THE ALIMENTARY CANAL.
The present Chapter completes the history of the primitive alimentary
canal, whose formation has already been described. In order to economise
space, no attempt has been made to give a full account of the alimentary
canal and its appendages, but only those points have been dealt with which
present any features of special interest.
The development of the following organs is described in order.
(1) The solid oesophagus.
(2) The postanal section of the alimentary tract.
(3) The cloaca and anus.
(4) The thyroid body.
(5) The pancreas.
(6) The liver.
(7) The subnotochordal rod.
_The solid oesophagus._
A curious point which has turned up in the course of my investigations is
the fact that for a considerable period of embryonic life a part of the
oesophagus remains quite solid and without a lumen. The part of the
oesophagus to undergo this peculiar change is that which overlies the
heart, and extends from the front end of the stomach to the branchial
region. At first, this part of the oesophagus has the form of a tube with a
well-developed lumen like the remainder of the alimentary tract, but at a
stage slightly younger than K its lumen becomes smaller, and finally
vanishes, and the original tube is replaced by a solid rod of uniform and
somewhat polygonal cells. A section of it in this condition is represented
in Pl. 11, fig. 8_a_.
At a slightly later stage its outermost cells become more columnar than the
remainder, and between stages K and L it loses its cylindrical form and
becomes much more flattened. By stage L the external layer of columnar
cells is more definitely established, and the central rounded cells are no
longer so numerous (Pl. 18, fig. 4, _soes_).
In the succeeding stages the solid part of the oesophagus immediately
adjoining the stomach is carried farther back relatively to the heart and
overlies the front end of the liver. A lumen is not however formed in it by
the close of stage Q, and beyond that period I have not carried my
investigations, and cannot therefore state the exact period at which the
lumen reappears. The limits of the solid part of the oesophagus are very
satisfactorily shewn in longitudinal and vertical sections.
The solidification of the oesophagus belongs to a class of embryological
phenomena which are curious rather than interesting, and are mainly worth
recording from the possibility of their turning out to have some
unsuspected morphological bearings.
Up to stage Q there are no signs of a rudimentary air-bladder.
_The postanal section of the alimentary tract._
An account has already been given (p. 307) of the posterior continuity of
the neural and alimentary canals, and it was there stated that Kowalevsky
was the discoverer of this peculiar arrangement. Since that account was
published, Kowalevsky has given further details of his investigations on
this point, and more especially describes the later history of the
hindermost section of the alimentary tract. He says[303]:
The two germinal layers, epiblast and hypoblast, are
continuous with each other at the border of the germinal
disc. The primitive groove or furrow appears at the border
of the germinal disc and is continued from the upper to the
lower side. By the closing of the groove there is formed the
medullary canal above, while the part of the groove on the
under surface directed below is chiefly converted into the
hind end of the alimentary tract. The connection of the two
tubes in Acanthias persists till the formation of the anus,
and the part of the nervous tube which lies under the chorda
passes gradually upwards to the dorsal side of the chorda,
and persists there for a long time in the form of a large
thin-walled vesicle.
Footnote 303: _Archiv f. Mic. Anat._ Vol. XIII. pp. 194, 195.
The last part of the description beginning at "The connection of" does not
hold good for any of the genera which I have had an opportunity of
investigating, as will appear from the sequel.
In a previous section[304] the history of the alimentary tract was
completed up to stage G.
Footnote 304: p. 303 et seq.
In stage H the point where the anus will (at a very much later period)
appear, becomes marked out by the alimentary tract sending down a
papilliform process towards the skin. This is shewn in Pl. 8, figs. _H_ and
_I, an_.
That part of the alimentary tract which is situated behind this point may,
for convenience, be called _the postanal section_. During stage H the
postanal section begins to develop a terminal dilatation or vesicle,
connected with the remainder of the canal by a narrower stalk. The relation
in diameter between the vesicle and the stalk may be gathered by a
comparison of figs. 3_a_ and 3_b_, Pl. 11. The diameter of the vesicle
represented in section in Pl. 11, fig. 3, is 0.328 Mm.
The walls both of the vesicle and stalk are formed of a fairly columnar
epithelium. The vesicle communicates in front by a narrow passage (Pl. 11,
fig. 3_a_) with the neural canal, and behind is continued into two horns
(Pl. 11, fig. 2, _al._) corresponding with the two caudal swellings spoken
of above (p. 288). Where the canal is continued into these two horns, its
walls lose their distinctness of outline, and become continuous with the
adjacent mesoblast.
