THE VASCULAR SYSTEM AND VASCULAR GLANDS.
The present chapter deals with the early development of the heart, the
development of the general circulatory system, especially the venous part
of it, and the circulation of the yolk-sack. It also contains an account of
two bodies which I shall call the suprarenal and interrenal bodies, which
are generally described as vascular glands.
_The heart._
The first trace of the heart becomes apparent during stage G, as a cavity
between the splanchnic mesoblast and the wall of the gut immediately behind
the region of the visceral clefts (Pl. 11, fig. 4, _ht._).
The body-cavity in the region of the heart is at first double, owing to the
two divisions of it not having coalesced; but even in the earliest
condition of the heart the layers of splanchnic mesoblast of the two sides
have united so as to form a complete wall below. The cavity of the heart is
circumscribed by a more or less complete epithelioid (endothelial) layer of
flattened cells, connected with the splanchnic wall of the heart by
protoplasmic processes. The origin of this lining layer I could not
certainly determine, but its connection with the splanchnic mesoblast
suggests that it is probably a derivative of this[314]. In front the cavity
of the heart is bounded by the approximation of the splanchnic mesoblast to
the wall of the throat, and behind by the stalk connecting the alimentary
canal with the yolk-sack.
Footnote 314: From observations on the development of the
heart in the Fowl, I have been able to satisfy myself that the
epithelioid lining of the heart is derived from the splanchnic
mesoblast. When the cavity of the heart is being formed by the
separation of the splanchnic mesoblast from the hypoblast, a
layer of the former remains close to the hypoblast, but
connected with the main mass of the splanchnic mesoblast by
protoplasmic processes. A second layer next becomes split from
the splanchnic mesoblast, connected with the first layer by the
above-mentioned protoplasmic processes. These two layers form
the epithelioid lining of the heart; between them is the cavity
of the heart, which soon loses the protoplasmic trabeculæ which
at first traverse it.
As development proceeds the ventral wall of the heart becomes bent inwards
on each side on a level with the wall of the gut (Plate 11, fig. 4), and
eventually becomes so folded in as to form for the heart a complete
muscular wall of splanchnic mesoblast. The growth inwards of the mesoblast
to form the dorsal wall of the heart does not, as might be expected, begin
in front and proceed backwards, but commences behind and is gradually
carried forwards.
From the above account it is clear that I have failed to find in
Elasmobranchii any traces of two distinct cavities coalescing to form the
heart, such as have been recently described in Mammals and Birds; and this,
as well as the other features of the formation of the heart in
Elasmobranchii, are in very close accordance with the careful description
given by Götte[315] of the formation of the heart in Bombinator. The
divergence which appears to be indicated in the formation of so important
an organ as the heart between Pisces and Amphibians on the one hand, and
Aves and Mammalia on the other, is certainly startling, and demands a
careful scrutiny. The most complete observations on the double formation of
the heart in Mammalia have been made by Hensen, Götte and Kölliker. These
observations lead to the conclusion (1) that the heart arises as two
independent splits between the splanchnic mesoblast and the hypoblast, each
with an epithelioid (endothelial) lining. (2) _That the heart is first
formed at a period when the folding in of the splanchnopleure to form the
throat has not commenced, and when therefore it would be impossible for it
to be formed as a single tube._
Footnote 315: Bischoff has recently stated,
_Historisch-kritische Bemerkungen ii. d. Entwicklung d.
Säugethiereier_, that Götte has found a double formation of the
heart in Bombinator. It may seem bold to question the accuracy
of Bischoff's interpretation of writings in his own language,
but I have certainly failed to gather this either from Dr
Götte's text or figures.
In Birds almost every investigator since von Baer has detected more or less
clearly the coalescence of two halves to form the unpaired heart[316]. Most
investigators have however believed that there was from the first an
unpaired anterior section of the heart, and that only the posterior part
was formed by the coalescence of two lateral halves. Professor Darlste His,
and more recently Kölliker, have stated that there is no such unpaired
anterior section of the heart. My own recent observations confirm their
conclusions as to the double formation of the heart, though I find that the
heart has from the first a Lambda-shaped form. At the apex of the Lambda
the two limbs are only separated by a median partition and are not
continuous with the aortic arches, which do not arise till a later
period[317]. In the Bird the heart arises just _behind_ the completed
throat, and a double formation of the heart appears, in fact, in all
instances to be _most distinctly correlated with the non-closure of the
throat_, a non-closure which it must be noted would render it impossible
for the heart to arise otherwise than as a double cavity.
Footnote 316: Vide _Elements of Embryology_, Foster and
Balfour, pp. 64-66.
