Chapter XXV (1)

THE ALIMENTARY CANAL AND ITS APPENDAGES, IN THE CHORDATA.

The alimentary canal in the Chordata is always formed of three sections, analogous to those so universally present in the Invertebrata. These sections are (1) the mesenteron lined by hypoblast; (2) the stomodæum or mouth lined by epiblast, and (3) the proctodæum or anal section lined like the stomodæum by epiblast.

_Mesenteron._

The early development of the epithelial wall of the mesenteron has already been described (Chapter XI.). It forms at first a simple hypoblastic tube extending from near the front end of the body, where it terminates blindly, to the hinder extremity where it is united with the neural tube by the neurenteric canal (fig. 420, _ne_). It often remains for a long time widely open in the middle towards the yolk-sack.

It has already been shewn that from the dorsal wall of the mesenteron the notochord is separated off nearly at the same time as the lateral plates of mesoblast (pp. 292-300).

The subnotochordal rod. At a period slightly subsequent to the formation of the notochord, and before any important differentiations in the mesenteron have become apparent, a remarkable rod-like body, which was first discovered by Götte, becomes split off from the dorsal wall of the alimentary tract in all the Ichthyopsida. This body, which has a purely provisional existence, is known as the subnotochordal rod.

It develops in Elasmobranch embryos in two sections, one situated in the head, and the other in the trunk.

The section in the trunk is the first to appear. The wall of the alimentary canal becomes thickened along the median dorsal line (fig. 412, _x_), 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 become gradually constricted off as a rod, which lies immediately dorsal to the alimentary tract, and ventral to the notochord (fig. 413, _x_).

[FIG. 412. TRANSVERSE SECTION THROUGH THE TAIL REGION OF A
PRISTIURUS EMBRYO OF THE SAME AGE AS FIG. 28 E.

_df._ dorsal fin; _sp.c._ spinal cord; _pp._ body cavity; _sp._
splanchnic layer of mesoblast; _so._ somatic layer of mesoblast;
_mp´._ portion of splanchnic mesoblast commencing to be
differentiated into muscles; _ch._ notochord; _x._ subnotochordal
rod arising as an outgrowth of the dorsal wall of the alimentary
tract; _al._ alimentary tract.]

[FIG. 413. TRANSVERSE SECTION THROUGH THE TRUNK OF AN EMBRYO
SLIGHTLY OLDER THAN FIG. 28 E.

_nc._ neural canal; _pr._ posterior root of spinal nerve; _x._
subnotochordal rod; _ao._ aorta; _sc._ somatic mesoblast; _sp._
splanchnic mesoblast; _mp._ muscle-plate; _mp´._ portion of
muscle-plate converted into muscle; _Vv._ portion of the vertebral
plate which will give rise to the vertebral bodies; _al._ alimentary
tract.]

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 to develop in the same way as that in the trunk, and the splitting off from the throat proceeds from before backwards.

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 (fig. 367, _x_).

When the subnotochordal rod attains its fullest development it terminates anteriorly some way in front of the auditory vesicle, 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 reach quite so far back as the caudal vesicle (fig. 424, _b x_). Very shortly after it has attained its maximum size it begins to atrophy in front. We may therefore conclude that its atrophy, like its development, takes place from before backwards. During the later embryonic stages not a trace of it is to be seen. It has also been met with in Acipenser, Lepidosteus, the Teleostei, Petromyzon, and the Amphibia, in all of which it appears to develop in fundamentally the same way as in Elasmobranchii. In Acipenser it appears to persist in the adult as the subvertebral ligament (Bridge, Salensky). It has not yet been found in a fully developed form in any amniotic Vertebrate, though a thickening of the hypoblast, which may perhaps be a rudiment of it, has been found by Marshall and myself in the Chick (fig. 110, _x_).

Eisig has instituted an interesting comparison between it and an organ which he has found in a family of Chætopods, the Capitellidæ. In these forms there is a tube underlying the alimentary tract for nearly its whole length, and opening into it in front, and probably behind. A remnant of such a tube might easily form a rudiment like the subnotochordal rod of the Ichthyopsida, and as Eisig points out the prolongation into the latter during its formation of the lumen of the alimentary tract distinctly favours such a view of its original nature. We can however hardly suppose that there is any direct genetic connection between Eisig's organ in the Capitellidæ and the subnotochordal rod of the Chordata.

Splanchnic mesoblast and mesentery. The mesenteron consists at first of a simple hypoblastic tube, which however becomes enveloped by a layer of splanchnic mesoblast. This layer, which is not at first continued over the dorsal side of the mesenteron, gradually grows in, and interposes itself between the hypoblast of the mesenteron, and the organs above. At the same time it becomes differentiated into two layers, viz. an outer epithelioid layer which gives rise to part of the peritoneal epithelium, and an inner layer of undifferentiated cells which in time becomes converted into the connective tissue and muscular walls of the mesenteron. The connective tissue layers become first formed, while of the muscular layers the circular is the first to make its appearance.

Coincidently with their differentiation the connective tissue stratum of the peritoneum becomes established.

_The Mesentery._ Prior to the splanchnic mesoblast growing round the alimentary tube above, the attachment of the latter structure to the dorsal wall of the body is very wide. On the completion of this investment the layer of mesoblast suspending the alimentary tract becomes thinner, and at the same time the alimentary canal appears to be drawn downwards and away from the vertebral column.

In what may be regarded as the thoracic division of the general pleuroperitoneal space, along that part of the alimentary canal which will form the oesophagus, this withdrawal is very slight, but it is very marked in the abdominal region. In the latter the at first straight digestive canal comes to be suspended from the body above by a narrow flattened band of mesoblastic tissue. This flattened band is the _mesentery_, shewn commencing in fig. 117, and much more advanced in fig. 119, _M_. It is covered on either side by a layer of flat cells, which form part of the general peritoneal epithelioid lining, while its interior is composed of indifferent tissue.

