DEVELOPMENT OF THE BRAIN. 679 



central canal of the spinal cord, and the columnar cells of the 

 epiblast form a lining of ciliated columnar epithelium. 



The epiblast at the lower part of the canal becomes converted 

 into the anterior gray columns, in connection with which arise 

 the anterior roots of the spinal nerves ; whilst at the upper part 

 the posterior gray columns are formed in connection with the pos- 

 terior roots of the spinal nerves and their ganglia. 



The white columns are thought by some authors to be derived 

 from the mesoblast surrounding the canal, but by others they are 

 assigned to the epiblast. 



The upper or dorsal canal becomes converted into a fissure by 

 the absorption of its root, and is thus changed into the posterior 

 fissure of the spinal cord. 



The anterior fissure is formed by the downgrowth 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, 

 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 downwards by two processes 

 growing forwards from the anterior cerebral vesicle, which 

 becomes divided into two parts, the anterior of which is subse- 

 quently developed into the cerebral hemispheres and the olfac- 

 tory lobes, while the hinder part receives the name of thalamen- 

 cephalon. 



The cavity of the thalamencephalon opens behind into the 