Footnote 317: Professor Bischoff (_loc. cit._) throws doubts
upon the double formation of the heart, and supports his views
by Dr Foster's and my failure to find any trace of a double
formation of the heart in the chick. Professor Bischoff must, I
think, have misunderstood our description, which contains a
clear account of the double formation of the heart.
In the instances in which the heart arises as a double cavity _it is formed
before the complete closure of the throat_, and in those in which it arises
as a single cavity _it is formed subsequently to the complete formation of
the throat_. There is thus a double coincidence which renders the
conclusion almost certain, _that the formation of the heart as two cavities
is a secondary change which has been brought about by variations in the
period of the closing in of the wall of the throat_.
If the closing in of the throat were deferred and yet the primitive time of
formation of the heart retained, it is clear that such a condition as may
be observed in Birds and Mammals must occur, and that the two halves of the
heart must be formed widely apart, and only eventually united on the
folding in of the wall of the throat. We may then safely conclude that the
double formation of the heart has no morphological significance, and does
not, as might at first sight be supposed, imply that the ancestral
Vertebrate had two tubes in the place of the present unpaired heart. I have
spoken of this point at considerable length, on account of the
morphological importance which has been attached to the double formation of
the heart. But the views above enunciated are not expressed for the first
time. In the _Elements of Embryology_ we say, p. 64, "The exact mode of
development (of the heart) appears according to our present knowledge to be
very different in different cases; and it seems probable that the
differences are in fact the result of variations in the mode of formation
and time of closure of the alimentary canal." Götte again in his great
work[318] appears to maintain similar views, though I do not perfectly
understand all his statements. In my review of Kölliker's Embryology[319]
this point is still more distinctly enunciated in the following passage:
"The primitive wide separation and complete independence of the two halves
of the heart is certainly surprising; but we are inclined, provisionally at
least, to regard it as a secondary condition due to the late period at
which the closing of the throat takes place in Mammals."
Footnote 318: _Entwicklungsgeschichte d. Unke_, pp. 779, 780,
781.
Footnote 319: _Journal of Anatomy and Physiology_, Vol. X. p.
794.
_The general circulation._
The chief points of interest in connection with the general circulation
centre round the venous system. The arterial arches present no
peculiarities: the dorsal aorta, as in all other Vertebrates, is at first
double (Pl. 11, fig. 6, _ao_), and, generally speaking, the arrangement of
the arteries accords with what is already known in other forms. The
evolution of the venous system deserves more attention.
The cardinal veins are comparatively late developments. There is at first
one single primitive vein continuous in front with the heart and underlying
the alimentary canal through its præanal and postanal sections. This vein
is shewn in section in Pl. 11, fig. 8, _V_. It may be called either the
subintestinal or splanchnic vein. At the cloaca, where the gut enlarges and
comes in contact with the skin, this vein is compelled to bifurcate
(Pl. 18, fig. 6,_d_, _v.cau._), and usually the two branches into which it
divides are unequal in size. The two branches meet again behind the cloaca
and take their course ventral to the postanal section of the gut, and
terminate close to the end of the tail, Pl. 18, fig. 6,_c_, _v.cau._ In the
tail they form what is usually known as the caudal vein. The venous system
of Scyllium or Pristiurus, during the early parts of stage K, presents the
simple constitution just described.
Before proceeding to describe the subsequent changes which take place in
it, it appears to me worth pointing out the remarkable resemblance which
the vascular system of an Elasmobranch presents at this stage to that of an
ordinary Annelid and Amphioxus. It consists, as does the circulatory
system, in Annelids, of a neural vessel (the aorta) and an intestinal
vessel, the blood flowing backwards in the latter and forwards in the
former. The two in Elasmobranchii communicate posteriorly by a capillary
system, and in front by the arterial arches, connected like the similar
vessels in Annelids with the branchiæ. Striking as is this resemblance,
there is a still closer resemblance between the circulation of the Scyllium
embryo at stage K and that of Amphioxus. The two systems are in fact
identical except in very small details. The subintestinal vessel, absent or
only represented by the caudal vein and in part by the ductus venosus in
higher Vertebrates and adult Fish, forms the main and only posterior venous
trunk of Amphioxus and the embryo Scyllium. The only noteworthy point of
difference between Amphioxus and the embryo Scyllium is the presence of a
portal circulation in the former, absent at this stage in the latter; but
even this is acquired in Scyllium before the close of stage K, and does not
therefore represent a real difference between the two types.
