CHAPTER XXV.

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.

In the next stage a cap of dentine has become formed, while in the

still older tooth this has become covered by a layer of enamel. As may

be gathered from this diagram, the primitive epithelial ridge from

which the enamel organ is formed is not necessarily absorbed on the

formation of a tooth, but is capable of giving rise to fresh enamel

organs. When the enamel organ has reached a certain stage of

development, its connection with the epithelial ridge is ruptured

(fig. 428).

The arrangement represented in fig. 428, in which successive enamel

organs are formed from the same epithelial ridge, is found in most

Vertebrata except the Teleostei. In the Teleostei, however (Tomes), a

fresh enamel organ grows inwards from the epithelium for each

successively formed tooth.

_The Proctodæum._

In all Vertebrata the cloacal section of the alimentary tract which

receives the urinogenital ducts is placed in communication with the

exterior by means of an epiblastic invagination, constituting a

proctodæum.

This invagination is not usually very deep, and in most instances the

boundary wall between it and the hypoblastic cloaca is not perforated

till considerably after the perforation of the stomodæum; in

Petromyzon, however, its perforation is effected before the mouth and

pharynx are placed in communication.

The mode of formation of the proctodæum, which is in general extremely

simple, is illustrated by fig. 420 _an_.

In most forms the original boundary between the epiblast of the

proctodæum and the hypoblast of the primitive cloaca becomes

obliterated after the two have become placed in free communication.

[FIG. 429. 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._ mesoblast; _an._ point where anus will be formed;

_p.c._ perivisceral cavity; _am._ amnion; _so._ somatopleure; _sp._

splanchnopleure.]

In Birds the formation of the proctodæum is somewhat more complicated

than in other types, owing to the outgrowth from it of the bursa

Fabricii.

The proctodæum first appears when the folding off of the tail end of

the embryo commences (fig. 429, _an_) and is placed near the front

(originally the apparent hind) end of the primitive streak. Its

position marks out the front border of the postanal section of the

gut.

The bursa Fabricii first appears on the seventh day (in the chick), as

a dorsal outgrowth of the proctodæum. The actual perforation of the

septum between the proctodæum and the cloacal section of the

alimentary tract is not effected till about the fifteenth day of

foetal life, and the approximation of the epithelial layers of the two

organs, preparatory to their absorption, is partly effected by the

tunneling of the mesoblastic tissue between them by numerous spaces.

The hypoblastic section of the cloaca of birds, which receives the

openings of the urinogenital ducts, is permanently marked off by a

fold from the epiblastic section or true proctodæum, with which the

bursa Fabricii communicates.

BIBLIOGRAPHY.

_Alimentary Canal and its appendages._

(561) B. Afanassiew. "Ueber Bau u. Entwicklung d. Thymus d. Säugeth."

_Archiv f. mikr. Anat._ Bd. XIV. 1877.

(562) Fr. Boll. _Das Princip d. Wachsthums._ Berlin, 1876.

(563) E. Gasser. "Die Entstehung d. Cloakenöffnung bei

Hühnerembryonen." _Archiv f. Anat. u. Physiol., Anat. Abth._ 1880.

(564) A. Götte. _Beiträge zur Entwicklungsgeschichte d. Darmkanals im

Hühnchen._ 1867.

(565) W. Müller. "Ueber die Entwickelung der Schilddrüse." _Jenaische

Zeitschrift_, Vol. VI. 1871.

(566) W. Müller. "Die Hypobranchialrinne d. Tunicaten." _Jenaische

Zeitschrift_, Vol. VII. 1872.

(567) S. L. Schenk. "Die Bauchspeicheldrüse d. Embryo."

_Anatomischphysiologische Untersuchungen._ 1872.

(568) E. Selenka. "Beitrag zur Entwicklungsgeschichte d. Luftsäcke d.

Huhns." _Zeit. f. wiss. Zool._ 1866.

(569) L. Stieda. _Untersuch. üb. d. Entwick. d. Glandula Thymus,

Glandula thyroidea, u. Glandula carotica._ Leipzig, 1881.

(570) C. Fr. Wolff. "De formatione intestinorum." _Nov. Comment. Akad.

Petrop._ 1766.

(571) H. Wölfler. _Ueb. d. Entwick. u. d. Bau d. Schilddrüse._ Berlin,

1880.

_Vide_ also Kölliker (298), Götte (296), His (232 and 297), Foster and

Balfour (295), Balfour (292), Remak (302), Schenk (303), etc.

_Teeth._

(572) T. H. Huxley. "On the enamel and dentine of teeth." _Quart. J.

of Micros. Science_, Vol. III. 1855.

(573) R. Owen. _Odontography._ London, 1840-1845.

(574) Ch. S. Tomes. _Manual of dental anatomy, human and comparative._

London, 1876.

(575) Ch. S. Tomes. "On the development of teeth." _Quart. J. of

Micros. Science_, Vol. XVI. 1876.

(576) W. Waldeyer. "Structure and development of teeth." _Stricker's

Histology._ 1870.

_Vide_ also Kölliker (298), Gegenbaur (294), Hertwig (306), etc.

INDEX TO VOLUME III.

Abdominal muscles, 675

Abdominal pore, 626, 749

Acipenser, development of, 102;

affinities of, 118;

comparison of gastrula of, 279;

pericardial cavity of, 627

Actinotrocha, 373

Air-bladder of Teleostei, 77;

Lepidosteus, 117;

blood supply of, 645;

general account of, 763;

homologies of, 766

Alciope, eye of, 480

Alisphenoid region of skull, 569

Alimentary canal and appendages, development of, 754

Alimentary tract of Ascidia, 18;

Molgula, 22;

Pyrosoma, 24;

Salpa, 31;

Elasmobranchii, 52;

Teleostei, 75;

Petromyzon, 93, 97;

Acipenser, 110;

Amphibia, 129, 136;

Chick, 167;

respiratory region of, 754;

temporary closure of oesophageal region of, 759

Allantois, development of in Chick, 191, 198;

blood-vessels of in Chick, 193;

Lacerta, 205, 209;

early development of in Rabbit, 229;

of Guinea-pig, 264;

origin of, 309.