In the succeeding stages up to K the tail grows longer and longer, and with
it grows the postanal section of the alimentary tract, without however
altering in any of its essential characters.
Its features at stage K are illustrated by an optical section of the tail
of an embryo (Pl. 18, fig. 5) and by a series of transverse sections
through the tail of another embryo in Pl. 18, figs. 6_a_, 6_b_, 6_c_, 6_d_.
In the optical section there is seen a terminal vesicle (_alv._) opening
into the neural canal, and connected with the remainder of the alimentary
tract. The terminal vesicle causes the end of the tail to be dilated, as is
shewn in Pl. 8, fig. _K_. The length of the postanal section extending from
the abdominal paired fins to the end of the tail (equal to rather less than
one-third of the whole length of the embryo), may be gathered from the same
figure.
The most accurate method of studying this part of the alimentary canal is
by means of transverse sections. Four sections have been selected for
illustration (Pl. 18, figs. 6_a_, 6_b_, 6_c_, and 6_d_) out of a
fairly-complete series of about one hundred and twenty.
Posteriorly (fig. 6_a_) there is present a terminal vesicle .25 Mm. in
diameter, and therefore rather smaller than in the earlier stage, whose
walls are formed of columnar epithelium, and which communicates dorsally by
a narrow opening with the neural canal; to this is attached a stalk in the
form of a tube, also lined by columnar epithelium, and extending through
about thirty sections (Pl. 18, fig. 6_b_). Its average diameter is about
.084 Mm. Overlying its front end is the subnotochordal rod (fig. 6_b_,
_x._), but this does not extend as far back as the terminal vesicle.
The thick-walled stalk of the vesicle is connected with the cloacal section
of the alimentary tract by a very narrow thin-walled tube (Pl. 18, 6_c_,
_al._). This for the most part has a fairly uniform calibre, and a diameter
of not more than .035 Mm. Its walls are formed of a flattened epithelium.
At a point not far from the cloaca it becomes smaller, and its diameter
falls to .03 Mm. In front of this point it rapidly dilates again, and,
after becoming fairly wide, opens on the dorsal side of the cloacal section
of the alimentary canal just behind the anus (fig. 6_d_).
Near the close of stage K at a point shortly behind the anus, where the
postanal section of the canal was thinnest in the early part of the stage,
the alimentary canal becomes solid (Pl. 11, fig. 9_d_), and a rupture here
occurs in it at a slightly later period.
In stage L the posterior part of the postanal section of the canal is
represented by a small rudiment near the end of the tail. The rudiment no
longer has a terminal vesicle, _nor does it communicate with the neural
canal_. It was visible in one series for about 40 sections, and was
continued forwards by a few granular cells, lying between the aorta and the
caudal vein. The portion of the postanal section of the alimentary tract
just behind the cloaca, was in the same embryo represented by a still
smaller rudiment of the dilated part which at an earlier period opened into
the cloaca.
Later than stage L no trace of the postanal section of the alimentary canal
has come under my notice, and I conclude that it vanishes without becoming
converted into any organ in the adult. Since my preliminary account of the
development of Elasmobranch Fishes was written, no fresh light appears to
have been thrown on the question of the postanal section of the alimentary
canal being represented in higher Vertebrata by the allantois.
_The cloaca and anus._
Elasmobranchii agree closely with other Vertebrates in the formation of the
cloaca and anus, and in the relations of the cloaca to the urinogenital
ducts.
The point where the anus, or more precisely the external opening of the
cloaca, will be formed, becomes very early marked out by the approximation
of the wall of the alimentary tract and external skin. This is shewn for
stages H and I in Pl. 8 _an_.
Between stages I and K the alimentary canal on either side of this point,
which we may for brevity speak of as the anus, is far removed from the
external skin, but at the anus itself the lining of the alimentary canal
and the skin are in absolute contact. There is, however, no involution from
the exterior, but, on the contrary, the position of the anus is marked by a
distinct prominence. Opposite the anus the alimentary canal dilates and
forms the cloaca.
During stage K, just in front of the prominence of the anus, a groove is
formed between two downgrowths of the body-wall. This is shewn in Pl. 11,
fig. 9_a_. During the same stage the segmental ducts grow downwards to the
cloaca, and open into it in the succeeding stage (Pl. 11, fig. 9_b_). Up to
stage K the cloaca is connected with the præanal section of the alimentary
canal in front, and the postanal section behind; the latter, however, by
stage L, as has been stated above, atrophies, with the exception of a very
small rudiment. In stage L the posterior part of the cloaca is on a level
with the hind end of the kidneys, and is situated behind the posterior
horns of the body-cavity, which are continued backwards to about the point
where the segmental ducts open into the cloaca, and though very small at
their termination rapidly increase in size anteriorly.
