, each of which is
constituted of a series of raised divisions--the ganglia--united by
shorter, less prominent divisions (fig. 14, _vg_). The nerve cords are
formed from before backwards, and are not at this stage found in the hinder
segments. _There is a distinct ganglionic thickening for the cheliceræ
quite independent of the procephalic lobes._
In the procephalic lobes the epiblast is much thickened, and is formed of
several rows of cells. The greater part of it is destined to give rise to
the supra-oesophageal ganglia.
During the various changes which have been described the blastoderm cells
have been continually dividing, and, together with their nuclei, have
become considerably smaller than at first. The yolk cells have in the
meantime remained much as before, and are, therefore, considerably larger
than the nuclei of the blastoderm cells. They are more numerous than in the
earlier stages, but are still surrounded by a protoplasmic body, which is
continued into a protoplasmic reticulum. The yolk is still divided up into
polygonal segments, but from sections it would appear that the nuclei are
more numerous than the segments, though I have failed to arrive at quite
definite conclusions on this point.
As development proceeds the appendages grow longer, and gradually bend
inwards. They become very soon divided by a series of ring-like
constrictions which constitute the first indications of the future joints
(Pl. 30, fig. 6). The full number of joints are not at once reached, but in
the ambulatory appendages five only appear at first to be formed. There are
four joints in the pedipalpi, while the cheliceræ do not exhibit any signs
of becoming jointed till somewhat later. The primitive presence of only
five joints in the ambulatory appendages is interesting, as this number is
permanent in Insects and in Peripatus.
The next stage figured forms the last of the third period (Pl. 30, figs. 7
and 7_a_). The ventral plate is still rolled round the egg (fig. 7), and
the end of the tail and the procephalic lobes nearly meet dorsally, so that
there is but a very slight development of the dorsal region. There are the
same number of segments as before, and the chief differences in appearance
between the present and the previous stage depend upon the fact (1) that
the median ventral integument between the nerve ganglia has become wider,
and at the same time thinner; (2) that the limbs have become much more
developed; (3) that the stomodæum is definitely established; (4) that the
procephalic lobes have undergone considerable development.
Of these features, the three last require a fuller description. The limbs
of the two sides are directed towards each other, and nearly meet in the
ventral line. The cheliceræ are two-jointed, and terminate in what appear
like rudimentary chelæ, a fact which perhaps indicates that the spiders are
descended from ancestors with chelate cheliceræ. The four embryonic
post-ambulatory appendages are now at the height of their development.
The stomodæum (Pl. 30, fig. 7, and Pl. 31, fig. 17, _st_) is a deepish pit
between the two procephalic lobes, and distinctly in front of the segment
of the cheliceræ. It is bordered in front by a large, well-marked, bilobed
upper lip, and behind by a smaller lower lip. The large upper lip is a
temporary structure, to be compared, perhaps, with the gigantic upper lip
of the embryo of Chelifer (cf. Metschnikoff). On each side of and behind
the mouth two whitish masses are visible, which are the epiblastic
thickenings which constitute the ganglia of the cheliceræ (Pl. 30, fig. 7,
_ch.g_).
The procephalic lobes (_pr.l_) now form two distinct masses, and each of
them is marked by a semicircular groove, dividing them into a narrower
anterior and a broader posterior division.
In the region of the trunk the general arrangement of the germinal layers
has not altered to any great extent. The ventral ganglionic thickenings are
now developed in all the segments in the abdominal as well as in the
thoracic region. The individual thickenings themselves, though much more
conspicuous than in the previous stage (Pl. 31, fig. 16, _v.c_), are still
integral parts of the epiblast. They are more widely separated than before
in the middle line. The mesoblastic somites retain their earlier
constitution (Pl. 31, fig. 16). Beneath the procephalic lobes the mesoblast
has, in most respects, a constitution similar to that of a mesoblastic
somite in the trunk. It is formed of two bodies, one on each side, each
composed of a splanchnic and somatic layer (Pl. 31, fig. 17, _sp._ and
_so_), enclosing between them a section of the body-cavity. But the
cephalic somites, unlike those of the trunk, are united by a median bridge
of mesoblast, in which no division into two layers can be detected. This
bridge assists in forming a thick investment of mesoblast round the
stomodæum (_st_).