The cardinal veins make their appearance before the close of stage K, and
very soon unite behind with the unpaired section of the caudal vein
(Pl. 11, fig. 9_b_, _p.cav._ and _v._). On this junction being effected
retrogressive changes take place in the original subintestinal vessel. It
breaks up in front into a number of smaller vessels; the lesser of the two
branches connecting it round the cloaca with the caudal vein first vanishes
(Pl. 11, fig. 9_a_, _v_), and then the larger; and the two cardinals are
left as the sole forward continuations of the caudal vein. This latter then
becomes prolonged forwards, and the two posterior cardinals open into it
some little distance in front of the hind end of the kidneys. By these
changes and by the disappearance of the postanal section of the gut the
caudal vein is made to appear as a superintestinal and not a subintestinal
vessel, and as the direct posterior continuation of the cardinal veins.
Embryology proves however that the caudal vein is a true subintestinal
vessel[320], and that its connection with the cardinals is entirely
secondary.
Footnote 320: The morphological importance of this point is
considerable. It proves, for instance, that the hæmal arches of
the vertebræ in the tail (vide pp. 373 and 374) potentially, at
any rate, encircle the gut and enclose the body-cavity as
completely as the ribs which meet in the median ventral line
may be said to do anteriorly.
The invariably late appearance of the cardinal veins in the embryo and
their absence in Amphioxus leads me to regard them as additions to the
circulatory system which appeared in the Vertebrata themselves, and were
not inherited from their ancestors. It would no doubt be easy to point to
vessels in existing Annelids which might be regarded as their equivalent,
but to do so would be in my opinion to follow an entirely false
morphological scent.
_The circulation of the yolk-sack._
The observations recorded on this subject are so far as I am acquainted
with them very imperfect, and in most cases the arteries and veins appear
to have been transposed.
Professor Wyman[321], however, gives a short description of the circulation
in Raja Batis, in which he rightly identifies the arteries, though he
regards the arterial ring which surrounds the vascular area as equivalent
to the venous sinus terminalis of the Bird.
Footnote 321: _Memoirs of the American Academy of Arts and
Sciences_, Vol. IX.
The general features of the circulation are clearly portrayed in the
somewhat diagrammatic figures on Pl. 9, in which the arteries are
represented red, and the veins blue[322].
Footnote 322: I may state that my determinations of the
arrangement of the circulation were made by actual observation
of the flow of the blood under the microscope.
I shall follow the figures on this plate in my descriptions.
Fig. 1 represents my earliest stage of the circulation of the yolk-sack. At
this stage there is visible a single aortic trunk passing forwards from the
embryo and dividing into two branches. No venous trunk could be detected
with the simple microscope, but probably venous channels were present in
the thickened edge of the blastoderm.
In fig. 2 the circulation was greatly advanced[323]. The blastoderm has now
nearly completely enveloped the yolk, and there remains only a small
circular space (_yk_) not enclosed by it. The arterial trunk is present as
before, and divides in front of the embryo into two branches which turn
backwards and nearly form a complete ring round the embryo. In general
appearance it resembles the sinus terminalis of the area vasculosa of the
Bird, but in reality bears quite a different relation to the circulation.
It gives off branches only on its inner side.
Footnote 323: My figure may be compared with that of Leydig,
_Rochen und Haie_, Plate III. fig. 6. Leydig calls the arterial
ring the sinus terminalis, and appears to regard it as venous,
but his description is so short that this point is not quite
clear.
A venous system of returning vessels is now fully developed, and its
relations are very remarkable. There is a main venous ring round the
thickened edge of the blastoderm, which is connected with the embryo by a
single stem which runs along the seam where the edges of the blastoderm
have coalesced. Since the venous trunks are only developed behind the
embryo, it is only the posterior part of the arterial ring which gives off
branches.
The succeeding stage, fig. 3, is also one of considerable interest. The
arterial ring has greatly extended, and now embraces nearly half the yolk,
and sends off trunks on its inner side along its whole circumference.
More important changes have taken place in the venous system. The
blastoderm has now completely enveloped the yolk, and as a result of this,
the venous ring no longer exists, but at the point where it vanished there
may be observed a number of smaller veins diverging in a brush-like fashion
from the termination of the unpaired trunk which originally connected the
venous ring with the heart. This point is indicated in the figure by the
letter _y_. The brush-like divergence of the veins is a still more marked
feature in a blastoderm of a succeeding stage (fig. 4).
The circulation in the succeeding stage (fig. 4) (projected in my figure)
only differs in details from that of the previous stage. The arterial ring
has become much larger, and the portion of the yolk not embraced (_x_) by
it is quite small. Instead of all the branches from the ring being of
nearly equal size, two of them are especially developed. The venous system
has undergone no important changes.