Nothing very worthy of note takes place in connection with the cloaca till
stage O. By this stage we have three important structures developed. (1) An
involution from the exterior to form the mouth of the cloaca or anus. (2) A
perforation leading into the cloaca at the hind end of this. (3) The
rudiments of the abdominal pockets. All of these structures are shewn in
Pl. 19, figs. 1_a_, 1_b_, 1_c_.
The mouth of the cloaca is formed by an involution of the skin, which is
deepest in front and becomes very shallow behind (Pl. 19, figs. 1_a_,
1_b_). At first only the mucous layer of the skin takes part in it, but
when the involution forms a true groove, both layers of the skin serve to
line it. At its posterior part, where it is shallowest, there is present,
at stage O, a slit-like longitudinal perforation, leading into the
posterior part of the cloaca (Pl. 19, fig. 1_c_) and forming its external
opening. Elsewhere the wall of the cloaca and cloacal groove are merely in
contact but do not communicate. On each side of the external opening of the
cloaca there is present an involution (Pl. 19, fig. 1_c_, _ab.p._) of the
skin, which resembles the median cloacal involution, and forms the rudiment
of an abdominal pocket. These two rudiments must not be confused with two
similar ones, which are present in all the three sections represented, and
mark out the line which separates the limbs from the trunk. These latter
are not present in the succeeding stages. The abdominal pockets are only
found in sections through the opening into the cloaca, and are only visible
in the hindermost of my three sections.
All the structures of the adult cloaca appear to be already constituted by
stage O, and the subsequent changes, so far as I have investigated them,
may be dealt with in very few words. The perforation of the cloacal
involution is carried slowly forwards, so that the opening into the cloaca,
though retaining its slit-like character, becomes continuously longer; by
stage Q its size is very considerable. The cloacal involution, relatively
to the cloaca, recedes backwards. In stage O its anterior end is situated
some distance in front of the opening of the segmental duct into the
cloaca; by stage P the front end of the cloacal involution is nearly
opposite this opening, and by stage Q is situated behind it.
As I have shewn elsewhere[305], the so-called abdominal pores of Scyllium
are simple pockets open to the exterior, but without any communication with
the body-cavity. By stage Q they are considerably deeper than in stage O,
and retain their original position near the hind end of the opening into
the cloaca. The opening of the urinogenital ducts into the cloaca will be
described in the section devoted to the urinogenital system.
Footnote 305: This Edition, No. VII. p. 152.
In Elasmobranchii, as in other Vertebrata, that part of the cloaca which
receives the urinogenital ducts, is in reality the hindermost section of
the gut and not the involution of epiblast which eventually meets this.
Thus the urinogenital ducts at first open into the alimentary canal and not
to the exterior. This fact is certainly surprising, and its meaning is not
quite clear to me.
The very late appearance of the anus may be noticed as a point in which
Elasmobranchii agree with other Vertebrata, notably the Fowl[306]. The
abdominal pockets, as might be anticipated from their structure in the
adult, are simple involutions of the epiblast.
Footnote 306: Vide Gasser, _Entwicklungsgeschichte der
Allantois, etc._
_The thyroid body._
The earliest trace of the thyroid body has come under my notice in a
Torpedo embryo slightly older than I. In this embryo it appeared as a
diverticulum from the ventral surface of the throat in the region of the
_mandibular arch_, and extended from the border of the mouth to the point
where the ventral aorta divided into the two aortic branches of the
mandibular arch. In front it bounded a groove (Pl. 15, fig. 5_a_, _Th._),
directly continuous with the narrow posterior pointed end of the mouth and
open to the throat, while behind it became a solid rod attached to the
ventral wall of the oesophagus (Pl. 15, fig. 5_b_, _Th._). In a Scyllium
embryo belonging to the early part of stage K, the thyroid gland presented
the same arrangement as in the Torpedo embryo just described, with the
exception that no solid posterior section of it was present.
Towards the close of stage K the thyroid body begins to elongate and become
solid, though it still retains its attachment to the wall of the
oesophagus. The solidification is effected by the columnar cells which line
the groove elongating and meeting in the centre. As soon as the lumen is by
these means obliterated, small cells make their appearance in the interior
of the body, probably budded off from the original columnar cells.