_See also_ 'Placenta' and 'Bladder'

Alternation of generations in Ascidians, origin of, 35;

in Botryllus, 35;

Pyrosoma, 36;

Salpa, 36;

Doliolum, 36

Alytes, branchial chamber of, 136;

yolk-sack of, 139;

branchiæ, 141;

Müllerian duct of, 710

Amblystoma, ovum of, 120;

larva of, 142, 143

Amia, ribs of, 561

Ammocoetes, 95;

metamorphosis of, 97;

eye of, 498

Amnion, early development of in Chick, 185;

later history of in Chick, 196;

Lacerta, 204, 210;

Rabbit, 229;

origin of, 307, 309

Amphibia, development of, 120;

viviparous, 121;

gastrula of, 277;

suctorial mouth of, 317;

cerebellum of, 426;

infundibulum of, 431;

pineal gland of, 433;

cerebrum of, 439;

olfactory lobes of, 444;

nares of, 553;

notochord and its sheath, 548;

vertebral column of, 554;

ribs of, 561;

branchial arches of, 574;

mandibular and hyoid arches of, 582;

columella of, 582;

pectoral girdle of, 605;

pelvic girdle of, 607;

limbs of, 619;

heart of, 638;

arterial system of, 645;

venous system of, 655;

excretory system of, 707;

vasa efferentia of, 711;

liver of, 769;

postanal gut of, 774;

stomodæum of, 778

Amphiblastula larva of Porifera, 344

Amphioxus, development of, 1;

gastrula of, 275;

formation of mesoblast of, 292;

development of notochord of, 293;

head of, 314;

spinal nerves of, 461;

olfactory organ of, 462;

venous system of, 651;

transverse abdominal muscle of, 673;

generative cells of, 748;

liver of, 769;

postanal gut of, 772;

stomodæum of, 777

Amphistylic skulls, 578

Angular bone, 594

Anterior abdominal vein, 653

Anura, development of, 121;

epiblast of, 125;

mesoblast of, 128;

notochord of, 128;

hypoblast of, 129;

general growth of embryo of, 131;

larva of, 134;

vertebral column of, 556;

mandibular arch of, 584

Anus of Amphioxus, 7;

Ascidia, 18;

Pyrosoma, 28;

Salpa, 31;

Elasmobranchii, 57;

Amphibia, 130, 132;

Chick, 167;

primitive, 324

Appendicularia, development of, 34

Aqueductus vestibuli, 519

Aqueous humour, 497

Arachnida, nervous system of, 409;

eye of, 481

Area, embryonic, of Rabbit, 218;

epiblast of, 219;

origin of embryo from, 228;

area opaca of Chick, 150;

epiblast, hypoblast, and mesoblast of, 159;

area pellucida of Chick, 150;

of Lacerta, 202 area vasculosa of Chick, 194;

mesoblast of, 160;

of Lizard, 209;

Rabbit, 228, 229

Arteria centralis retinæ, 503

Arterial system of Petromyzon, 97;

constitution of in embryo, 643;

of Fishes, 644;

of Amphibia, 645;

of Amniota, 647

Arthropoda, head of, 313;

nervous system of, 409;

eye of, 480;

excretory organs of, 688

Articular bone of Teleostei, 581;

of Sauropsida, 588

Ascidia, development of, 9

Ascidians. _See_ 'Tunicata'

Ascidiozooids, 25

Atrial cavity of Amphioxus, 7;

Ascidia, 18;

Pyrosoma, 24

Atrial pore of Amphioxus, 7;

Ascidia, 20;

Pyrosoma, 28;

Salpa, 32

Auditory capsules, ossifications in, 595, 596

Auditory involution of Elasmobranchii, 57;

Teleostei, 73;

Petromyzon, 89, 92;

Acipenser, 106;

Lepidosteus, 114;

Amphibia, 127;

Chick, 170

Auditory nerve, development of, 459

Auditory organs, of Ascidia, 15;

of Salpa, 31;

of Ammocoetes, 98;

Ganoidei, 108, 114;

of Amphibia, 127;

of Aves, 170;

general development of, 512;

of aquatic forms, 512;

of land forms, 513;

of Coelenterata, 513;

of Mollusca, 515;

of Crustacea, 516;

of Vertebrata, 517;

of Cyclostomata, 89, 92, 518;

of Teleostei, Lepidosteus and Amphibia, 518;

of Mammalia, 519;

accessory structures of, 527;

of Tunicata, 528

Auriculo-ventricular valves, 642

Autostylic skulls, 579

Aves, development of, 145;

cerebellum of, 426;

mid-brain of, 427;

infundibulum of, 431;

pineal gland of, 434;

pituitary body of, 436;

cerebrum of, 439;

olfactory lobes of, 444;

spinal nerves of, 449;

cranial nerves of, 455;

vagus of, 458;

glossopharyngeal of, 458;

vertebral column of, 557;

ossification of vertebral column of, 558;

branchial arches of, 572, 573;

pectoral girdle of, 603;

pelvic girdle of, 608;

heart of, 637;

arterial system of, 647;

venous system of, 658;

muscle-plates of, 670;

excretory organs of, 714;

mesonephros of, 715;

pronephros of, 718;

Müllerian duct of, 718, 720;

nature of pronephros of, 721;

connection of Müllerian duct with Wolffian in, 720;

kidney of, 722;

lungs of, 764;

liver of, 769;

postanal gut of, 774

Axolotl, 142, 143;

ovum of, 120;

mid-brain of, 427;

mandibular arch of, 583

Basilar membrane, 524

Basilar plate, 565

Basipterygium, 612

Basisphenoid region of skull, 569

Bilateral symmetry, origin of, 373-376

Bile duct, 770

Bladder, Amphibia, 131;

of Amniota, 726

Blastodermic vesicle, of Rabbit, first development of, 217;

of 7th day, 222;