The primitive simplicity in the arrangement of the mesentery is usually afterwards replaced by a more complicated disposition, owing to the subsequent elongation and consequent convolution of the intestine and stomach.

The layer of peritoneal epithelium on the ventral side of the stomach is continued over the liver, and after embracing the liver, becomes attached to the ventral abdominal wall (fig. 380). Thus in the region of the liver the body cavity is divided into two halves by a membrane, the two sides of which are covered by the peritoneal epithelium, and which encloses the stomach dorsally and the liver ventrally. The part of the membrane between the stomach and liver is narrow, and constitutes a kind of mesentery suspending the liver from the stomach: it is known to human anatomists as the lesser omentum.

The part of the membrane connecting the liver with the anterior abdominal wall constitutes the falciform or suspensory ligament of the liver. It arises by a secondary fusion, and is not a remnant of a primitive ventral mesentery (_vide_ pp. 624 and 625).

The mesentery of the stomach, or mesogastrium, enlarges in Mammalia to form a peculiar sack known as the greater omentum.

The mesenteron exhibits very early a trifold division. An anterior portion, extending as far as the stomach, becomes separated off as the respiratory division. On the formation of the anal invagination the portion of the mesenteron behind the anus becomes marked off as the postanal division, and between the postanal section and the respiratory division is a middle portion forming an intestinal and cloacal division.

_The respiratory division of the mesenteron._

This section of the alimentary canal is distinguished by the fact that its walls send out a series of paired diverticula, which meet the skin, and after a perforation has been effected at the regions of contact, form the branchial or visceral clefts.

In Amphioxus the respiratory region extends close up to the opening of the hepatic diverticulum, and therefore to a position corresponding with the commencement of the intestine in higher types. In the craniate Vertebrata the number of visceral clefts has become reduced, but from the extension of the visceral clefts in Amphioxus, combined with the fact that in the higher Vertebrata the vagus nerve, which is essentially the nerve of the branchial pouches, supplies in addition the walls of the oesophagus and stomach, it may reasonably be concluded, as has been pointed out by Gegenbaur, that the true respiratory region primitively included the region which in the higher types forms the oesophagus and stomach.

In Ascidians the respiratory sack is homologous with the respiratory tract of Amphioxus.

The details of the development of the branchial clefts in the different groups of Vertebrata have already been described in the systematic part of this work.

In all the Ichthyopsida the walls of a certain number of clefts become folded; and in the mesoblast within these folds a rich capillary network, receiving its blood from the branchial arteries, becomes established. These folds constitute the true internal gills.

In addition to internal gills _external branchial processes_ covered by epiblast are placed on certain of the visceral arches in the larva of Polypterus, Protopterus and many Amphibia. The external gills have probably no genetic connection with the internal gills.

The so-called external gills of the embryos of Elasmobranchii are merely internal gills prolonged outwards through the gill clefts.

The posterior part of the primitive respiratory division of the mesenteron becomes, in all the higher Vertebrata, the oesophagus and stomach. With reference to the development of these parts the only point worth especially noting is the fact that in Elasmobranchii and Teleostei their lumen, though present in very young embryos, becomes at a later stage completely filled up, and thus the alimentary tract in the regions of the oesophagus and stomach becomes a solid cord of cells (fig. 23 A, _oes_): as already suggested (p. 61) it seems not impossible that this feature may be connected with the fact that the oesophageal region of the throat was at one time perforated by gill clefts.

In addition to the gills two important organs, viz. the thyroid body and the lungs, take their origin from the respiratory region of the alimentary tract.

[FIG. 414. DIAGRAMMATIC VERTICAL SECTION OF A JUST-HATCHED LARVA OF
PETROMYZON. (From Gegenbaur; after Calberla.)

_o._ mouth; _o´._ olfactory pit; _v._ septum between stomodæum and
mesenteron; _h._ thyroid involution; _n._ spinal cord; _ch._
notochord; _c._ heart; _a._ auditory vesicle.]

Thyroid body. In the Ascidians the origin of a groove-like diverticulum of the ventral wall of the branchial sack, bounded by two lateral folds, and known as the endostyle or hypopharyngeal groove, has already been described (p. 18). This groove remains permanently open to the pharyngeal sack, and would seem to serve as a glandular organ secreting mucus. As was first pointed out by W. Müller there is present in Amphioxus a very similar and probably homologous organ, known as the hypopharyngeal groove.

[FIG. 415. DIAGRAMMATIC TRANSVERSE SECTIONS THROUGH THE BRANCHIAL
REGION OF YOUNG LARVÆ OF PETROMYZON. (From Gegenbaur; after
Calberla.)

_d._ branchial region of throat.]

In the higher Vertebrata this organ never retains its primitive condition in the adult state. In the larva of Petromyzon there is, however, present a ventral groove-like diverticulum of the throat, extending from about the second to the fourth visceral cleft. This organ is shewn in longitudinal section in fig. 414, _h_, and in transverse section in fig. 415, and has been identified by W. Müller (Nos. 565 and 566) with the hypopharyngeal groove of Amphioxus and Ascidians. It does not, however, long retain its primitive condition, but its opening becomes gradually reduced to a pore, placed between the third and fourth of the permanent clefts (fig. 416, _th_). This opening is retained throughout the Ammocoete condition, but the organ becomes highly complicated, with paired anterior and posterior horns and a median spiral portion. In the adult the connection with the pharynx is obliterated, and the organ is partly absorbed and partly divided up into a series of glandular follicles, and eventually forms _the thyroid body_.