The gland continues to grow in length, and by stage L assumes a long
sack-like form with a layer of columnar cells bounding it externally, and a
core of rounded cells filling up its interior. Anteriorly it is still
attached to the throat, and its posterior extremity lies immediately below
the end of the ventral aorta. The cells of the gland contain numerous
yellowish concretionary pigment bodies, which are also present in the later
stages.
Up to stage P the thyroid gland retains its original position. Its form and
situation are shewn in Pl. 19, fig. 3, _th._, in longitudinal and vertical
section for a stage between O and P. The external layer of columnar cells
has now vanished, and the gland is divided up by the ingrowth of
connective-tissue septa into a number of areas or lobules--the rudiments of
the future follicles. These lobules are perfectly solid without any trace
of a lumen. A capillary network following the septa is present.
By stage Q the rudimentary follicles are more distinctly marked, but still
without a lumen, and a connective-tissue sheath indistinctly separated from
the surrounding tissue has been formed. My sections do not shew a junction
between the gland and the epithelium of the throat; but the two are so
close together, that I am inclined to think that such a junction still
exists. It is certainly present up to stage P.
Dr Müller[307], in his exhaustive memoir on the thyroid body, gives an
account of its condition in two Acanthias embryos. In his earliest embryo
(which, judging from the size, is perhaps about the same age as my latest)
the thyroid body is disconnected from the throat, yet contains a lumen, and
is not divided up into lobules. It is clear from this account, that there
must be considerable differences of detail in the development of the
thyroid body in Acanthias and Scyllium.
Footnote 307: _Jenaische Zeitschrift_, Vol. VI.
In the Bird Dr Müller's figures shew that the thyroid body develops in the
region of the hyoid arch, whereas, in Elasmobranchii, it develops in the
region of the mandibular arch. Dr Götte's[308] account of this body in
Bombinator accords very completely with my own, both with reference to the
region in which it develops, and its mode of development.
Footnote 308: _Entwicklungsgeschichte d. Unke._
_The pancreas._
The pancreas arises towards the close of stage K as a somewhat rounded
hollow outgrowth from the dorsal side of that part of the gut which from
its homologies may be called the duodenum. In the region where the pancreas
is being formed the appearances presented in a series of transverse
sections are somewhat complicated (Pl. 18, fig. 1), owing to the several
parts of the gut and its appendages which may appear in a single section,
but I have detected no trace of other than a single outgrowth to form the
pancreas.
By stage L the original outgrowth from the gut has become elongated
longitudinally, but transversely compressed: at the same time its opening
into the duodenum has become somewhat narrowed.
Owing to these changes the pancreas presents in longitudinal and vertical
section a funnel-shaped appearance (Pl. 19, fig. 4). From the expanded
dorsal part of the funnel, especially from its anterior end, numerous small
tubular diverticula grow out into the mesoblast. The apex of the funnel
leads into the duodenum. From this arrangement it results that at this
period the original outgrowth from the duodenum serves as a receptacle into
which each ductule of the embryonic gland opens separately. I have not
followed in detail the further growth of the gland. It is, however, easy to
note that while the ductules grow longer and become branched, vascular
processes grow in between them, and the whole forms a compact glandular
body in the mesentery on the dorsal side of the alimentary tract, and
nearly on a level with the front end of the spiral valve. The funnel-shaped
receptacle loses its original form, and elongating, assumes the character
of a duct.
From the above account it follows that the glandular part of the pancreas,
and not merely its duct, is derived from the original hypoblastic outgrowth
from the gut. This point is extremely clear in my preparations, and does
not, in spite of Schenk's observations to the contrary[309], appear to me
seriously open to doubt.
Footnote 309: _Lehrbuch d. vergleichenden Embryologie._
_The liver._
The liver arises during stage I as a ventral outgrowth from the duodenum
immediately in front of the opening of the umbilical canal (duct of the
yolk-sack) into the intestine. Almost as soon as it is formed this
outgrowth develops two lateral diverticula opening into a median canal.
The two diverticula are the rudimentary lobes of the liver, and the median
duct is the rudiment of the common bile-duct (ductus choledochus) and
gall-bladder (Pl. 11, fig. 9).