Guinea-pig, 263;

meaning of, 291

Blastoderm of Pyrosoma, 24;

Elasmobranchii, 41;

Chick, 150;

Lacerta 202

Blastopore, of Amphioxus, 2;

of Ascidia, 11;

Elasmobranchii, 42, 54, 62;

Petromyzon, 87;

Acipenser, 104;

Amphibia, 125, 130;

Chick, 153;

Rabbit, 216;

true Mammalian, 226;

comparative history of closure of, 284, 288;

summary of fate of, 340;

relation of to primitive anus, 324

Blood-vessels, development of, 633

Body cavity, of Ascidia, 21;

Molgula, 21;

Salpa, 31;

Elasmobranchii, 47;

of Teleostei, 75;

Petromyzon, 94;

Chick, 169;

development of in Chordata, 325;

views on origin of, 356-360, 377;

of Invertebrata, 623;

of Chordata, 624;

of head, 676

Bombinator, branchial chamber of, 136;

vertebral column of, 556

Bonellia, excretory organs of, 687

Bones, origin of cartilage bones, 542;

origin of membrane bones, 543;

development of, 543;

homologies of membrane bones, 542;

homologies of cartilage bones, 545

Brachiopoda, excretory organs of, 683;

generative ducts of, 749

Brain, of Ascidia, 11, 15;

Elasmobranchii, 56, 59, 60;

Teleostei, 77;

Petromyzon, 89, 92;

Acipenser, 105;

Lepidosteus, 113;

early development of in Chick, 170;

flexure of in Chick, 175;

later development of in Chick, 176;

Rabbit, 229;

general account of development of, 419;

flexure of, 420;

histogeny of, 422

Branchial arches, præoral, 570;

disappearance of posterior, 573;

dental plates of in Teleostei, 574;

relation of to head cavities, 571;

_see_ 'Visceral arches'

Branchial chamber of Amphibia, 136

Branchial clefts, of Amphioxus, 7;

of Ascidia, 18, 20;

Molgula, 23;

Salpa, 32;

of Elasmobranchii, 57, 59-61;

Teleostei, 77;

Petromyzon, 91, 96;

Acipenser, 105;

Lepidosteus, 114, 116;

Amphibia, 132, 133;

Chick, 178;

Rabbit, 231;

præoral, 312, 318;

of Invertebrata, 326;

origin of, 326

Branchial rays, 574

Branchial skeleton, development of, 572, 592;

of Petromyzon, 96, 312, 571;

of Ichthyopsida, 572;

dental plates of in Teleostei, 574;

relation of to head cavities, 572

Branchiæ, external of Elasmobranchii, 61, 62;

of Teleostei, 77;

Acipenser, 107;

Amphibia, 127, 133, 135

Brood-pouch, of Salpa, 29;

Teleostei, 68;

Amphibia, 121

Brown tubes of Gephyrea, 686

Bulbus arteriosus, of Fishes, 638;

Amphibia, 639

Bursa Fabricii, 167, 779

Canalis auricularis, 639

Canalis reuniens, 521

Capitellidæ, excretory organs of, 683

Carcharias, placenta of, 66

Cardinal vein, 652

Carnivora, placenta of, 250

Carpus, development of, 620

Cartilage bones of skull, 595;

homologies of, 595

Cat, placenta of, 250

Caudal swellings of Elasmobranchii, 46, 55;

Teleostei, 72;

Chick, 162, 170

Cephalic plate of Elasmobranchii, 55

Cephalochorda, development of, 1

Cephalopoda, eyes of, 473-477

Cerebellum, Petromyzon, 93;

Chick, 176;

general account of development of, 424, 425

Cerebrum of Petromyzon, 93, 97;

Chick, 175;

general development of, 429, 438;

transverse fissure of, 443

Cestoda, excretory organs of, 681

Cetacea, placenta, 255

Chætognatha, nervous system of, 349;

eye of, 479;

generative organs of, 743;

generative ducts of, 749

Chætopoda, head of, 313;

eyes of, 479;

excretory organs of, 683;

generative organs of, 743;

generative ducts of, 749

Charybdæa, eye of, 472

Cheiroptera, placenta of, 244

Cheiropterygium, 618;

relation of to ichthyopterygium, 621

Chelonia, development of, 210;

pectoral girdle of, 603;

arterial system of, 649

Chick, development of, 145;

general growth of embryo of, 170;

rotation of embryo of, 173;

foetal membranes of, 185;

epiblast of, 150, 166;

optic nerve and choroid fissure of, 500

Chilognatha, eye of, 481

Chilopoda, eye of, 481

Chimæra, lateral line of, 539;

vertebral column of, 548;

nares of, 533

Chiromantis, oviposition of, 121

Chorda tympani, development of, 460

Chordata, ancestor of, 311;

branchial system of, 312;

evidence from Ammocoetes, 312;

head of, 312;

mouth of, 318;

table of phylogeny of, 327

Chorion, 237;

villi of, 237, 257

Choroid coat, Ammocoetes, 99;

general account of, 487

Choroid fissure, of Vertebrate eye, 486, 493;

of Ammocoetes, 498;

comparative development of, 500;

of Chick, 501;

of Lizards, 501;

of Elasmobranchii, 502;

of Teleostei, 503;

Amphibia, 503;

Mammals, 503, 504

Choroid gland, 320

Choroid pigment, 489

Choroid plexus, of fourth ventricle, 425;

of third ventricle, 432;

of lateral ventricle, 442

Ciliated sack of Ascidia, 18;

Pyrosoma, 26;

Salpa, 31

Ciliary ganglion, 461

Ciliary muscle, 490

Ciliary processes, 488;

comparative development of, 506

Clavicle, 600

Clitoris, development of, 727

Clinoid ridge, 569

Cloaca, 766

Coccygeo-mesenteric vein, 661

Cochlear canal, 519

Coecilia, development of, 143;

pronephros of, 707;

mesonephros of, 709;