In fig. 5 the circulation is represented at a still later stage. The
arterial ring has come to embrace the whole yolk, and as a result of this,
has in its turn vanished as did the venous ring before it. At this stage of
the circulation there is present a single arterial and a single venous
trunk. The arterial trunk is a branch of the dorsal aorta, and the venous
trunk originally falls into the heart together with the subintestinal or
splanchnic vein, but on the formation of the liver enters this and breaks
up into capillaries in it. The venous trunk leaves the body on the right
side, and the arterial on the left.
The most interesting point to be noticed in connection with the yolk-sack
circulation of Scyllium is the fact of its being formed on a completely
different type to that of the Amniotic Vertebrates.
THE VASCULAR GLANDS.
There are in Scyllium two structures which have gone under the name of the
suprarenal body. The one of these is an unpaired rod-like body lying
between the dorsal aorta and the caudal vein in the region of the posterior
end of the kidneys. This body I propose to call _the interrenal body_. The
other is formed by a series of paired bodies situated dorsal to the
cardinal veins on branches of the aorta, and arranged segmentally. These
bodies I shall call _the suprarenal bodies_. I propose treating the
literature of these bodies together, since they have usually been dealt
with in this way, and indeed regarded as parts of the same system. As I
hope to shew in the sequel, the origin of these bodies is very different.
The interrenal body appears to be developed from the mesoblast; while my
researches on the suprarenal bodies confirm the brilliant investigations of
Leydig, shewing that they are formed out of the sympathetic ganglia.
The most important investigations on these bodies have been made by
Leydig[324]. In his first researches, _Rochen u. Haie_, pp. 71, 72, he
gives an account of the position and histology of what is probably my
interrenal body[325].
Footnote 324: _Rochen und Haie and Untersuchung. ü. Fische u.
Reptilien._
Footnote 325: I do not feel sure that Leydig's unpaired
suprarenal body is really my interrenal body, or at any rate it
alone. The point could no doubt easily be settled with fresh
specimens, but these I unfortunately cannot at present obtain.
My doubts rest partly on the fact that, in addition to my
interrenal body, other peculiar masses of tissue (which may be
called lymphoid in lieu of a better name) are certainly present
around some of the larger vessels of the kidneys which are not
identical in structure and development with my interrenal body,
and partly that Stannius' statements (to be alluded to
directly) rather indicate the existence of a second unpaired
body in connection with the kidneys, though I do not fully
understand his descriptions.
The position and relations of the interrenal body vary somewhat according
to Leydig in different cases. He makes the following statement about its
histology. "Fat molecules form the chief mass of the body, which causes its
white, or ochre-yellow colour, and one finds freely embedded in them clear
vesicular nuclei." He then proceeds to state that this structure is totally
dissimilar to that of the Mammalian suprarenal body, and gives it as his
opinion that it is not the same body as this. In his later researches[326]
he abandons this opinion, and adopts the view that the interrenal body is
part of the same system as the suprarenal bodies to be subsequently spoken
of. Leydig describes the suprarenal bodies as paired bodies segmentally
arranged along the ventral side of the spinal column situated on the
successive arteriæ axillares, and in close connection with one or more
sympathetic ganglia. He finds them formed of lobes, consisting of closed
vesicles full of nuclei and cells. Numerous nerve-fibres are also described
as present. With reference to the real meaning of these bodies he expresses
a distinct view. He says[327], "As the pituitary body is an integral part
of the brain, so are the suprarenal bodies part of the sympathetic system."
He re-affirms with still greater emphasis the same view in his _Fische u.
Reptilien_. Though these views have not obtained much acceptance, and the
accuracy of the histological data on which they are grounded has been
questioned, yet I hope to shew in the sequel not only that Leydig's
statements are in the main true, but that development proves his
conclusions to have been well founded.
Footnote 326: _Fische u. Reptilien_, p. 14.
Footnote 327: _Rochen u. Haie_, p. 18.
Stannius alludes[328] to both these bodies, and though he does not
contribute much to Leydig's previous statements, yet he accepts Leydig's
position with reference to the relation of the sympathetic and suprarenal
bodies[329].
Footnote 328: _Vergleichende Anatomie_, II. Auflage.
Footnote 329: Stannius' description is not quite intelligible,
but appears to point to the existence of a third kind of body
connected with the kidney. From my own observations (vide
above), I am inclined to regard it as probable that such a
third body exists.
