the roof develops into the hemispheres proper. The cavities of these lobes become the lateral ventricles, and are connected by

communicated with those of the ventricles.

Middle Cerebral Vesicle.-By the cranial flexure the brain is bent at the junction of the first and second cerebral vesicles;

the first is thus turned downward, leaving the second as the most anterior part of the brain.

The upper walls of the middle cerebral vesicle are developed into the corpora quadrigemina.

The cavity of this vesicle persists as a narrow channel, forming a communication between the third ventricle in front and the fourth ventricle behind, and receives the name in the adult brain of the iter a tertio ad quartum ventriculum. The crura cerebri arise from the lower wall of this middle vesicle.

Posterior Cerebral Vesicle.-This is divided into an anterior and a posterior part. From the roof of the anterior division arises the cerebellum, and from its floor the pons Varolii.

The posterior division gives rise to the medulla oblongata.

The cavity of this vesicle is called the fourth ventricle. It is continuous with the central canal of the spinal cord. Its upper wall is thinned and forms the valve of Vieussens. It communicates with the subarachnoid space through the foramen of Majendie.

THE ALIMENTARY CANAL AND ITS APPENDAGES.

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 conmunicates 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 allantois, 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

Alimentary canal of an embryo while the rudimentary mid-gut is still in continuity with

the yolk sac. (Kölliker after Bischoff.)

lower larger yolk sac, the connection between the two forming the ductus vitello-intestinalis.

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

Position of the various parts of the alimentary canal at different stages. five weeks; B. Of eight weeks; C. Of ten weeks. (Allen Thomson.) A. Embryo of 1. Pharynx with the lungs; s. Stomach; i. Small intestine; i. Large intestine; g. Genital duct; u. Bladder; cl. Cloaca; c. Cæcum; vi. Ductus vitello-intestinalis; si. Urogenital sinus; v. Yolk sac. FIG. 292.

Longitudinal section of a foetal sheep. (Cudiat.)

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

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.

The primitive intestine placed at the inferior aspect of the embryo, just below the protovertebræ, 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, being developed into the peritoneal processes of the organ.

The lower part of the fore gut is of much smaller calibre than

FIG. 293.

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 column forms the mesentery.

The large intestine is at first a

Diagram of the alimentary canal of straight tube lying to the left of the

a chick at the fourth day. (Foster

and Balfour, after Götte.)

lg. Diverticulum of one lung. St. Stomach. 1. Liver. p. Pancreas.

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.