Müllerian duct of, 710

Coelenterata, larvæ of, 367;

eyes of, 471;

auditory organs of, 513;

generative organs of, 741

Columella auris, 529;

of Amphibia, 582;

of Sauropsida, 588

Commissures, of spinal cord, 417;

of brain, 431, 432, 439, 443

Coni vasculosi, 724

Conus arteriosus, of Fishes, 638;

of Amphibia, 638

Coracoid bone, 599

Cornea, of Ammocoetes, 99;

general development of, 495;

corpuscles of, 496;

comparative development of, 499;

of Mammals, 499

Coronoid bone, 595

Corpora geniculata interna, 428

Corpora quadrigemina, 428

Corpora striata, development of, 437

Corpus callosum, development of, 443

Corti, organ of, 522;

structure of, 525;

fibres of, 525;

development of, 526

Cranial flexure, of Elasmobranchii, 58, 60;

of Teleostei, 77;

Petromyzon, 93, 94;

of Amphibia, 131, 132;

Chick, 174;

Rabbit, 231;

characters of, 321;

significance of, 322

Cranial nerves, development of, 455;

relation of to head cavities, 461;

anterior roots of, 462-464;

view on position of roots of, 466

Crocodilia, arterial system of, 649

Crura cerebri, 429

Crustacea, nervous system of, 411;

eye of, 481;

auditory organs of, 515;

generative cells of, 745;

generative ducts of, 751

Cupola, 524

Cutaneous muscles, 676

Cyathozooid, 25

Cyclostomata, auditory organs of, 517;

olfactory organ of, 532;

notochord and vertebral column of, 546, 549;

abdominal pores of, 626;

segmental duct of, 700;

pronephros of, 700;

mesonephros of, 700;

generative ducts of, 733, 749;

venous system of, 651;

excretory organs of, 700

Cystignathus, oviposition of, 122

Dactylethra, branchial chamber of, 136;

branchiæ of, 136;

tadpole of, 140

Decidua reflexa, of Rat, 242;

of Insectivora, 243;

of Man, 245

Deiter's cells, 526

Dental papilla, 777

Dental capsule, 777

Dentary bone, 595

Dentine, 780

Descemet's membrane, 496

Diaphragm, 631;

muscle of, 676

Dipnoi, nares of, 534;

vertebral column of, 548;

membrane bones of skull of, 592;

heart of, 638;

arterial system of, 645;

excretory system of, 707;

stomodæum of, 777

Diptera, eye of, 481

Discophora, excretory organs of, 687

Dog, placenta of, 248

Dohrn, on relations of Cyclostomata, 84;

on ancestor of Chordata, 311, 319

Doliolum, development of, 28

Ductus arteriosus, 649

Ductus Botalli, 648

Ductus Cuvieri, 654

Ductus venosus Arantii, 663

Dugong, heart of, 642

Dysticus, eye of, 481

Ear, _see_ 'Auditory organ'

Echinodermata, secondary symmetry of larva of, 380;

excretory organs of, 689;

generative ducts of, 752

Echinorhinus, lateral line of, 539;

vertebral column of, 548

Echiurus, excretory organs of, 686

Ectostosis, 543

Edentata, placenta of, 248, 250, 256

Eel, generative ducts of, 703

Egg-shell of Elasmobranchii, 40;

Chick, 146

Elasmobranchii, development of, 40;

viviparous, 40;

general features of development of, 55;

gastrula of, 281;

development of mesoblast of, 294;

notochord of, 294;

meaning of formation of mesoblast of, 295;

restiform tracts of, 425;

optic lobes of, 427;

cerebellum of, 425;

pineal gland of, 432;

pituitary body of, 435;

cerebrum of, 438;

olfactory lobes of, 444;

spinal nerves, 449;

cranial nerves of, 457;

sympathetic nervous system of, 466;

nares of, 533;

lateral line of, 539;

vertebral column of, 549;

ribs of, 560;

parachordals of, 567;

mandibular and hyoid arches of, 576;

pectoral girdle of, 600;

pelvic girdle of, 607;

limbs of, 609;

pericardial cavity of, 627;

arterial system of, 644;

venous system of, 651;

muscle-plates of, 668;

excretory organs of, 690;

constitution of excretory organs in adult of, 697;

spermatozoa of, 747;

swimming-bladder of, 763;

intestines of, 767;

liver of, 769;

postanal gut of, 772

Elæoblast of Pyrosoma, 28;

Salpa, 30

Elephant, placenta of, 249

Embolic formation of gastrula, 333

Enamel organ, 777

Endolymph of ear, 522

Endostosis, 543

Endostyle of Ascidia, 18, 759;

Pyrosoma, 25;

Salpa, 32

Epiblast, of Elasmobranchii, 47;

Teleostei, 71, 75;

Petromyzon, 86;

Lepidosteus, 112;

Amphibia, 122, 125;

Chick, 149, 166;

Lacerta, 203;

Rabbit, 216, 219;

origin of in Rabbit, 221;

comparative account of development of, 300

Epibolic formation of gastrula, 334

Epichordal formation of vertebral column, 556

Epicrium glutinosum, 143

Epidermis, in Coelenterata, 393;

protective structures of, 394

Epididymis, 724

Epigastric vein, 653

Episkeletal muscles, 676

Episternum, 602

Epoophoron, 725

Ethmoid bone, 597

Ethmoid region of skull, 570

ethmo-palatine ligament of Elasmobranchs, 576

Euphausia, eye of, 483

Eustachian tube, of Amphibia, 135;

Chick, 180;