The general text-books of Histology, Kölliker's work, and Eberth's article
in Stricker's _Histology_, do not give much information on this subject;
but Eberth, without apparently having examined the point, questions the
accuracy of Leydig's statements with reference to the anatomical relations
of the sympathetic ganglia and suprarenal bodies.
The last author who has dealt with this subject is Professor Semper[330].
He records observations both on the anatomy and development of these
organs. His anatomical observations are in the main confirmatory of those
of Leydig, but he shews still more clearly than did Leydig the segmental
arrangement of the suprarenal bodies. He definitely regards the interrenal
and suprarenal bodies as parts of the same system, and states that in many
forms they are continuous (p. 228):
Footnote 330: "Urogenitalsystem d. Plagiostomen." _Arb.
zool.-zoot. Inst. z. Würzburg_, Vol. II.
"Hier freilich gehen sie bei manchen Formen...in einen Körper über,
welcher zwischen den Enden d. beiden Nieren liegend dicht an der
einfachen Caudalvene sitzt."
With reference to their development he says: "They arise then also
completely independently of the kidneys, as isolated segmentally arranged
groups of mesoderm cells between the convolutions of the segmental organs;
only anteriorly do they stretch beyond them, and extend quite up to the
pericardium."
To Semper's statements I shall return, but now pass on to my own
observations. The paired suprarenal bodies are dealt with first.
_The suprarenal bodies._
My observations on these bodies in the adult Scyllium have only been made
with specimens hardened in chromic acid, and there are many points which
deserve a fuller investigation than I have been able to give them.
The general position and relations of the suprarenal bodies have been fully
given by Leydig and Semper, and I have nothing to add to their statements.
They are situated on branches of the aorta, segmentally arranged, and
extend on each side of the vertebral column from close behind the heart to
the posterior part of the body-cavity. The anterior pair are the largest,
and are formed apparently from the fusion of two bodies[331]. When these
bodies are examined microscopically, their connection with the sympathetic
ganglia becomes at once obvious. Bound up in the same sheath as the
anterior one is an especially large ganglion already alluded to by Leydig,
and sympathetic ganglia are more or less distinctly developed in connection
with all the others. There is however considerable irregularity in the
development and general arrangement of the sympathetic ganglia, which are
broken up into a number of small ganglionic swellings, on some of which an
occasional extra suprarenal body is at times developed. As a rule it may be
stated that there is a much smaller ganglionic development in connection
with the posterior suprarenal bodies than with the anterior.
Footnote 331: There is a very good figure of them in Semper's
paper, Pl. XXI. fig. 3.
The different suprarenal bodies exhibit variations in structure mainly
dependent on the ganglion cells and nerves in them, and their typical
structure is best exhibited in a posterior one, in which there is a
comparatively small development of nervous elements.
A portion of a section through one of these is represented on Pl. 19,
fig. 6, and presents the following features. Externally there is present a
fibrous capsule, which sends in the septa, imperfectly dividing up the body
into a series of alveoli or lobes. Penetrating and following the septa
there is a rich capillary network. The parenchyma of the body itself
exhibits a well-marked distinction in the majority of instances into a
cortical and medullary substance. The cortical substance is formed of
rather irregular columnar cells, for the most part one row deep, arranged
round the periphery of the body. Its cells measure on about an average
.03 Mm. in their longest diameter. The medullary substance is more or less
distinctly divided into alveoli, and is formed of irregularly polygonal
cells; and though it is difficult to give an estimate of their size on
account of their irregularity, .021 Mm. may be taken as probably about the
diameter of an average cell. The character of the cortical and medullary
cells is nearly the same, and the cells of the two strata appear rather to
differ in shape than in any other essential point. The protoplasm of both
has a markedly yellow tinge, giving to the suprarenal bodies a yellowish
brown colour. The nuclei are small compared to the size of the cells, being
about .009 Mm. in both cortical and medullary cells. In the anterior
suprarenal body there is a less marked distinction between the cortical and
the medullary layers, and a less pronounced yellow coloration of the whole,
than in the posterior bodies. The suprarenal bodies are often partially or
completely surrounded by a lymphoid tissue, which is alluded to in the
account of their development.
