VI.] THE BRAIN. 117 



SO completely has the front end of the neural canal 

 been folded over the end of the notochord. The com- 

 mencement of this cranial flexure gives the body of an 

 embryo of the third day somewhat the appearance of a 

 retort, the head oi the embryo corresponding to the 

 bulb. On the fourth day the flexure is still greater 

 than on the third, but on the fifth and succeeding days 

 it becomes less obvious, owing to the filling up of the 

 parts of the skuU. 



The brain. The vesicle of the cerebral hemispheres, 

 which on the second day began to grow out from the 

 front of the fore-brain, increases rapidly in size during 

 the third day, growing out laterally, so as to form two 

 vesicles, so much so that by the end of the day it (Fig. 

 37, OH, Fig. 38) is as large or larger than the original 

 vesicle from which it sprang, and forms the most con- 

 spicuous part of the brain. In its growth it pushes 

 aside the optic vesicles, and thus contributes largely to 

 the roundness which the head is now acquiring. Each 

 lateral vesicle possesses a cavity, which afterwards 

 becomes one of the lateral ventricles. These cavities are 

 continuous behind with the cavity of the fore-brain. 



Owing to the development of the cerebral vesicle the 

 original fore-brain no longer occupies the front position 

 (Fig. 37, FB, Fig. 38, lb), and ceases to be the con- 

 spicuous object that it was. Inasmuch as its walls will 

 hereafter be developed into the parts surrounding the 

 so-called third ventricle of the brain, we shall hence- 

 forward speak of it as the vesicle of the third ventricle, 

 or thalamencephalon. 



On the summit of the thalamencephalon there may 

 now be seen a small conical projection, the rudiment of 