The existence of a section of the body-cavity in the præoral region is a
fact of some interest, especially when taken in connection with the
discovery, by Kleinenberg, of a similar structure in the head of Lumbricus.
The procephalic lobe represents the præoral lobe of Chætopod larvæ, but the
prolongation of the body-cavity into it does not, in my opinion,
necessarily imply that it is equivalent to a post-oral segment.
The epiblast of the procephalic lobes is a thick layer several cells deep,
but without any trace of a separation of the ganglionic portion from the
epidermis.
The nuclei of the yolk have increased in number, but the yolk, in other
respects, retains its earlier characters.
The next period in the development is that in which the body of the embryo
gradually acquires the adult form. The most important event which takes
place during this period is the development of the dorsal region of the
embryo, which, up to its commencement, is practically non-existent. As a
consequence of the development of the dorsal region, the embryo, which has
hitherto had what may be called a dorsal flexure, gradually unrolls itself,
and acquires a ventral flexure. This change in the flexure of the embryo is
in appearance a rather complicated phenomenon, and has been somewhat
differently described by the two naturalists who have studied it in recent
times.
For Claparède the prime cause of the change of flexure is the translation
dorsalwards of the limbs. He compares the dorsal region of the embryo to
the arc of a circle, the two ends of which are united by a cord formed by
the line of insertion of the limbs. He points out that if you bring the
middle of the cord, so stretched between the two ends of the arc, nearer to
the summit of the arc, you necessarily cause the two ends of the arc to
approach each other, or, in other words, if the insertion of the limbs is
drawn up dorsally, the head and tail must approach each other ventrally.
Barrois takes quite a different view to that of Claparède, which will
perhaps be best understood if I quote a translation of his own words. He
says: "At the period of the last stage of the embryonic band (the stage
represented in Pl. 31, fig. 7, in the present paper) this latter completely
encircles the egg, and its posterior extremity nearly approaches the
cephalic region. Finally, the germinal bands, where they unite at the anal
lobe (placed above on the dorsal surface), form between them a very acute
angle. During the following stages one observes the anal segment separate
further and further from the cephalic region, and approach nearer and
nearer to the ventral region. This displacement of the anal segment
determines, in its turn, a modification in the divergence of the anal
bands; the angle which they form at their junction tends to become more
obtuse. The same processes continue regularly till the anal segment comes
to occupy the opposite extremity to the cephalic region, a period at which
the two germinal bands are placed in the same plane and the two sides of
the obtuse angle end by meeting in a straight line. If we suppose a
continuation of the same phenomenon it is clear that the anal segment will
come to occupy a position on the ventral surface, and the germinal bands to
approach, but in the inverse way, so as to form an angle opposite to that
which they formed at first. This condition ends the process by which the
posterior extremity of the embryonic band, at first directed towards the
dorsal side, comes to bend in towards the ventral region."
Neither of the above explanations is to my mind perfectly satisfactory. The
whole phenomenon appears to me to be very simple, and to be caused by the
elongation of the dorsal region, _i.e._ the region on the dorsal surface
between the anal and procephalic lobes. Such an elongation necessarily
separates the anal and procephalic lobes; but, since the ventral plate does
not become shortened in the process, and the embryo cannot straighten
itself on account of the egg-shell, it necessarily becomes flexed, and such
flexure can only be what I have already called a ventral flexure. If there
were but little food yolk this flexure would cause the whole embryo to be
bent in, so as to have the ventral surface concave, but instead of this the
flexure is confined at first to the two bands which form the ventral plate.
