THE INTESTINAL CANAL. 681 



The upper walls of the second cerebral vesicle are developed 

 into the corpora quadrigeraina. 



The cavity of this vesicle persists as a narrow channel, and 

 forms a communication between the third ventricle in. front and 

 the fourth ventricle behind, and receives the name in the adult 

 brain of the Her a tertio ad quartum ventriculum. The crura cere- 

 bri arise from the lower wall of this middle vesicle. 



The third cerebral vesicle is divided into an anterior and a pos- 

 terior part. From the upper part of the anterior division arises 

 the cerebellum, and from its lower part 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 Vieussens. It communi- 

 cates with the subarachnoid space through the foramen of Ma- 

 jendie. 



THE ALIMENTARY CANAL AND ITS APPENDAGES. 



When the blastoderm is bent at its anterior extremity to form 

 the cephalic fold, it closes in and forms the anterior boundary of 

 a short canal, the upper wall of which is formed by the general 

 blastoderm, and its lower wall by that part of the splanchno- 

 pleure which runs backward, leaving the somatopleure to form 

 the pleuroperitoneal space. It then turns forward to meet with 

 the uncleft mesoblast, forming the wall of the yelk sack, which 

 communicates freely with this rudimentary part of the alimentary 

 tract. 



This canal becomes closed in for a considerable extent, and is 

 then 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- 