The most interesting features of my sections of the anterior bodies are the
relations they bring to light between the sympathetic ganglia and the
suprarenal bodies. In the case of one of the posterior suprarenal bodies, a
small ganglion is generally found attached to both ends of the body, and
invested in the same sheath; in addition to this a certain number of
ganglion cells (very conspicuous by their size and other characters) are to
be found scattered through the body. In the anterior suprarenal bodies the
development of ganglion cells is very much greater. If a section is taken
through the region where the large sympathetic ganglion (already mentioned)
is attached to the body, one half of the section is composed mainly of
sympathetic ganglion cells and nerve fibres, and the other of suprarenal
tissue, but the former spread in considerable numbers into the latter. A
transverse section through the suprarenal body in front of, or behind this
point, is still more instructive. One of these is represented in Pl. 19,
fig. 7. The suprarenal tissue is not inserted, but fills up the whole space
within the outline of the body. At one point a nerve (_n_) is seen to
enter. In connection with this are a number of ganglion cells, the exact
distribution of which has been reproduced. They are scattered irregularly
throughout the suprarenal body, but are more concentrated at the smaller
than at the large end. It is this small end which, in succeeding sections,
is entirely replaced by a sympathetic ganglion. Wavy fibres (which I take
to be nervous) are distributed through the suprarenal body in a manner
which, roughly speaking, is proportional to the number of ganglion cells.
At the large end of the body, where there are few nerve cells, the typical
suprarenal structure is more or less retained. Where the nerve fibres are
more numerous at the small end of the section, they give to the tissue a
somewhat peculiar appearance, though the individual suprarenal cells retain
their normal structure. In a section of this kind the ganglion and nerves
are clearly so intimately united with the suprarenal body as not to be
separable from it.
The question naturally arises as to whether there are cells of an
intermediate character between the ganglion cells and the cells of the
suprarenal body. I have not clearly detected any such, but my observations
are of too limited a character to settle the point in an adverse sense.
The embryological part of my researches on these bodies is in reality an
investigation of later development of the sympathetic ganglia. The earliest
stages in the development of these have already been given[332], and I take
them up here as they appear during stage L, and shall confine my
description to the changes they undergo in the anterior part of the trunk.
They form during stage L irregular masses of cells with very conspicuous
branches connecting them with the spinal nerves (Pl. 18, fig. 3). There may
be noticed at intervals solid rods of cells passing from the bodies to the
aorta, Pl. 18, fig. 2. These rods are the rudiments of the aortic branches
to which the suprarenal bodies are eventually attached.
Footnote 332: _Antea_, pp. 394-396.
In a stage between M and N the trunks connecting these bodies with the
spinal nerves are much smaller and less easy to see than during stage L. In
some cases moreover the nerves appear to attach themselves more definitely
to a central and inner part of the ganglia than to the whole of them. This
is shewn in Pl. 19, fig. 8, and I regard it as the first trace of a
division of the primitive ganglia into a suprarenal part and a ganglionic
part. The branches from the aorta have now a definite lumen, and take a
course through the centre of these bodies, as do the aortic branches in the
adult.
By stage O these bodies have acquired a distinct mesoblastic investment,
which penetrates into their interior, and divides it, especially in the
case of the anterior bodies, into a number of distinct alveoli. These
alveoli are far more distinct in some parts of the bodies than in others.
The nerve-trunks uniting the bodies with the spinal nerves are (at least in
specimens hardened in picric and chromic acids) very difficult to see, and
I have failed to detect that they are connected with special parts of the
bodies, or that the separate alveoli differ much as to the nature of their
constituent cells. The aortic branches to the bodies are larger than in the
previous stage, and the bodies themselves fairly vascular.
By stage Q (Pl. 19, fig. 9) two distinct varieties of cells are present in
these bodies. One of these is large, angular, and strikingly resembles the
ganglion cells of the spinal nerves at the same period. This variety is
found in separate lobules or alveoli on the inner border of the bodies. I
take them to be true ganglion cells, though I have not seen them in my
sections especially connected with the nerves. The cells of the second
variety are also aggregated in special lobules, and are very markedly
smaller than the ganglionic cells. They form, I imagine, the cells of the
true suprarenal tissue. At this and the earlier stage lymphoid tissue, like
that surrounding the suprarenal bodies in the adult, is found adjacent to
these bodies.
Stage Q forms my last embryonic stage, and it may perhaps be asked on what
grounds I regard these bodies as suprarenal bodies at all and not as simple
sympathetic ganglia.
My determination mainly rests on three grounds: (1) That a branch from the
aorta penetrates these bodies and maintains exactly the same relations to
them that the same branches of the aorta do in the adult to the true
suprarenal bodies. (2) That the bodies are highly vascular. (3) That in my
last stage they become divided into a ganglionic and a non-ganglionic part,
with the same relations as the ganglia and suprarenal tissue in the adult.
These grounds appear to me to afford ample justification for my
determinations, and the evidence adduced above appears to me to render it
almost certain that the suprarenal tissue is a product of the primitive
ganglion and not introduced from the mesoblast without, though it is not to
be denied that a more complete investigation of this point than it has been
possible for me to make would be very desirable.
