472 EMBRYOLOGY OF THE CHICK 



THE DIFFERENTIATION OF THE BRAIN REGION 



At twenty-seven hours, the more cephalic end of the neural tube has 

 become considerably enlarged as compared with the more caudal por- 

 tion. The walls are thicker and the lumen larger. This portion is to 

 become the brain proper, the portion in which the lumen has not en- 

 larged becoming the spinal cord. A picture of the brain at this time 

 (Figs. 278 and 282) will show three primary vesicles or lumen-enlarge- 



Fig. 277. 



Diagrammatic ventral view of a 35-36 hour chick embryo. Compare with Figures 

 279 and 280. (Modified from Prentiss.) 



ments together with what these three vesicles later become. The most 

 anterior of the three primary vesicles is known as the fore-brain, or 

 prosencephalon. The mid portion is called the mid-brain or mesencepha- 

 lon, while the most posterior vesicle forms the hind-brain or rhomben- 

 cephalori. The rhombencephalon is continuous with the spinal cord. 



As all further developments of the brain arise from these three pri- 

 mary regions, it is of the utmost importance that these primary regions 

 be grasped fully. 