Rabbit, 232;

general development of, 528

Excretory organs, general constitution of, 680;

of Platyelminthes, 680;

of Mollusca, 681;

of Polyzoa, 682;

of Brachiopoda, 683;

of Chætopoda, 683;

of Gephyrea, 686;

of Discophora, 687;

of Arthropoda, 688;

of Nematoda, 689;

of Echinodermata, 689;

constitution of in Craniata, 689;

of Elasmobranchii, 690;

constitution of in adult Elasmobranch, 697;

of Petromyzon, 700;

of Myxine, 701;

of Teleostei,701;

of Ganoidei, 704;

of Dipnoi, 707;

of Amphibia, 707;

of Amniota, 713;

comparison of Vertebrate and Invertebrate, 737

Excretory system, of Elasmobranchii, 49;

Teleostei, 78;

Petromyzon, 95, 98;

Acipenser, 99;

Amphibia, 133

Exoccipital bone, 595

Exoskeleton, dermal, 393-395;

epidermal, 393-396

External generative organs, 726

Extra-branchial skeleton, 572

Eye, of Ascidia, 16;

Salpa, 31;

Elasmobranchii, 56, 57, 58;

Teleostei, 73;

Petromyzon, 92, 98;

Aves, 170;

Rabbit, 229;

general development of, 470;

evolution of, 470, 471;

simple, 480;

compound, 481;

aconous, 482;

pseudoconous, 482;

of Invertebrata, 471;

of Vertebrata, 483;

comparative development of Vertebrate, 497;

of Ammocoetes, 497;

of Tunicata, 507;

of Chordata, general views on, 508;

accessory eyes of Fishes, 509;

muscles of, 677

Eyelids, development of, 506

Falciform ligament, 757

Falx cerebri, 439

Fasciculi teretes, of Elasmobranchii, 426

Feathers, development of, 396

Fenestra rotunda and ovalis, 529

Fertilization, of Amphioxus, 2;

of Urochorda, 9;

Salpa, 29;

Elasmobranchii, 46;

of Teleostei, 68;

Petromyzon, 84;

Amphibia, 120;

Chick, 145;

Reptilia, 202;

meaning of, 331

Fifth nerve, development of, 460

Fifth ventricle, 443

Fins, of Elasmobranchii, 62;

Teleostei, 78;

Petromyzon, 94, 95;

Acipenser, 109;

Lepidosteus, 118;

relation of paired to unpaired, 611, 612;

development of pelvic, 614;

development of pectoral, 615;

views on nature of paired fins, 616

Fissures of spinal cord, 417

Foetal development, 360;

secondary variations in, 361

Foot, 618

Foramen of Munro, 430, 438

Foramen ovale, 642

Fore-brain, of Elasmobranchii, 55, 59, 60;

Petromyzon, 93;

general development of, 428

Formative cells, of Chick, 154

Fornix, development of, 443

Fornix of Gottsche, 428

Fourth nerve, 464

Frontals, 592

Frontonasal process of Chick, 179

Gaertner's canals, 724

Gall-bladder, 770

Ganoidei, development of, 102;

relations of, 118;

nares of, 534;

notochord of, 546;

vertebral column of, 546, 553;

ribs of, 561;

pelvic girdle of, 606;

arterial$ system of, 645;

excretory organs of, 704;

generative ducts of, 734

Gastropoda, eye of, 472

Gastrula, of Amphioxus, 2;

of Ascidia, 10;

Elasmobranchii, 43, 44;

Petromyzon, 86;

Acipenser, 103;

Amphibia, 123;

comparative development of, in Invertebrata, 275;

comparison of Mammalian, 291;

phylogenetic meaning of, 333;

ontogeny of (general), 333;

phylogeny of, 338-343;

secondary types of, 341

Geckos, vertebral column of, 557

Generative cells, development of, 741;

origin of in Coelenterata, 741;

of Invertebrata, 743;

of Vertebrata, 746

Generative ducts, of Teleostei, 704, 735;

of Ganoids, 704;

of Cyclostomata, 733;

origin of, 733;

of Lepidosteus, 735, 750;

development and evolution of, 748;

of Coelenterata, 748;

of Sagitta, 749;

of Tunicata, 749;

Chætopoda, Gephyrea, etc., 749;

of Mollusca, 751;

of Discophora, 751;

of Echinodermata, 752

Generative system of Elasmobranchii, 51

Gephyrea, nervous system of, 412;

excretory organs of, 686;

generative cells of, 743;

generative ducts of, 749

Germinal disc, of Elasmobranchii, 40;

Teleostei, 68;

Chick, 147

Germinal epithelium, 746

Germinal layers, summary of organs derived from, in Vertebrata, 304;

historical account of views of, 332;

homologies of in the Metazoa, 345

Germinal wall of Chick, 152, 159;

structure and changes of, 160

Geryonia, auditory organ of, 515

Gill of Salpa, 31

Giraldes, organ of, 725

Glands, epidermic, development of, 397

Glomerulus, external, of Chick, 716

Glossopharyngeal nerve, development of, 456, 457

Grey matter of spinal cord, 417;

of brain, 423

Growth in length of Vertebrate embryo, 306

Guinea-pig, primitive streak of, 223;

notochord of, 226;

placenta of, 242;

development of, 262

Gymnophiona, _see_ 'Coecilia'

Habenula perforata, 525

Hairs, development of, 396

Halichærus, placenta of, 250

Hand, 619

Head, comparative account of, 313;

segmentation of, 314

Head cavities, of Elasmobranchii, 50;

Petromyzon, 90, 96;

Amphibia, 127;

general development of, 676

Head-fold of Chick, 157, 167

Head kidney, _see_ 'Pronephros'

Heart, of Pyrosoma, 25;

Elasmobranchii, 50, 58;

Petromyzon, 94, 97;

Acipenser, 106;

Chick, 170;

first appearance of in Rabbit, 230;

general development of, 633;

of Fishes, 635, 637;

of Mammalia, 638;

of Birds, 637, 639;

meaning of development of, 637;

of Amphibia, 638;

of Amniota, 639;

change of position of, 643

Hind-brain, Elasmobranchii, 55, 59, 60;

