772 GENERATION AND DEVELOPMENT. 



Development of the Nervous System. 



The mode in which the rudimentary structures of the 

 cerebro- spinal nervous system are formed, has been already 

 stated (p. 744). The dorsal laminae, the inner borders of 

 which close in and form the canal of the spinal cord, seem 

 to leave a fissure in the situation of the medulla oblongata. 

 Between this and Ihe most anterior extremity of the canal, 

 three vesicular enlargements, the vesicles of the brain, are 

 developed (see fig. 210), and from these again are developed 

 the following parts : 



From the anterior primary vesicle the optic thalami, 

 corpora striata, the third ventricle, and the cerebral 

 hemispheres, together with some other parts in connec- 

 tion with those above named, as the corpus callosum, 

 fornix, etc. 



From the middle primary vesicle the corpora quadrige- 

 niina and crura cerebri, with the aqueduct of Sylvius. 



From the posterior primary vesicle the cerebellum, pons 

 Varolii, medulla oblongata, etc. 



Development of the Organs of Sense. 



The eye is in part developed as a protruded portion of 

 the first primary cerebral vesicle ; while passing backwards, 

 and pressing on the front of this process or primary optic 



Fig. 229.* 



* Fig. 229. Longitudinal section of the primary optic vesicle in the 

 chick magnified (from Remak). A, from an embryo of sixty-five hours ; 

 B, a few hours later ; C, of the fourth day ; c, the corneous layer or 

 epidermis, presenting in A, the open depression for the lens, which is 



