THE EMBRYOLOGY OF THE EYE. 3 



blast a linear furrow forms upon its inner 

 surface, and the portion of epiblast surround- 

 ing this furrow becomes thickened so as to 

 dip down into the mesoblast and push the 

 furrow before it. At length this furrow 

 closes and, becoming separated from the ex- 

 ternal epiblastic layer, forms a long narrow 

 tube, the neural canal, which is the begin- 

 ning of the cerebro-spinal axis (Fig. A). 



Fig. a. 



This tube becomes dilated at the extremity 

 where the brain is to be formed, and constric- 

 tions divide the dilated portion into three 

 parts, called the anterior, middle, and poste- 

 rior p7Hmary cerebral vesicles. Later the 

 anterior and the posterior primary vesicles 

 each divide again, forming thus five secondary 

 cerebral vesicles. 