Petromyzon, 93;

general account of, 424

Hippocampus major, development of, 442

Hirudo, development of blood-vessels of, 633;

excretory organs of, 688

Horse, placenta of, 253

Hyaloid membrane, 492

Hylodes, oviposition of, 121;

metamorphosis of, 137

Hyobranchial cleft, 572

Hyoid arch, of Chick, 179;

general account of, 572, 575;

modifications of, 573, 577;

of Elasmobranchii, 576;

of Teleostei, 577;

of Amphibia, 582;

of Sauropsida, 588;

of Mammalia, 589

Hyomandibular bar of Elasmobranchii, 576, 577;

of Teleostei, 579;

of Amphibia, 582

Hyomandibular cleft, of Petromyzon, 91;

Chick, 179;

general account of, 572

Hyostylic skulls, 582

Hypoblast of Elasmobranchii, 51;

Teleostei, 71, 75;

Petromyzon, 86;

Acipenser, 104;

Lepidosteus, 113;

Amphibia, 122, 129;

Chick, 151, 167;

Lacerta, 203;

Rabbit, 215, 216, 219;

origin of in Rabbit, 220

Hyposkeletal muscles, 675

Hyrax, placenta of, 249

Incus, 529, 590

Infraclavicle, 600

Infundibulum of Petromyzon, 92;

Chick, 175;

general development of, 430

Insectivora, placenta of, 243

Insects, nervous system of, 410;

eye of, 481;

generative organs of, 745;

generative ducts of, 751

Intercalated pieces of vertebral column, 551

Interclavicle, homologies of, 602

Intermediate cell-mass of Chick, 183

Intermuscular septa, 672

Interorbital septum, 570

Interrenal bodies, 665

Iris, 489;

comparative development of, 506

Iris of Ammocoetes, 98

Island of Reil, 444

Jacobson's organ, 537

Jugal bone, 594

Kidney, _see_ 'Metanephros'

Labia majora, development of, 727

Labial cartilages, 597

Labium tympanicum, 525;

vestibulare, 525

Lacertilia, general development of, 202;

nares of, 537;

pectoral girdle of, 603;

pelvic girdle of, 607;

arterial system of, 649

Lacrymal bone, 593

Lacrymal duct, 506

Lacrymal glands, 506

Læmargus, vertebral column of, 548

Lagena, 524

Lamina spiralis, 524

Lamina terminalis, 438

Larva of Amphioxus, 2;

of Ascidia, 15-21;

Teleostei, 81;

Petromyzon, 89, 95;

Lepidosteus, 117, 318;

Amphibia, 134, 142;

types of, in the Invertebrata, 363

Larvæ, nature, origin, and affinities of, 360-386;

secondary variations of less likely to be retained, 362;

ancestral history more fully recorded in, 362;

secondary variations in development of, 363;

ontogenetic record of secondary variations in, 361;

of freshwater and land animals, 362;

types of, 362;

phosphorescence of, 364;

of Coelenterata, 367;

table of, 365;

of Invertebrata, 367 et seq.

Larynx, 766

Lateral line sense organs, 538;

comparison of, with invertebrate, 538;

development of, in Teleostei, 538;

development of, in Elasmobranchii, 539

Lateral ventricle, 438;

anterior cornu of, 440;

descending cornu of, 440;

choroid plexus of, 443

Layers, formation of, in Elasmobranchii, 41, 56;

Teleostei, 71;

Petromyzon, 85;

Acipenser, 103;

Lepidosteus, 111;

Amphibia, 121;

Chick, 150, 152;

Lacerta, 202;

Rabbit, 215-227;

comparison of Mammalia with lower forms, 226, 289;

comparison of formation of in Vertebrata, 275;

origin and homologies of, in the Metazoa, 331

Leech, _see_ 'Hirudo'

Lemuridæ, placenta, 256

Lens, of Elasmobranchii, 57, 58;

Petromyzon, 94, 99;

Acipenser, 106;

Lepidosteus, 115;

Amphibia, 127;

Chick, 177;

of Vertebrate eyes, 485;

general account of, 493;

capsule of, 493;

comparative development of, 499;

of Amphibia, Teleostei, Lepidosteus, 499

Lepidosteus, development of, 111;

larva of, 117;

relations of, 119;

spinal nerves of, 455;

ribs of, 561;

generative ducts of, 704, 735;

swimming-bladder of, 763

Ligamentum pectinatum, 490

Ligamentum suspensorium, 557, 558

Ligamentum vesicæ medium, 239

Limbs, of Elasmobranchii, 59;

Teleostei, 80;

first appearance of in Chick, 184;

Rabbit, 232;

muscles of, 673;

of Fishes, 609;

relation of, to unpaired fins of Fishes, 611, 612;

of Amphibia, 618

Liver of Teleostei, 78;

Petromyzon, 95, 96;

Acipenser, 110;

Amphibia, 130;

general account of, 769

Lizard, development of, 202;

general growth of embryo of, 208;

Müllerian duct of, 721

Lizzia, eye of, 471

Lobi inferiores, 431

Lungs of Amphibia, 137;

development of, 763;

homology of, 766

Lymphatic system, 664

Malleus, 529, 591;

views on, 591

Malpighian bodies, development of accessory in Elasmobranchs, 695

Mammalia, development of, 214;

comparison of gastrula of, 291;

cerebellum of, 427;

infundibulum of, 431;

pineal gland of, 434;

pituitary body of, 436;

cerebrum of, 439;

spinal nerves of, 449;

sympathetic of, 466;

vertebral column of, 558;

branchial arches of, 573, 574;

mandibular and hyoid arches of, 589;

pectoral girdle of, 604;

pelvic girdle of, 608;

heart of, 636;

arterial system of, 647;

venous system of, 661;

muscle-plates of, 671;

mesonephros of, 714;

testicular network of, 724;

urinogenital sinus of, 727;

spermatozoa of, 747;

lungs of, 765;

intestines of, 768;

liver of, 769;

postanal gut of, 774;

stomodæum of, 775

Mammary gland, development of, 398

Man, placenta of, 244;

general account of development of, 265;

characters of embryo of, 270

Mandibular arch of Elasmobranchii, 62, 576;