Professor Semper states that he only made a very slight embryological
investigation of these bodies, and probably has only carefully studied
their later stages. He has accordingly overlooked the branches connecting
them with the spinal nerves, and has not therefore detected the fact that
they develop as parts of the sympathetic nervous system. I feel sure that
if he re-examines his sections of younger embryos he will not fail to
discover the nerve-branches described by me. His descriptions apart from
this point accord fairly well with my own. The credit of the discovery that
these bodies are really derivatives of the sympathetic nervous system is
entirely Leydig's: my observations do no more than confirm his remarkable
observations and well-founded conclusions.
_Interrenal body._
My investigations on the interrenal body in the adult are even less
complete than those on the suprarenal bodies. I find the body forming a
small rod elliptical in section in the posterior region of the kidney
between the dorsal aorta and unpaired caudal vein. Some little distance
behind its front end (and probably not at its thickest point) it measured
in one example, of which I have sections, a little less than a millimetre
in its longest diameter. Anteriorly it overlaps the suprarenal bodies, and
I failed to find any connection between them and it. On this point my
observations do not accord with those of Professor Semper. I have however
only been able to examine hardened specimens.
It is, vide Pl. 18, fig. 8, invested by a fairly thick tunica propria,
which sends in septa, dividing it into rather well-marked lobules or
alveoli. These are filled with polygonal cells, which form the true
parenchyma of the body. These cells are in my hardened specimens not
conspicuous by the number of oil-globules they contain, as might have been
expected from Leydig's description[333]. They are rather granular in
appearance, and are mainly peculiar from the somewhat large size of the
nucleus. The diameter of an average cell is about .015 Mm., and that of the
nucleus about .01 to .012. The nuclei are remarkably granular. The septa of
the body are provided with a fairly rich capillary network.
Footnote 333: Perhaps the body I am describing is not
identical with Leydig's posterior suprarenal body. I do not, as
mentioned above, feel satisfied that it is so from Leydig's
description.
At the first glance there is some resemblance in structure between the
tissues of the suprarenal and interrenal bodies, but on a closer inspection
this resemblance resolves itself into both bodies being divided up into
lobules by connective-tissue septa. There is in the interrenal body no
distinction between cortical and medullary layers as in the suprarenal. The
cells of the two bodies have very different characters, as is demonstrated
by a comparison of the relative diameters of the nuclei and the cells. The
cells of the suprarenal bodies are considerably larger than those of the
interrenal (.021 to .03 as compared to .015), yet the nuclei of the larger
cells of the former body do not equal in size those of the smaller cells of
the latter (.009 as compared to .01).
My observations both on the coarser anatomy and on the histology of the
interrenal body in the adult point to its being in no way connected with
the suprarenal bodies, and are thus in accordance with the earlier and not
the later views of Leydig.
The embryology of this body (under the title of suprarenal body) was first
described in my preliminary account of the development of the Elasmobranch
Fishes[334]. A short account of its embryonic structure was given, and I
stated that although I had not fully proved the point, yet I believed it to
be derived from the wall of the alimentary canal. As will be shewn in the
sequel this belief was ill-founded, and the organ in question is derived
from the mesoblast. Allusion has also been made to it by Professor Semper,
who figures it at an early stage of development, and implies that it arises
in the mesoblast and in connection with the suprarenal body. It appears at
stage K as a rod-like aggregate of mesoblast cells, rather more closely
packed than their neighbours, between the two kidneys near their hinder
ends (Plate 11, fig. 9_a_, _su_). The posterior and best marked part of it
does not extend further forwards than the front end of the large intestine,
and reaches backwards nearly as far as the hinder end of the kidneys. This
part of the body lies between the caudal vein and dorsal aorta.
Footnote 334: _Quarterly Journal of Microscopic Science_,
October, 1874. [This edition No. V.]
At about the point where the unpaired caudal vein divides into the two
cardinals, the interrenal body becomes less well marked off from the
surrounding tissue, though it may be traced forward for a considerable
distance in the region of the small intestine. It retains up to stage Q its
original extension, but the anterior part becomes quite definite though
still of a smaller calibre than the posterior. In one of my examples of
stage O the two divisions were separated by a small interval, and not as in
other cases continuous. I have not determined whether this was an
accidental peculiarity or a general feature. I have never seen any signs of
the interrenal body becoming continuous with the suprarenal bodies, though,
as in the adult, the two bodies overlap for a considerable distance.