From the consideration of the above facts W. Müller was led to the conclusion _that the thyroid body of the Craniata was derived from the endostyle or hypopharyngeal groove_. In all the higher Vertebrata the thyroid body arises as a diverticulum of the ventral wall of the throat in the region either of the mandibular or hyoid arches (fig. 417, _Th_), which after being segmented off becomes divided up into follicles.

In Elasmobranch embryos it appears fairly early as a diverticulum from the ventral surface of the throat in the region of the _mandibular arch_, extending from the border of the mouth to the point where the ventral aorta divides into the two aortic branches of the mandibular arch (fig. 417, _Th_). Somewhat later it becomes in Scyllium and Torpedo solid, though still retaining its attachment to the wall of the oesophagus. It continues to grow in length, and becomes divided up into a number of solid branched lobules separated by connective tissue septa. Eventually its connection with the throat becomes lost, and the lobules develop a lumen. In Acanthias the lumen of the gland is retained (W. Müller) till after its detachment from the throat. It preserves its embryonic position through life. In Amphibia it originates, as in Elasmobranchii, from the region of the mandibular arch; but when first visible it forms a double epithelial wall connecting the throat with the nervous layer of the epidermis. It subsequently becomes detached from the epidermis, and then has the usual form of a diverticulum from the throat. In most Amphibians it becomes divided into two lobes, and so forms a paired body. The peculiar connection between the thyroid diverticulum and the epidermis in Amphibia has been noted by Götte in Bombinator, and by Scott and Osborn in Triton. It is not very easy to see what meaning this connection can have.

[FIG. 416. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A LARVA
OF PETROMYZON.

The larva had been hatched three days, and was 4.8 mm. in length.
The optic and auditory vesicles are supposed to be seen through the
tissues. The letter _tv_ pointing to the base of the velum is where
Scott believes the hyomandibular cleft to be situated.

_c.h._ cerebral hemisphere; _th._ optic thalamus; _in._
infundibulum; _pn._ pineal gland; _mb._ mid-brain; _cb._ cerebellum;
_md._ medulla oblongata; _au.v._ auditory vesicle; _op._ optic
vesicle; _ol._ olfactory pit; _m._ mouth; _br.c._ branchial pouches;
_th._ thyroid involution; _v.ao._ ventral aorta; _ht._ ventricle of
heart; _ch._ notochord.]

In the Fowl (W. Müller) the thyroid body arises at the end of the second or beginning of the third day as an outgrowth from the hypoblast of the throat, opposite the point of origin of the anterior arterial arch. This outgrowth becomes by the fourth day a solid mass of cells, and by the fifth ceases to be connected with the epithelium of the throat, becoming at the same time bilobed. By the seventh day it has travelled somewhat backwards, and the two lobes have completely separated from each other. By the ninth day the whole is invested by a capsule of connective tissue, which sends in septa dividing it into a number of lobes or solid masses of cells, and by the sixteenth day it is a paired body composed of a number of hollow branched follicles, each with a 'membrana propria,' and separated from each other by septa of connective tissue. It finally travels back to the point of origin of the carotids.

[FIG. 417. SECTION THROUGH THE HEAD OF AN ELASMOBRANCH EMBRYO, AT
THE LEVEL OF THE AUDITORY INVOLUTION.

_Th._ rudiment of thyroid body; _aup._ auditory pit; _aun._ ganglion
of auditory nerve; _iv.v._ roof of fourth ventricle; _a.c.v._
anterior cardinal vein; _aa._ aorta; _I.aa._ aortic trunk of
mandibular arch; _pp._ head cavity of mandibular arch; _Ivc._
alimentary pouch which will form the first visceral cleft.]

Amongst Mammalia the thyroid arises in the Rabbit (Kölliker) and Man (His) as a hollow diverticulum of the throat at the bifurcation of the foremost pair of aortic arches. It soon however becomes solid, and is eventually detached from the throat and comes to lie on the ventral side of the larynx or windpipe. The changes it undergoes are in the main similar to those in the lower Vertebrata. It becomes partially constricted into two lobes, which remain however united by an isthmus[278]. The fact that the thyroid sometimes arises in the region of the first and sometimes in that of the second cleft is probably to be explained by its rudimentary character.

[278] Wölfler (No. 571) states that in the Pig and Calf the
thyroid body is formed as a pair of epithelial vesicles, which
are developed as outgrowths of the walls of the first pair of
visceral clefts. He attempts to explain the contradictory
observations of other embryologists by supposing that they have
mistaken the ventral ends of visceral pouches for an unpaired
outgrowth of the throat. Stieda (No. 569) also states that in the
Pig and Sheep the thyroid arises as a paired body from the
epithelium of a pair of visceral clefts, at a much later period
than would appear from the observations of His and Kölliker. In
view of the comparative development of this organ it is difficult
to accept either Wölfler's or Stieda's account. Wölfler's attempt
to explain the supposed errors of his predecessors is certainly
not capable of being applied in the case of Elasmobranch Fishes,
or of Petromyzon; and I am inclined to think that the method of
investigation by transverse sections, which has been usually
employed, is less liable to error than that by longitudinal
sections which he has adopted.

The Thymus gland. The thymus gland may conveniently be dealt with here, although its origin is nearly as obscure as its function. It has usually been held to be connected with the lymphatic system. Kölliker was the first to shew that this view was probably erroneous, and he attempted to prove that it was derived in the Rabbit from the walls of one of the visceral clefts, mainly on the ground of its presenting in the embryo an epithelial character.

Stieda (No. 569) has recently verified Kölliker's statements. He finds that in the Pig and the Sheep the thymus arises as a paired outgrowth from the epithelial remnants of a pair of visceral clefts. Its two lobes may at first be either hollow (Sheep) or solid (Pig), but eventually become solid, and unite in the median line. Stieda and His hold that in the adult gland, the so-called corpuscles of Hassall are the remnants of the embryonic epithelial part of the gland, and that the lymphatic part of it is of mesoblastic origin; but Kölliker believes the lymphatic cells to be direct products of the embryonic epithelial cells.