Petromyzon, 91;

Acipenser, 106, 116;

Chick, 179;

general account of, 572, 575;

modification of to form jaws, 573, 575;

of Teleostei, 580;

of Amphibia, 582;

Sauropsida, 588;

Mammalia, 589

Mandibular bar, evolution of, 311, 321

Manis, placenta of, 256

Marsupial bones, 608

Marsupialia, foetal membranes of, 240;

cerebellum of, 426;

corpus callosum of, 443;

uterus of, 726

Maxilla, 594

Meatus auditorius externus, of Chick, 181;

development of, 527

Meckelian cartilage, of Elasmobranchii, 576;

of Teleostei, 581;

of Amphibia, 584, 585;

of Sauropsida, 588;

of Mammalia, 590

Mediastinum anterior and posterior, 630

Medulla oblongata, of Chick, 176;

general development of, 425

Medullary plate of Amphioxus, 4, 5;

of Ascidia, 11;

Elasmobranchii, 44, 47, 55;

Teleostei, 72;

Petromyzon, 88;

Acipenser, 104;

Lepidosteus, 111;

Amphibia, 126, 127, 131;

Chick, 159;

Lacerta, 204;

Rabbit, 223, 227, 228;

primitive bilobed character of, 303, 317

Medusæ, auditory organs of, 513

Membrana capsulo-pupillaris, 494, 504, 507

Membrana elastica externa, 546

Membrana limitans of retina, 491

Membrana tectoria, 522, 525

Membrane bones, of Amphibia, 582;

of Sauropsida, 588;

of Mammalia, 590;

of mandibular arch, 593;

of pectoral girdle, 599, 602;

origin of, 592;

homologies of, 593

Membranous labyrinth, development of in Man, 519

Menobranchus, branchial arches of, 142

Mesenteron of Elasmobranchii, 43;

Teleostei, 75;

Petromyzon, 85;

Acipenser, 104;

Amphibia, 123, 124, 129;

Chick, 167;

general account of, 754

Mesentery, 626, 756

Mesoblast, of Amphioxus, 6;

Ascidia, 17, 20;

Pyrosoma, 24;

Salpa, 30;

Elasmobranchii, 44, 47;

Teleostei, 75;

Petromyzon, 86;

Acipenser, 105;

Lepidosteus, 113;

Amphibia, 125, 128, 129;

of Chick, 154, 167;

double origin of in Chick, 154, 158, 159;

origin of from lips of blastopore in Chick, 158;

of area vasculosa of Chick, 160;

Lacerta, 203;

origin of in Rabbit, 218, 223;

of area vasculosa in Rabbit, 227;

comparative account of formation of, 292;

discussion of development of in Vertebrata, 297;

meaning of development of in Amniota, 298;

phylogenetic origin of, 346;

summary of ontogeny of, 349-352;

views on ontogeny of, 352-360

Mesoblastic somites, of Amphioxus, 6;

Elasmobranchii, 48, 55;

Petromyzon, 88;

Acipenser, 105;

Lepidosteus, 114;

Amphibia, 129, 131;

Chick, 161, 180;

Rabbit, 228;

development of in Chordata, 325;

meaning of development of, 331;

of head, 676

Mesogastrium, 758

Mesonephros, of Teleostei, 78, 702;

Petromyzon, 95, 98, 700;

Acipenser, 110, 705;

Amphibia, 134, 708;

Chick, 184, 714;

general account of, 690;

development of in Elasmobranchs 691;

of Cyclostomata, 700;

Ganoidei, 705;

sexual and non-sexual part of in Amphibia, 710;

of Amniota, 713, 724;

summary and general conclusions as to, 729;

relation of to pronephros, 731

Mesopterygium, 616

Metagenesis of Ascidians, 34

Metamorphosis of Amphibia, 137, 140

Metanephros, 690;

development of in Elasmobranchii, 697;

of Amphibia, 712;

of Amniota, 713;

of Chick, 722;

of Lacertilia, 723;

phylogeny of, 736

Metapterygium, 616

Metapterygoid, of Elasmobranchii, 576;

of Teleostei, 581

Metazoa, evolution of, 339, 342;

ancestral form of, 333, 345

Mid-brain, of Elasmobranchii, 55, 58, 59;

Petromyzon, 92;

general account of development of, 427

Moina, generative organs of, 745

Molgula, development of, 22

Mollusca, nervous system of, 414;

eyes of, 472;

auditory organs of, 515;

excretory organs of, 681

Monotremata, foetal membranes of, 240;

cerebellum of, 426;

corpus callosum of, 443;

cerebrum of, 443;

urinogenital sinus of, 726

Mormyrus, generative ducts of, 704

Mouth, of Amphioxus, 7;

of Ascidia, 18;

Pyrosoma, 27;

Salpa, 31;

Elasmobranchii, 57, 60, 61, 62;

Petromyzon, 92, 94, 95, 99;

Acipenser, 107;

Lepidosteus, 118;

Amphibia, 129, 132, 134;

Rabbit, 231;

origin of, 317

Mouth, suctorial, of Petromyzon, 99;

Acipenser, 107;

Lepidosteus, 116, 317;

Amphibia, 133, 141, 317

Müllerian duct, 690;

of Elasmobranchs, 693;

of Ganoids, 704;

of Amphibia, 710;

of Aves, 717, 720;

opening of into cloaca, 727;

origin of, 733;

summary of development of, 733;

relation of to pronephros, 733

Muscle-plates, of Amphioxus, 6;

Elasmobranchii, 49, 668;

Teleostei, 670;

Petromyzon, 94;

Chick, 183, 670;

general development of, 669;

of Amphibia, 670;