By stage K the hepatic diverticula have begun to bud out a number of small
hollow knobs. These rapidly increase in length and number, and form the
so-called hepatic cylinders. They anastomose and unite together, so that by
stage L there is constructed a regular network. As the cylinders increase
in length their lumen becomes very small, but appears never to vanish
(Pl. 19, fig. 5).
The mode of formation of the liver parenchyma by hollow and not solid
outgrowths agrees with the suggestion made in the _Elements of Embryology_,
p. 133, and also with the results of Götte on the Amphibian liver. Schenk
has thrown doubts upon the hypoblastic nature of the secreting tissue of
the liver, but it does not appear to me, from my own investigations, that
this point is open to question.
Coincidently with the formation of the hepatic network, the umbilical vein
(Pl. 11, fig. 9, _u.v._) which unites with the subintestinal or splanchnic
vein (Pl. 11, fig. 8, _V._) breaks up into a series of channels, which form
a second network in the spaces of the hepatic network. These vascular
channels of the liver appear to me to have from the first distinct walls of
delicate spindle-shaped cells, and I have failed to find a stage similar to
that described by Götte for Amphibians in which the blood-channels are
simply lacunar spaces in the hepatic parenchyma.
The changes of the median duct of the liver are of rather a passive nature.
By stage O its anterior end has dilated into a distinct gall-bladder, whose
duct receives in succession the hepatic ducts, and so forms the ductus
choledochus. The ductus choledochus opens on the ventral side of the
intestine immediately in front of the commencement of the spiral valve.
It may be noted that the liver and pancreas are corresponding ventral and
dorsal appendages of the part of the alimentary tract immediately in front
of its junction with the yolk-sack.
_The subnotochordal rod._
The existence of this remarkable body in Vertebrata was first made known by
Dr Götte[310], who not only demonstrated its existence, but also gave a
correct account of its development. Its presence in Elasmobranchii and mode
of development were mentioned by myself in my preliminary account of the
development of these fishes[311], and it has been independently observed
and described by Professor Semper[312]. No plausible suggestion as to its
function has hitherto been made, and it is therefore a matter of some
difficulty to settle with what group of organs it ought to be treated. In
the presence of this difficulty it seemed best to deal with it in this
chapter, since it is unquestionably developed from the wall of the
alimentary canal.
Footnote 310: _Archiv für Micros. Anatomie_, Bd. V., and
_Entwicklungsgeschichte d. Unke_.
Footnote 311: _Quarterly Journal of Microscopic Science_, Oct.,
1874. [This Edition, No. V.]
Footnote 312: "Stammverwandtschaft d. Wirbelthiere u.
Wirbellosen" and "Das Urogenitalsystem d. Plagiostomen," _Arb.
Zool.-Zoot. Institut. z. Würzburg_, Bd. II.
At its full growth this body forms a rod underlying the notochord, and has
nearly the same longitudinal extension as this. It is indicated in most of
my sections by the letter _x_. We may distinguish two sections of it, the
one situated in the head, the other in the trunk. The junction between the
two occurs at the hind border of the visceral clefts.
The section in the trunk is the first to develop. It arises during stage H
in the manner illustrated in Pl. 11, figs. 1 and 1_a_. The wall of the
alimentary canal becomes thickened (Pl. 11, fig. 1) along the median dorsal
line, or else produced into a ridge into which there penetrates a narrow
prolongation of the lumen of the alimentary canal. In either case the cells
at the extreme summit of the thickening become gradually constricted off as
a rod, which lies immediately dorsal to the alimentary tract, and ventral
to the notochord. The shape of the rod varies in the different regions of
the body, but it is always more or less elliptical in section. Owing to its
small size and soft structure it is easily distorted in the process of
preparing sections.
In the hindermost part of the body its mode of formation differs somewhat
from that above described. In this part the alimentary wall is very thick
and undergoes no special growth prior to the formation of the
subnotochordal rod; on the contrary, a small linear portion of the wall
becomes scooped out along the median dorsal line, and eventually separates
from the remainder as the rod in question. In the trunk the splitting off
of the rod takes place from before backwards, so that the anterior part of
it is formed before the posterior.
The section of the subnotochordal rod in the head would appear from my
observations on Pristiurus to develop in the same way as in the trunk, and
the splitting off from the throat proceeds from before backwards (Pl. 15,
fig. 4_a_, _x_).