The posterior visceral clefts in the course of their atrophy give rise to various more or less conspicuous bodies of a pseudo-glandular nature, which have been chiefly studied by Remak[279].

[279] For details on these organs _vide_ Kölliker,
_Entwicklungsgeschichte_, p. 881.

Swimming bladder and lungs. A swimming bladder is present in all Ganoids and in the vast majority of Teleostei. Its development however is only imperfectly known.

In the Salmon and Carp it arises, as was first shewn by Von Baer, as an outgrowth of the alimentary tract, shortly in front of the liver. In these forms it is at first placed on the dorsal side and slightly to the right, and grows backwards on the dorsal side of the gut, between the two folds of the mesentery.

The absence of a pneumatic duct in the Physoclisti would appear to be due to a post-larval atrophy.

In Lepidosteus the air-bladder appears to arise, as in the Teleostei, as an invagination of the dorsal wall of the oesophagus.

In advanced embryos of Galeus, Mustelus and Acanthias, Miklucho-Maclay detected a small diverticulum opening on the dorsal side of the oesophagus, which he regards as a rudiment of a swimming bladder. This interpretation must however be regarded as somewhat doubtful.

_The lungs._ The lungs originate in a nearly identical way in all the Vertebrate forms in which their development has been observed. They are essentially buds or processes of the ventral wall of the primitive oesophagus.

At a point immediately behind the region of the visceral clefts the cavity of the alimentary canal becomes compressed laterally, and at the same time constricted in the middle, so that its transverse section (fig. 418 1) is somewhat hourglass-shaped, and shews an upper or dorsal chamber _d_, joining on to a lower or ventral chamber _l_ by a short narrow neck.

The hinder end of the lower tube enlarges (fig. 418 2), and then becomes partially divided into two lobes (fig. 418 3). All these parts at first freely communicate, but the two lobes, partly by their own growth, and partly by a process of constriction, soon become isolated posteriorly; while in front they open into the lower chamber of the oesophagus (fig. 422).

[FIG. 418. FOUR DIAGRAMS ILLUSTRATING THE FORMATION OF THE LUNGS.
(After Götte.)

_a._ mesoblast; _b._ hypoblast; _d._ cavity of digestive canal; _l._
cavity of the pulmonary diverticulum.

In (1) the digestive canal has commenced to be constricted into an
upper and lower canal; the former the true alimentary canal, the
latter the pulmonary tube; the two tubes communicate with each
other in the centre.
In (2) the lower (pulmonary) tube has become expanded.
In (3) the expanded portion of the tube has become constricted into
two tubes, still communicating with each other and with the
digestive canal.
In (4) these are completely separated from each other and from the
digestive canal, and the mesoblast has also begun to exhibit
externally changes corresponding to the internal changes which
have been going on.]

By a continuation forwards of the process of constriction the lower chamber of the oesophagus, carrying with it the two lobes above mentioned, becomes gradually transformed into an independent tube, opening in front by a narrow slit-like aperture into the oesophagus. The single tube in front is the rudiment of the trachea and larynx, while the two diverticula behind become (fig. 419, _lg_) the bronchial tubes and lungs.

While the above changes are taking place in the hypoblastic walls of the alimentary tract, the splanchnic mesoblast surrounding these structures becomes very much thickened; but otherwise bears no marks of the internal changes which are going on, so that the above formation of the lungs and trachea cannot be seen from the surface. As the paired diverticula of the lungs grow backwards, the mesoblast around them takes however the form of two lobes, into which they gradually bore their way.

There do not seem to be any essential differences in the mode of formation of the above structures in the types so far observed, viz. Amphibia, Aves and Mammalia. Writers differ as to whether the lungs first arise as paired diverticula, or as a single diverticulum; and as to whether the rudiments of the lungs are established before those of the trachea. If the above account is correct it would appear that any of these positions might be maintained. Phylogenetically interpreted the ontogeny of the lungs appears however to imply that this organ was first an unpaired structure and has become secondarily paired, and that the trachea was relatively late in appearing.

[FIG. 419. SECTION THROUGH THE CARDIAC REGION OF AN EMBRYO OF
LACERTA MURALIS OF 9 MM. TO SHEW THE MODE OF FORMATION OF THE
PERICARDIAL CAVITY.

_ht._ heart; _pc._ pericardial cavity; _al._ alimentary tract; _lg._
lung; _l._ liver; _pp._ body cavity; _md._ open end of Müllerian
duct; _wd._ Wolffian duct; _vc._ vena cava inferior; _ao._ aorta;
_ch._ notochord; _mc._ medullary cord.]

The further development of the lungs is at first, in the higher types at any rate, essentially similar to that of a racemose gland. From each primitive diverticulum numerous branches are given off. In Aves and Mammalia (fig. 355) they are mainly confined to the dorsal and lateral parts. These branches penetrate into the surrounding mesoblast and continue to give rise to secondary and tertiary branches. In the mesoblast around them numerous capillaries make their appearance, and the further growth of the bronchial tubes is supposed by Boll to be due to the mutual interaction of the hitherto passive mesoblast and of the hypoblast.

The further changes in the lungs vary somewhat in the different forms.

The air sacks are the most characteristic structures of the avian lung. They are essentially the dilated ends of the primitive diverticula or of their main branches.

In Mammalia (Kölliker, No. 298) the ends of the bronchial tubes become dilated into vesicles, which may be called the primary air-cells. At first, owing to their development at the ends of the bronchial branches, these are confined to the surface of the lungs. At a later period the primary air-cells divide each into two or three parts, and give rise to secondary air-cells, while at the same time the smallest bronchial tubes, which continue all the while to divide, give rise at all points to fresh air-cells. Finally the bronchial tubes cease to become more branched, and the air-cells belonging to each minute lobe come in their further growth to open into a common chamber. Before the lungs assume their function the embryonic air-cells undergo a considerable dilatation.