The histology of the interrenal body in the embryonic periods is very
simple. At first it is formed of cells differing from those around in being
more circular and more closely packed. By stage L its cells have acquired a
character of their own. They are still spherical or oval, but have more
protoplasm than before, and their nucleus becomes very granular. At the
same time the whole body becomes invested by a tunic of spindle-shaped
mesoblast cells. By stage O it begins to be divided into a number of
separate areas or lobes by septa formed of nucleated fibres. These become
more distinct in the succeeding stages up to Q (Pl. 18, fig. 7), and in
them a fair number of capillaries are formed.
From the above description it is clear that embryology lends no more
countenance than does anatomy to the view that the interrenal bodies belong
to the same system as the suprarenal, and it becomes a question with which
(if of either) of these two bodies the suprarenal bodies of the higher
Vertebrata are homologous. This question I shall not attempt to answer in a
definite way. My own decided belief is that the suprarenal bodies of
Scyllium are homologous with the suprarenal bodies of Mammalia, and a good
many points both in their structure and position might be urged in favour
of this view. In the mean time, however, it appears to me better to wait
before expressing a definite opinion till the embryonic development of the
suprarenal bodies has been worked out in the higher Vertebrata.
EXPLANATION OF PLATE 19.
COMPLETE LIST OF REFERENCE LETTERS.
_Nervous System._
_n._ Nerve. _spn._ Spinal nerve. _syg._ Sympathetic ganglion.
_Alimentary Canal._
_cl._ Cloaca. _incl._ Cloacal involution. _oeep._ OEsophageal epithelium.
_pan._ Pancreas. _th._ Thyroid body.
_General._
_abp._ Abdominal pocket (pore). _aur._ Auricle. _cav._ Cardinal vein.
_cauv._ Caudal vein. _ly._ Lymphoid tissue. _mm._ Muscles. _od._ Oviduct.
_pc._ Pericardium. _pp._ body-cavity. _sr._ Suprarenal body. _u._ Ureter.
_vao._ Ventral aorta (anterior continuation of bulbus arteriosus). _ven._
Ventricle. _wd._ Wolffian duct.
Figs. 1_a_, 1_b_, 1_c_. Three sections through the cloacal region of an
embryo belonging to stage O. 1_a_ is the anterior of the three sections.
Zeiss A, ocul. 2. Reduced one-third.
1_a_ shews the cloacal involution at its deepest part abutting on the
cloacal section of the alimentary tract.
1_b_ is a section through a point somewhat behind this close to the opening
of the Wolffian ducts into the cloaca.
1_c_ shews the opening to the exterior in the posterior part of the cloaca,
and also the rudiments of the two abdominal pockets (_abp_).
Fig. 2. Section through the cloacal region of an embryo belonging to stage
P. Zeiss A, ocul. 2.
The figure shews the solid anterior extremity of the cloacal involution.
Fig. 3. Longitudinal vertical section through the thyroid body in a stage
between O and P. Zeiss a a, ocul. 1.
The figure shews the solid thyroid body (_th_) connected in front with
throat, and terminating below the bulbus arteriosus.
Fig. 4. Pancreas (_pan_) and adjoining part of the alimentary tract in
longitudinal section, from an embryo between stages L and M. Zeiss A, ocul.
2.
Fig. 5. Portion of liver network of stage L. Zeiss C, ocul. 2. The section
is intended to illustrate the fact that the tubules or cylinders of which
the liver is composed are hollow and not solid. Between the liver tubules
are seen blood spaces with distinct walls, and blood corpuscles in their
interior.
Fig. 6. Section through part of one of the suprarenal bodies of an adult
Scyllium hardened in chromic acid. Zeiss C, ocul. 2. The section shews the
columnar cells forming the cortex and the more polygonal cells of the
medulla.
Fig. 7. Transverse section through the anterior suprarenal body of an adult
Scyllium. Zeiss B, ocul. 2. Reduced one-third. The tissue of the suprarenal
body has not been filled in, but only the sympathetic ganglion cells which
are seen to be irregularly scattered through the substance of the body. The
entrance of the nerve (_n_) is shewn, and indications are given of the
distribution of the nerve-fibres.
Fig. 8. Section through the sympathetic ganglion of a Scyllium embryo
between stages M and N, shewing the connecting trunk between the suprarenal
body and the spinal nerve (_spn_), and the appearance of an indication in
the ganglion of a portion more directly connected with the nerve. Zeiss D,
ocul. 2.
Fig. 9. Section through one of the anterior sympathetic ganglia of an
embryo of stage Q, shewing its division into a true ganglionic portion
(_syg_), and a suprarenal body (_sr_). Zeiss C, ocul. 2.