In Torpedo, this rod develops very much later in the head than in the
trunk; and indeed my conclusion that it develops in the head at all is only
based on grounds of analogy, since in my oldest Torpedo embryo (just
younger than K) there is no trace of it present. In a Torpedo embryo of
stage I the subnotochordal rod of the trunk terminated anteriorly by
uniting with the wall of the throat. The junction was effected by a narrow
pedicle, so that the rod appeared mushroom-shaped in section, the stalk
representing the pedicle of attachment.
On the formation of the dorsal aorta, the subnotochordal rod becomes
separated from the wall of the gut and the aorta interposed between the
two.
The subnotochordal rod attains its fullest development during stage K.
Anteriorly it terminates at a point well in front of the ear, though a
little behind the end of the notochord; posteriorly it extends very nearly
to the extremity of the tail and is almost co-extensive with the postanal
section of the alimentary tract, though it does not quite reach so far back
as the caudal vesicle (Pl. 18, fig. 6_b_, _x_). In stage L it is still
fairly large in the tail, though it has begun to atrophy anteriorly. We may
therefore conclude that its atrophy, like its development, takes place from
before backwards. In the succeeding stages I have failed to find any trace
of it, and conclude, as does Professor Semper, that it disappears
completely.
Götte[313] is of opinion that the subnotochordal rod is converted into the
dorsal lymphatic trunk, and regards it as the anterior continuation of the
postanal gut, which he believes to be also converted into a lymphatic
trunk. My observations afford no support to these views, and the fact
already mentioned, that the subnotochordal rod is nearly co-extensive with
the postanal section of the gut, renders it improbable that both these
structures are connected with the lymphatic system.
Footnote 313: _Entwicklungsgeschichte d. Unke_, p. 775.
EXPLANATION OF PLATE 18.
COMPLETE LIST OF REFERENCE LETTERS.
_Nervous System._
_ar._ Anterior root of spinal nerve. _nc._ Neural canal. _pr._ Posterior
root of spinal nerve. _spn._ Spinal nerve. _syg._ Sympathetic ganglion.
_Alimentary Canal._
_al._ Alimentary canal. _alv._ Caudal vesicle of the postanal gut. _clal._
Cloacal section of alimentary canal. _du._ Duodenum. _hpd._ Ductus
choledochus. _pan._ pancreas. _soes._ Solid oesophagus. _spv._ Intestine
with rudiment of spiral valve. _umc._ Umbilical canal.
_General._
_ao._ Dorsal aorta. _aur._ Auricle of heart. _cav._ Cardinal vein. _ch._
Notochord. _eppp._ Epithelial lining of the body-cavity. _ir._ Interrenal
body. _me._ Mesentery. _mp._ Muscle-plate. _mpl_. Muscle-plate sending a
prolongation into the limb. _po._ Primitive ovum. _pp._ Body-cavity. _sd._
Segmental duct. _st._ Segmental tube. _ts._ Tail swelling. _vcau._ Caudal
vein. _x._ Subnotochordal rod.
Fig. 1. Transverse section through the anterior abdominal region of an
embryo of a stage between K and L. Zeiss B, ocul. 2. Reduced one-third.
The section illustrates the junction of a sympathetic ganglion with a
spinal nerve and the sprouting of the muscle-plates into the limbs (_mpl_).
Fig. 2. Transverse section through the abdominal region of an embryo
belonging to stage L. Zeiss B, ocul. 2. Reduced one-third.
The section illustrates the junction of a sympathetic ganglion with a
spinal nerve, and also the commencing formation of a branch from the aorta
(still solid) which will pass through the sympathetic ganglion, and forms
the first sign of the conversion of part of a sympathetic ganglion into one
of the suprarenal bodies.
Fig. 3. Longitudinal and vertical section of an embryo of a stage between L
and M, shewing the successive junctions of the spinal nerves and
sympathetic ganglia.
Fig. 4. Section through the solid oesophagus during stage L. Zeiss A, ocul.
1. The section is taken through the region of the heart, so that the cavity
of the auricle (_aur_) lies immediately below the oesophagus.
Fig. 5. Optical section of the tail of an embryo between stages I and K,
shewing the junction between the neural and alimentary canals.
Fig. 6. Four sections through the caudal region of an embryo belonging to
stage K, shewing the condition of the postanal section of the alimentary
tract. Zeiss A, ocul. 2. An explanation of these figures is given on p.
449.
Fig. 7. Section through the interrenal body of a Scyllium embryo belonging
to stage Q. Zeiss C, ocul. 2.
Fig. 8. Portion of a section of the interrenal body of an adult Scyllium.
Zeiss C, ocul. 2.