_The trachea and larynx._ The development of the trachea and larynx does not require any detailed description. The larynx is formed as a simple dilatation of the trachea. The cartilaginous structures of the larynx are of the same nature as those of the trachea.

It follows from the above account that the whole pulmonary structure is the result of the growth by budding of a system of branched hypoblastic tubes in the midst of a mass of mesoblastic tissue, the hypoblastic elements giving rise to the epithelium of the tubes, and the mesoblast providing the elastic, muscular, cartilaginous, vascular, and other connective tissues of the tracheal and bronchial walls.

There can be no doubt that the lungs and air-bladder are homologous structures, and the very interesting memoir of Eisig on the air-bladder of the Chætopoda[280] shews it to be highly probable that they are the divergent modifications of a primitive organ, which served as a reservoir for gas secreted in the alimentary tract, the gas in question being probably employed for respiration when, for any reason, ordinary respiration by the gills was insufficient.

[280] H. Eisig, "Ueb. d. Vorkommen eines schwimmblasenähnlichen
Organs bei Anneliden." _Mittheil. a. d. zool. Station z. Neapel_,
Vol. II. 1881.

Such an organ might easily become either purely respiratory, receiving its air from the exterior, and so form a true lung; or mainly hydrostatic, forming an air-bladder, as in Ganoidei and Teleostei.

It is probable that in the Elasmobranchii the air-bladder has become aborted, and the organ discovered by Micklucho-Maclay may perhaps be a last remnant of it.

The middle division of the mesenteron. The middle division of the mesenteron, forming the intestinal and cloacal region, is primitively a straight tube, the intestinal region of which in most Vertebrate embryos is open below to the yolk-sack.

Cloaca. In the Elasmobranchii, the embryos of which probably retain a very primitive condition of the mesenteron, this region is not at first sharply separated from the postanal section behind. Opposite the point where the anus will eventually appear a dilatation of the mesenteron arises, which comes in contact with the external skin (fig. 28 E, _an_). This dilatation becomes the hypoblastic section of the cloaca. It communicates behind with the postanal gut (fig. 424 D), and in front with the intestine; and _may be defined as the dilated portion of the alimentary tract which receives the genital and urinary ducts and opens externally by the proctodæum_.

In Acipenser and Amphibia the cloacal region is indicated as a ventral diverticulum of the mesenteron even before the closure of the blastopore. It is shewn in the Amphibia at an early stage in fig. 73, and at a later period, when in contact with the skin at the point where the anal invagination is about to appear, in fig. 420.

[FIG. 420. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF
BOMBINATOR. (After Götte.)

_m._ mouth; _an._ anus; _l._ liver; _ne._ neurenteric canal; _mc._
medullary canal; _ch._ notochord; _pn._ pineal gland.]

In the Sauropsida and Mammalia the cloaca appears as a dilatation of the mesenteron, which receives the opening of the allantois almost as soon as the posterior part of the mesenteron is established.

The eventual changes which it undergoes have been already dealt with in connection with the urinogenital organs.

Intestine. The region in front of the cloaca forms the intestine. In certain Vertebrata it nearly retains its primitive character as a straight tube; and in these types its anterior part is characterised by the presence of a peculiar fold, which in a highly specialised condition is known as the spiral valve. This structure appears in its simplest form in Ammocoetes. It there consists of a fold in the wall of the intestine, giving to the lumen of this canal a semilunar form in section, and taking a half spiral.

In Elasmobranchii a similar fold to that in Ammocoetes first makes its appearance in the embryo. This fold is from the first not quite straight, but winds in a long spiral round the intestine. In the course of development it becomes converted into a strong ridge projecting into the lumen of the intestine (fig. 388, _l_). The spiral it makes becomes much closer, and it thus acquires the form of the adult spiral valve. A spiral valve is also found in Chimæra and Ganoids. No rudiment of such an organ is found in the Teleostei, the Amphibia, or the higher Vertebrata.

The presence of this peculiar organ appears to be a very primitive Vertebrate character. The intestine of Ascidians exhibits exactly the same peculiarity as that of Ammocoetes, and we may probably conclude from embryology that the ancestral Chordata were provided with a straight intestine having a fold projecting into its lumen, to increase the area of the intestinal epithelium.

In all forms in which there is not a spiral valve, with the exception of a few Teleostei, the intestine becomes considerably longer than the cavity which contains it, and therefore necessarily more or less convoluted.

The posterior part usually becomes considerably enlarged to form the rectum or in Mammalia the large intestine.

In Elasmobranchii there is a peculiar gland opening into the dorsal side of the rectum, and in many other forms there is a cæcum at the commencement of the rectum or of the large intestine.

In Teleostei, the Sturgeon and Lepidosteus there opens into the front end of the intestine a number of cæcal pouches known as the pancreatic cæca. In the adult Sturgeon these pouches unite to form a compact gland, but in the embryo they arise as a series of isolated outgrowths of the duodenum.

Connected with the anterior portion of the middle region of the alimentary canal, which may be called the duodenum, are two very important and constant glandular organs, the liver and the pancreas.

The liver. The liver is the earliest formed and largest glandular organ in the embryo.

It appears in its simplest form in Amphioxus as a single unbranched diverticulum of the alimentary tract, immediately behind the respiratory region, which is directed forwards and placed on the left side of the body.

[FIG. 421. SECTION THROUGH THE VENTRAL PART OF THE TRUNK OF A YOUNG
EMBRYO OF SCYLLIUM AT THE LEVEL OF THE UMBILICAL CORD.