Aves, 670;

of Mammalia, 671;

origin of muscles from, 672

Muscles, of Ascidia, 11, 17;

development of from muscle-plates, 672;

of limbs, 673;

of head, 676;

of branchial arches, 678;

of eye, 678

Muscular fibres, epithelial origin of, 667

Muscular system, development of, 667;

of Chordata, 668

Mustelus, placenta of, 66

Myoepithelial cells, 667

Mysis, auditory organ of, 517

Myxine, ovum of, 100;

olfactory organ of, 533;

portal sinus of, 652;

excretory system of, 701

Nails, development of, 397

Nares, of Acipenser, 108;

of Ichthyopsida, 534;

development of in Chick, 535;

development of in Lacertilia, 537;

development of in Amphibia, 537

Nasal bones, 592

Nasal pits, Acipenser, 108;

Chick, 176;

general development of, 531

Nematoda, excretory organs of, 689;

generative organs of, 745;

generative ducts of, 752

Nemertines, nervous system of, 311;

excretory organs of, 681

Nerve cord, origin of ventral, 378

Nerves, spinal, 449;

cranial, 455-466

Nervous system, central,

general account of development of in Vertebrata, 415;

conclusions as to, 445;

sympathetic, 466

Nervous system, of Amphioxus, 4;

Ascidia, 15, 16;

Molgula, 22;

Pyrosoma, 24, 25;

Salpa, 30, 31;

Elasmobranchii, 44;

Teleostei, 77;

Petromyzon, 89, 93;

Acipenser, 105;

Amphibia, 126;

comparative account of formation of central, 301;

of Sagitta, 349;

origin of in Coelenterata, 349;

of præoral lobe, 377, 380;

evolution of, 400-405;

development of in Invertebrates, 406;

of Arthropoda, 408;

of Gephyrea, 412;

Mollusca, 414

Neural canal, of Ascidia, 10;

Teleostei, 72;

Petromyzon, 88;

Acipenser, 105;

Lepidosteus, 114;

Amphibia, 126, 131;

Chick, 166, 171;

Lacerta, 208;

closure of in Frog and Amphioxus, 279;

closure of in Elasmobranchii, 284;

phylogenetic origin of, 316

Neural crest, 449, 456, 457

Neurenteric canal, of Amphioxus, 4, 5;

Ascidia, 10;

Elasmobranchii, 54;

Petromyzon, 88;

Acipenser, 105;

Lepidosteus, 113;

Aves, 162;

Lacerta, 203, 206;

general account of, 323;

meaning of, 323

Newt, ovum of, 120;

development of, 125;

general growth of, 141

Notidanus, vertebral column of, 548;

branchial arches of, 572

Notochord of Amphioxus, 6;

Ascidia, 11, 17;

Elasmobranchii, 51;

Teleostei, 74;

Petromyzon 86, 94;

Acipenser, 104;

Lepidosteus, 113;

Amphibia, 128, 129;

Chick, 157;

canal of, in Chick, 163;

Lacerta, 204, 205;

Guinea-pig, 226;

comparative account of formation of, 292, 325;

sheath of, 545;

later histological changes in, 546;

cartilaginous sheath of, 547;

in head, 566;

absence of in region of trabeculæ, 567

Notodelphys, brood-pouch of, 121;

branchiæ of, 140

Nototrema, brood-pouch of, 121

Nucleus pulposus, 559

Oceania, eye of, 471

Occipital bone, 595

OEsophagus, solid, of Elasmobranchii, 61, 759;

of Teleostei, 78

Olfactory capsules, 571

Olfactory lobes, development of, 444

Olfactory nerves, Ammocoetes, 99;

general development of, 464

Olfactory organ, of aquatic forms, 531;

Insects and Crustacea, 531;

of Tunicata, 532;

of Amphioxus, 532;

of Vertebrata, 533;

Petromyzon, 533;

of Myxine, 533

Olfactory sacks, of Elasmobranchii, 60;

Teleostei, 73;

Petromyzon, 92, 97;

Acipenser, 106, 108;

Lepidosteus, 116;

Chick, 176

Oligochæta, excretory organs of, 683

Olivary bodies, 426

Omentum, lesser and greater, 757

Onchidium, eye of, 478

Opercular bones, 593

Operculum, of Teleostei, 77;

Acipenser, 107;

Lepidosteus, 117, 118;

Amphibia, 135

Ophidia, development of, 210;

arterial system of, 649;

venous system of, 656

Optic chiasma, 430, 493

Optic cup, retinal part of, 488;

ciliary portion of, 489

Optic lobes, 428

Optic nerve, development of, 492;

comparative development of, 500

Optic thalami, development of, 431

Optic vesicle, of Elasmobranchii, 57-59;

Teleostei, 74, 499;

Petromyzon, 89, 92;

Acipenser, 106;

Lepidosteus, 115;

Chick, 170;

Rabbit, 229;

general development of, 429;

formation of secondary, 487;

obliteration of cavity of, 488;

comparative development of, 499;

of Lepidosteus and Teleostei, 499.

_See also_ 'Eye'

Ora serrata, 488

Orbitosphenoid region of skull, 570

Organs, classification of, 391;

derivation of from germinal layers, 392

Orycteropus, placenta of, 249

Otic process of Axolotl, 583;

of Frog, 585 et seq.

Otoliths, 512

Oviposition, of Amphioxus, 1;

Elasmobranchii, 40;

Teleostei, 68;

Petromyzon, 84;

Amphibia, 121;

Reptilia, 202

Ovum, of Amphioxus, 1;

Pyrosoma, 23;

Elasmobranchii, 40;

Teleostei, 68;

Petromyzon, 83;

Myxine, 100;

Acipenser, 102;

Lepidosteus, 111;

Amphibia, 120;

Chick, 146;

Reptilia, 202;

Mammalia, 214;

of Porifera, 741;

migration of in Coelenterata, 742;

Vertebrata, 746

Palatine bone, of Teleostei, 580;

origin of, 594

Pancreas, Acipenser, 110;

general development of, 770

Pancreatic cæca, of Teleostei, etc. 768

Papillæ, oral, of Acipenser, 108;

Lepidosteus, 116

Parachordals, 565, 566

Parasphenoid bone, 594