These bands are bent in the natural way (Pl. 30, fig. 8_b_), but the yolk
forms a projection, a kind of yolk-sack as Barrois calls it, distending the
thin integument between the two ventral bands. This yolk-sack is shewn in
surface view in Pl. 30, fig. 8, and in section in Pl. 32, fig. 18. At a
later period, when the yolk has become largely absorbed in the formation of
various organs, the true nature of the ventral flexure becomes apparent,
and the abdomen of the young Spider is found to be bent over so as to press
against the ventral surface of the thorax (Pl. 30, fig. 9). This flexure is
shewn in section in Pl. 32, fig. 21.
At the earliest stage of this period of which I have examples, the dorsal
region has somewhat increased, though not very much. The limbs have grown
very considerably and _now cross in the middle line_.
The ventral ganglia, though not the supra-oesophageal, have become
separated from the epiblast.
The yolk nuclei, each surrounded by protoplasm as before, are much more
numerous.
In other respects there are no great changes in the internal features.
In my next stage, represented in Pl. 30, figs. 8_a_, and 8_b_, a very
considerable advance has become effected. In the first place the dorsal
surface has increased in length to rather more than one half the
circumference of the ovum. The dorsal region has, however, not only
increased in length, but also in definiteness, and a series of transverse
markings (figs. 8_a_ and _b_), which are very conspicuous in the case of
the four anterior abdominal segments (the segments with rudimentary
appendages), have appeared, indicating the limits of segments dorsally. The
terga of the somites may, in fact, be said to have become formed. The
posterior terga (fig. 8_a_) are very narrow compared to the anterior.
The caudal protuberance is more prominent than it was, and somewhat
bilobed; it is continued on each side into one of the bands, into which the
ventral plate is divided. These bands, as is best seen in side view
(fig. 8_b_), have a ventral curvature, or, perhaps more correctly, are
formed of two parts, which meet at a large angle open towards the ventral
surface. The posterior of these parts bears the four still very conspicuous
provisional appendages, and the anterior the six pairs of thoracic
appendages. The four ambulatory appendages are now seven-jointed, as in the
adult, but though longer than in the previous stage they do not any longer
_cross or even meet in the middle line_, but are, on the contrary,
separated by a very considerable interval. This is due to the great
distension by the yolk of the ventral part of the body, in the interval
between the two parts of the original ventral plate. The amount of this
yolk may be gathered from the section (Pl. 32, fig. 18). The pedipalpi
carry a blade on their basal joint. The cheliceræ no longer appear to
spring from an independent postoral segment.
There is a conspicuous lower lip, but the upper is less prominent than
before. Sections at this stage shew that the internal changes have been
nearly as considerable as the external.
The dorsal region is now formed of a (1) flattened layer of epiblast cells,
and a (2) fairly thick layer of large and rather characteristic cells which
any one who has studied sections of spider's embryos will recognize as
derivatives of the yolk. These cells are not, therefore, derived from
prolongations of the somatic and splanchnic layers of the already formed
somites, but are new formations derived from the yolk. They commenced to be
formed at a much earlier period, and some of them are shewn in the
longitudinal section (Pl. 31, fig. 15). In the next stage these cells
become differentiated into the somatic and splanchnic mesoblast layers of
the dorsal region of the embryo.
In the dorsal region of the abdomen the heart has already become
established. So far as I have been able to make out it is formed from a
solid cord of the cells of the dorsal region. The peripheral layer of this
cord gives rise to the walls of the heart, while the central cells become
converted into the corpuscles of the blood.
The rudiment of the heart is in contact with the epiblast above, and there
is no greater evidence of its being derived from the splanchnic than from
the somatic mesoblast; it is, in fact, formed before the dorsal mesoblast
has become differentiated into two layers.
In the abdomen three or four transverse septa, derived from the splanchnic
mesoblast, grow a short way into the yolk. They become more conspicuous
during the succeeding stage, and are spoken of in detail in the description
of that stage. In the anterior part of the thorax a longitudinal and
vertical septum is formed, which grows downwards from the median dorsal
line, and divides the yolk in this region into two parts. In this septum
there is formed at a later stage a vertical muscle attached to the
suctorial part of the stomodæum.
The mesoblastic somites of the earlier stage are but little modified; and
there are still prolongations of the body-cavity into the limbs (Pl. 32,
fig. 18).