_b._ pectoral fin; _ao._ dorsal aorta; _cav._ cardinal vein; _ua._
vitelline artery; _uv._ vitelline vein united with subintestinal
vein; _al._ duodenum; _l._ liver; _sd._ opening of segmental duct
into the body-cavity; _mp._ muscle-plate; _um._ umbilical canal.]

In all true Vertebrata the gland has a much more complicated structure. It arises as a ventral outgrowth of the duodenum (fig. 420, _l_). This outgrowth may be at first single, and then grow out into two lobes, as in Elasmobranchii (fig. 421) and Amphibia, or have from the first the form of two somewhat unequal diverticula, as in Birds (fig. 422), or again as in the Rabbit (Kölliker) one diverticulum may be first formed, and a second one appear somewhat later. The hepatic diverticula, whatever may be their primitive form, grow into a special thickening of the splanchnic mesoblast.

From the primitive diverticula there are soon given off a number of hollow buds (fig. 421) which rapidly increase in length and number, and form the so-called hepatic cylinders. They soon anastomose and unite together, and so constitute an irregular network. Coincidently with the formation of the hepatic network the united vitelline and visceral vein or veins (_u.v_), in their passage through the liver, give off numerous branches, and gradually break up into a plexus of channels which form a secondary network amongst the hepatic cylinders. In Amphibia these channels are stated by Götte to be lacunar, but in Elasmobranchii, and probably Vertebrata generally, they are from the first provided with distinct though delicate walls.

It is still doubtful whether the hepatic cylinders are as a rule hollow or solid. In Elasmobranchii they are at first provided with a large lumen, which though it becomes gradually smaller never entirely vanishes. The same seems to hold good for Amphibia and some Mammalia. In Aves the lumen of the cylinders is even from the first much more difficult to see, and the cylinders are stated by Remak to be solid, and he has been followed in this matter by Kölliker. In the Rabbit also Kölliker finds the cylinders to be solid.

The embryonic hepatic network gives rise to the parenchyma of the adult liver, with which in its general arrangement it closely agrees. The blood-channels are at first very large, and have a very irregular arrangement; and it is not till comparatively late that the hepatic lobules with their characteristic vascular structures become established.

[FIG. 422. DIAGRAM OF THE DIGESTIVE TRACT OF A CHICK UPON THE FOURTH
DAY. (After Götte.)

The black line indicates the hypoblast. The shaded part around it is
the splanchnic mesoblast.

_lg._ lung; _st._ stomach; _p._ pancreas; _l._ liver.]

The biliary ducts are formed either from some of the primitive hepatic cylinders, or, as would seem to be the case in Elasmobranchii and Birds (fig. 422), from the larger diverticula of the two primitive outgrowths.

The gall-bladder is so inconstant, and the arrangement of the ducts opening into the intestine so variable, that no general statements can be made about them. In Elasmobranchii the primitive median diverticulum (fig. 421) gives rise to the ductus choledochus. Its anterior end dilates to form a gall-bladder.

In the Rabbit a ductus choledochus is formed by a diverticulum from the intestine at the point of insertion of the two primitive lobes. The gall-bladder arises as a diverticulum of the right primitive lobe.

The liver is relatively very large during embryonic life and has, no doubt, important functions in connection with the circulation.

The pancreas. So far as is known the development of the pancreas takes place on a very constant type throughout the series of craniate Vertebrata, though absent in some of the Teleostean fishes and Cyclostomata, and very much reduced in most Teleostei and in Petromyzon.

It arises nearly at the same time as the liver in the form of a hollow outgrowth from the dorsal side of the intestine nearly opposite but slightly behind the hepatic outgrowth (fig. 422, _p_). It soon assumes, in Elasmobranchii and Mammalia, somewhat the form of an inverted funnel, and from the expanded dorsal part of the funnel there grow out numerous hollow diverticula into the passive splanchnic mesoblast.

As 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. The funnel-shaped receptacle loses its original form, and elongating, assumes the character of a duct.

From the above mode of development it is clear that the glandular cells of the pancreas are derived from the hypoblast.

Into the origin of the varying arrangements of the pancreatic ducts it is not possible to enter in detail. In some cases, _e.g._ the Rabbit (Kölliker), the two lobes and ducts arise from a division of the primitive gland and duct. In other cases, _e.g._ the Bird, a second diverticulum springs from the alimentary tract. In a large number of instances the primitive condition with a single duct is retained.

Postanal section of the mesenteron. In the embryos of all the Chordata there is a section of the mesenteron placed behind the anus. This section invariably atrophies at a comparatively early period of embryonic life; but it is much better developed in the lower forms than in the higher. At its posterior extremity it is primitively continuous with the neural tube (fig. 420), as was first shewn by Kowalevsky.

The canal connecting the neural and alimentary canals has already been described as the neurenteric canal, and represents the remains of the blastopore.

In the Tunicata the section of the mesenteron, which in all probability corresponds to the postanal gut of the Vertebrata, is that immediately following the dilated portion which gives rise to the branchial cavity and permanent intestine. It has already been shewn that from the dorsal and lateral portions of this section of the primitive alimentary tract the notochord and muscles of the Ascidian tadpole are derived. The remaining part of its walls forms a solid cord of cells (fig. 423, _al´_), which either atrophies, or, according to Kowalevsky, gives rise to blood-vessels.

[FIG. 423. TRANSVERSE OPTICAL SECTION OF THE TAIL OF AN EMBRYO OF
PHALLUSIA MAMMILLATA. (After Kowalevsky.)

The section is from an embryo of the same age as fig. 8 IV.

_ch._ notochord; _n.c._ neural canal; _me._ mesoblast; _al´._
hypoblast of tail.]

In Amphioxus the postanal gut, though distinctly developed, is not very long, and atrophies at a comparatively early period.

In Elasmobranchii this section of the alimentary tract is very well developed, and persists for a considerable period of embryonic life. The following is a history of its development in the genus Scyllium.

