DEVELOPMENT OF THE BRAIN. 



677 



The anterior fissure is formed by the down-growth of the ante- 

 rior columns, which diverge, leaving between them an interval 

 which becomes occupied by the pia mater. 



The commissures are not formed between the lateral halves of 

 the cord until later. The gray commissure appears first. 



The Brain. 



Anterior Cerebral Vesicle. As already mentioned, the brain is 

 formed from the primitive neural canal, the anterior part of 

 which becomes dilated into three little swellings called the ante- 

 rior, middle and posterior cerebral vesicles. From the anterior, 



Diagram of the cerebral vesicles of the brain of a chick at the second day. (Cadiat.) 

 1, 2, 3, Cerebral vesicles. 0. Optic vesicles. 



or first cerebral vesicle, at an early period spring two processes, 

 which become the optic vesicles. These ultimately become de- 

 veloped into the retina and other nervous parts of the eye, with 

 the history of which the changes occurring in them will be de- 

 scribed. 



The optic vesicles are displaced downward by two processes 

 growing forward from the anterior cerebral vesicle, which becomes 

 divided into two parts, the anterior of which is subsequently de- 

 veloped into the cerebral hemispheres and the olfactory lobes, 

 while the hinder part receives the name of thalamencephalon. 



