When the blastoderm is bent at its anterior extremity to form the cephalic fold, it closes and forms the anterior boundary of a short canal, the upper wall of which is formed by the general blastoderm, and the lower by that part of the splanchnopleure which runs backward, leaving the somatopleure to form the pleuroperitoneal space. It then turns forward to meet the uncleft mesoblast, forming the wall of the yolk sac, which communicates freely with this rudimentary part of the alimentary tract.

This canal becomes closed in for a considerable extent, and is called the fore gut. It is the precursor of the pharynx, the lungs, the oesophagus, the stomach, and the duodenum. The mouth, which at this period is unformed, is developed later by an involution of the epiblast and the removal of the tissue between the fore gut and the buccal cavity.

The tail fold, in a somewhat similar manner, shuts off a canal called the hind gut, which becomes developed into the posterior part of the alimentary canal. This hind gut, until the further development of the bladder, etc., is in connection with the allan-tois, which arises as a bud from the lower part of the rudimentary hind gut.

Between these two canals an intermediate one is formed by the splanchnopleure, which, at a distance from its origin, becomes constricted, and shuts off an upper canal, the mid-gut, from the lower larger yolk sac, the connection between the two forming the ductus vitello-intestinalis.

The Alimentary Canal And Its Appendages 307Alimentary canal of an embryo while the rudimentary mid gut is still in continuity with the yolk sac.

Fig. 290. Alimentary canal of an embryo while the rudimentary mid-gut is still in continuity with the yolk sac. (Kolliker, after Bischoff).

A. View from below.

a. Pharyngeal plates.

b. The pharynx.

c.c. Diverticula forming the lungs. d. The stomach.

f. Diverticula of the liver.

g. Membrane torn from the yolk sac. h. Hind-gut.

B. Longitudinal Section.

a. Diverticulum of a lung.

b. Stomach. e. Liver.

d. Yolk sac.

Thus the primitive alimentary canal consists of an anterior and a posterior blind canal, which are closed below, and a third intermediate between these, which opens at its lower surface into the yolk sac.

As the placental circulation becomes more developed, the yolk sac shrinks and atrophies, until it is represented by a fold of tissue connected with the primitive intestine. The ductus vitello-intestinalis accordingly becomes obliterated, and thus the mid-gut is closed at its lower aspect.

Position of the various parts of the alimentary canal at different stages.

Fig. 291. Position of the various parts of the alimentary canal at different stages. A. Embryo of five weeks; B. Of eight weeks; C. Of ten weeks. {Allen Thomson).

I. Pharynx with the lungs; s. Stomach; i. Small intestine; i'. Large intestine; g. Genital duct; u. Bladder; cI. Cloaca; c. Caecum; vi. Ductus vitello-intestinalis; si. Urogenital sinus; v. Yolk sac.

Longitudinal section of a fcetal sheep. {Cadiat).

Fig. 292. Longitudinal section of a fcetal sheep. {Cadiat).

a. Pericardium; b. Commencement of diaphragm; c. Heart; d. Branchial arches; e. Pharynx; f. Origin of lung; g. Liver.

The primitive intestine placed at the inferior aspect of the embryo, just below the protovertebrae, is lined by hypoblast and covered by mesoblast. The cephalic or anterior extremity of the canal is formed by uncleft mesoblast; the rest of the canal is formed by the splanchnopleural layer.

A dilatation of a part of the fore gut gives origin to the primitive stomach; this is quite straight at first, lying below the vertebral column, with which it is connected by mesoblast. After a time the stomach becomes turned to the right side, so that the left surface of the organ lies anteriorly and the right posteriorly, the mesoblast connecting it with the vertebral column, beiifg developed into the peritoneal processes of the organ.

The lower part of the fore gut is of much smaller calibre than the dilated portion forming the stomach; it becomes the duodenum, in connection with which arise two important viscera, the liver and the pancreas. The mid-gut and hind gut form the small and large intestines, these being at first one straight tube, of which the small intestine has the larger calibre. The small intestine, as it grows, falls into folds, and the mesoblast connecting it to the vertebral columns forms the mesentery.

The large intestine is at first a straight tube lying to the left of the embryo; it becomes bent, and part of the tube is directed toward the right side; this develops another flexure, the portion of intestine below which grows downward. That part remaining on the left side forms the rectum, the sigmoid flexure, and the descending colon; while that between the flexures becomes the transverse colon, and that on the right side the ascending colon.

Diagram of the alimentary canal of a chick at the fourth day.

Fig.293. Diagram of the alimentary canal of a chick at the fourth day. (Foster and Balfour, after G'otte).

Ig. Diverticulum of one lung. St. Stomach. l. Liver. p. Pancreas.

The caecum is developed from the ascending colon, the ileocecal valve shutting off one part of the intestinal canal from the other. The vermiform appendix originates from the inferior extremity of the caecum, which, owing to its feeble growth, is of much smaller calibre than the upper part.

The epithelial lining of the intestines is derived from the hypoblast, and the muscular, vascular, connective tissue and serous coverings are mesoblastic in their origin.

The liver is developed from two diverticula of the duodenum, in connection with which arise cylinders of cells. The hypoblast develops into the liver cells and the cells lining the ducts, the mesoblast furnishing the vascular and connective tissue parts of the organ. The two diverticula are connected by a transverse piece, and form the right and left lobes of the liver.

The process connecting the liver to the duodenum forms the common bile duct, and from this the gall bladder is developed as an outgrowth.

The vessels of the embryo, which are in relation to the liver, will be described under the vascular system.

The pancreas arises as an outgrowth from the duodenum, its constituent parts originating in a manner similar to those of the liver.

The spleen is derived from the mesoblast, and is developed in one of the peritoneal processes of the stomach.

The lungs are developed in connection with the oesophagus, of which they are early outgrowths.

The canal of the fore gut at a certain point becomes laterally constricted, its transverse section presenting an hour-glass shape, consisting of an upper and lower dilated portion, united by a central constricted neck. The lower of these cavities becomes subdivided by the outgrowth of the lateral portions and the upgrowth of a part of the lower wall which forms a central septum, so that the fore gut is composed of an upper undivided tube, giving off two appendages.

These appendages consist of hypoblastic tissue, and as they grow into the surrounding mesoblast they divide and subdivide, until at last they consist of very minute lobules, which terminate in dilated extremities. The undivided canal forms the permanent trachea, the appendages the main bronchi, while their minute subdivisions are the bronchioles, which terminate in the dilated alveoli.

The hypoblast forms the delicate lining membrane of the air passages, and the mesoblast gives rise to the supporting tissue holding them together, to the blood vessels, the muscular, cartilaginous and connective tissue of the bronchial tubes.

The pleurae surrounding the lungs are, like the other serous membranes, mesoblastic in their origin.