Shortly after the stage when the anus has become marked out by the alimentary tract sending down a papilliform process towards the skin, the postanal gut begins to develop a terminal dilatation or vesicle, connected with the remainder of the canal by a narrower stalk.

The walls both of the vesicle and stalk are formed of a fairly columnar epithelium. The vesicle communicates in front by a narrow passage with the neural canal, and behind is continued into two horns corresponding with the two caudal swellings previously spoken of (p. 55). 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, as the tail grows longer and longer, the postanal section of the alimentary tract grows with it, without however undergoing alteration in any of its essential characters. At the period of the maximum development, it has a length of about 1/3 of that of the whole alimentary tract.

Its features at a stage shortly before the external gills have become prominent are illustrated by a series of transverse sections through the tail (fig. 424). The four sections have been selected for illustration out of a fairly-complete series of about one hundred and twenty.

Posteriorly (A) there is present a terminal vesicle (_alv_) .25 mm. in diameter, which communicates dorsally by a narrow opening with the neural canal (_nc_); to this is attached a stalk in the form of a tube, also lined by columnar epithelium, and extending through about thirty sections (B _al_). Its average diameter is about .084 mm., and its walls are very thick. Overlying its front end is the subnotochordal rod (_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 (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 flattened epithelial cells. 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 (D _al_).

[FIG. 424. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL OF AN
EMBRYO OF THE SAME AGE AS FIG. 28 F.

A. is the posterior section.

_nc._ neural canal; _al._ postanal gut; _alv._ caudal vesicle of
postanal gut; _x._ subnotochordal rod; _mp._ muscle-plate; _ch._
notochord; _cl.al._ cloaca; _ao._ aorta; _v.cau._ caudal vein.]

Very shortly after the stage to which the above figures belong, at a point a little behind the anus, where the postanal section of the canal was thinnest in the previous stage, it becomes solid, and a rupture here occurs in it at a slightly later period.

The atrophy of this part of the alimentary tract having once commenced proceeds rapidly. The posterior part first becomes reduced to a small rudiment near the end of the tail. There is no longer a terminal vesicle, nor a neurenteric canal. The portion of the postanal section of the alimentary tract, just behind the cloaca, is for a short time represented by a small rudiment of the dilated part which at an earlier period opened into the cloaca.

In Teleostei the vesicle at the end of the tail, discovered by Kupffer, (fig. 34, _hyv_) is probably the equivalent of the vesicle at the end of the postanal gut in Elasmobranchii.

In Petromyzon and in Amphibia there is a well-developed postanal gut connected with a neurenteric canal which gradually atrophies. It is shewn in the embryo of Bombinator in fig. 420.

[FIG. 425. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR
END OF AN EMBRYO BIRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS.

_ep._ epiblast; _Sp.c._ spinal canal; _ch._ notochord; _n.e._
neurenteric canal; _hy._ hypoblast; _p.a.g._ postanal gut; _pr._
remains of primitive streak folded in on the ventral side; _al._
allantois; _me._ splanchnic mesoblast; _an._ point where anus will
be formed; _p.c._ perivisceral cavity; _am._ amnion; _so._
somatopleure; _sp._ splanchnopleure.]

Amongst the amniotic Vertebrata the postanal gut is less developed than in the Ichthyopsida. A neurenteric canal is present for a short period in various Birds (Gasser, etc.) and in the Lizard, but disappears very early. There is however, as has been pointed out by Kölliker, a well-marked postanal gut continued as a narrow tube from behind the cloaca into the tail both in the Bird (fig. 425, _p.a.g._) and Mammals (the Rabbit), but especially in the latter. It atrophies early as in lower forms.

The morphological significance of the postanal gut and of the neurenteric canal has already been spoken of in Chapter XII., p. 323.

_The Stomodæum._

The anterior section of the permanent alimentary tract is formed by an invagination of epiblast, constituting a more or less considerable pit, with its inner wall in contact with the blind anterior extremity of the alimentary tract.

In Ascidians this pit is placed on the dorsal surface (fig. 9, _o_), and becomes the permanent oral cavity of these forms. In the larva of Amphioxus it is stated to be formed unsymmetrically (_vide_ p. 5), but further observations on its development are required.

[FIG. 426. LONGITUDINAL SECTION THROUGH THE BRAIN OF A YOUNG
PRISTIURUS EMBRYO.

_cer._ unpaired rudiment of the cerebral hemispheres; _pn._ pineal
gland; _In._ infundibulum; _pt._ ingrowth from mouth to form the
pituitary body; _mb._ mid-brain; _cb._ cerebellum; _ch._ notochord;
_al._ alimentary tract; _Iaa._ artery of mandibular arch.]

In the true Vertebrata it is always formed on the ventral surface of the head, immediately behind the level of the fore-brain (fig. 426), and is deeper in Petromyzon (fig. 416, _m_) than in any other known form.

From the primary buccal cavity or stomodæum there grows out the pituitary pit (fig. 426, _pt_), the development of which has already been described (p. 435).

The wall separating the stomodæum from the mesenteron always becomes perforated, usually at an early stage of development, and though in Petromyzon the boundary between the two cavities remains indicated by the velum, yet in the higher Vertebrata all trace of this boundary is lost, and the original limits of the primitive buccal cavity become obliterated; while a secondary buccal cavity, partly lined by hypoblast and partly by epiblast, becomes established.

This cavity, apart from the organs which belong to it, presents important variations in structure. In most Pisces it retains a fairly simple character, but in the Dipnoi its outer boundary becomes extended so as to enclose the ventral opening of the nasal sack, which thenceforward constitutes the posterior nares.

In Amphibia and Amniota the posterior nares also open well within the boundary of the buccal cavity.

In the Amniota further important changes take place.

In the first place a plate grows inwards from each of the superior maxillary processes (fig. 427, _p_), and the two plates, meeting in the middle line, form a horizontal septum dividing the front part of the primitive buccal cavity into a dorsal respiratory section (_n_), containing the opening of the posterior nares, and a ventral cavity, forming the permanent mouth. The two divisions thus formed open into a common cavity behind. The horizontal septum, on the development within it of an osseous plate, constitutes the hard palate.

[FIG. 427. DIAGRAM SHEWING THE DIVISION OF THE PRIMITIVE BUCCAL
CAVITY INTO THE RESPIRATORY SECTION ABOVE AND THE TRUE MOUTH BELOW.
(From Gegenbaur.)

_p._ palatine plate of superior maxillary process; _m._ permanent
mouth; _n._ posterior part of nasal passage; _e._ internasal
septum.]

An internasal septum (fig. 427, _e_) may more or less completely divide the dorsal cavity into two canals, continuous respectively with the two nasal cavities.

In Mammalia a posterior prolongation of the palate, in which an osseous plate is not formed, constitutes the soft palate.

The second change in the Amniota, which also takes place in some Amphibia, is caused by the section of the mesenteron into which the branchial pouches open, becoming, on the atrophy of these structures, converted into the posterior part of the buccal cavity.

The organs derived from the buccal cavity are the tongue, the various salivary glands, and the teeth; but the latter alone will engage our attention here.

The teeth. The teeth are to be regarded as a special product of the oral mucous membrane. It has been shewn by Gegenbaur and Hertwig that in their mode of development they essentially resemble the placoid scales of Elasmobranchii, and that the latter structures extend in Elasmobranchii for a certain distance into the cavity of the mouth.

As pointed out by Gegenbaur, the teeth are therefore to be regarded as more or less specialised placoid scales, whose presence in the mouth is to be explained by the fact that the latter structure is lined by an invagination of the epidermis; The most important developmental point of difference between teeth and placoid scales consists in the fact, that in the case of the former there is a special ingrowth of epiblast to meet a connective tissue papilla which is not found in the latter.

Although the teeth are to be regarded as primitively epiblastic structures, they are nevertheless found in Teleostei and Ganoidei on the hyoid and branchial arches; and very possibly the teeth on some other parts of the mouth are developed in a true hypoblastic region.

The teeth are formed from two distinct organs, viz. an epithelial cap and a connective tissue papilla.

The general mode of development, as has been more especially shewn by the extended researches of Tomes, is practically the same for all Vertebrata, and it will be convenient to describe it as it takes place in Mammalia.

Along the line where the teeth are about to develop, there is formed an epithelial ridge projecting into the subjacent connective tissue, and derived from the innermost columnar layer of the oral epithelium. At the points where a tooth is about to be formed this ridge undergoes special changes. It becomes in the first place somewhat thickened by the development of a number of rounded cells in its interior; so that it becomes constituted of (1) an external layer of columnar cells, and (2) a central core of rounded cells; both of an epithelial nature. In the second place the organ gradually assumes a dome-shaped form (fig. 428, _e_), and covers over a papilla of the subepithelial connective tissue (_p_) which has in the meantime been developed.

[FIG. 428. DIAGRAM SHEWING THE DEVELOPMENT OF THE TEETH. (From
Gegenbaur.)

_p._ dental papilla; _e._ enamel organ.]

From the above epithelial structure, which may be called the enamel organ, and from the papilla it covers, which may be spoken of as the dental papilla, the whole tooth is developed. After these parts have become established there is formed round the rudiment of each tooth a special connective tissue capsule; known as the dental capsule.

Before the dental capsule has become definitely formed the enamel organ and the dental papilla undergo important changes. The rounded epithelial cells forming the core of the enamel organ undergo a peculiar transformation into a tissue closely resembling ordinary embryonic connective tissue, while at the same time the epithelium adjoining the dental papilla and covering the inner surface of the enamel organ, acquires a somewhat different structure to the epithelium on the outer side of the organ. Its cells become very markedly columnar, and form a very regular cylindrical epithelium. This layer alone is concerned in forming the enamel. The cells of the outer epithelial layer of the enamel organ become somewhat flattened, and the surface of the layer is raised into a series of short papillæ which project into the highly vascular tissue of the dental sheath. Between the epithelium of the enamel organ and the adjoining connective tissue there is everywhere present a delicate membrane known as the membrana præformativa.

The dental papilla is formed of a highly vascular core and a non-vascular superficial layer adjoining the inner epithelium of the enamel organ. The cells of the superficial layer are arranged so as almost to resemble an epithelium.

The first formation of the hard structures of the tooth commences at the apex of the dental papilla. A calcification of the outermost layer of the papilla sets in, and results in the formation of a thin layer of dentine. Nearly simultaneously a thin layer of enamel is deposited over this, from the inner epithelial layer of the enamel organ (fig. 428). Both enamel and dentine continue to be deposited till the crown of the tooth has reached its final form, and in the course of this process the enamel organ is reduced to a thin layer, and the whole of the outer layer of the dental papilla is transformed into dentine--while the inner portion remains as the pulp.

The root of the tooth is formed later than the crown, but the enamel organ is not prolonged over this part, so that it is only formed of dentine.

By the formation of the root the crown of the tooth becomes pushed outwards, and breaking through its sack projects freely on the surface.

The part of the sack which surrounds the root of the tooth gives rise to the cement, and becomes itself converted into the periosteum of the dental alveolus.

The general development of the enamel organs and dental papillæ is shewn in the diagram (fig. 428). From the epithelial ridge three enamel organs are represented as being developed. Such an arrangement may occur when teeth are successively replaced. The lowest and youngest enamel organ (_e_) has assumed a cap-like form enveloping a dental papilla, but no calcification has yet taken place.