4 8 



NERVOUS SYSTEM OF VERTEBRATES. 



functional efficiency. It is worthy of notice that in the cen- 

 tral gray matter in lower vertebrates a large part of the cells 

 have central processes which reach, and perhaps take part in, 

 the internal limiting membrane. These cells seem to show that 

 the bodies of cells formed from neuroblasts migrate toward 

 >the periphery, but their inner ends remain connected with the 

 internal limiting membrane and become drawn out into long 

 central processes. Other cells situated at the extreme periphery 

 in the fish brain are probably derived from peripheral cells of 



FIG. 24. A horizontal section. of the nucleus praeopticus of the sturgeon. Golgi 

 method. The cells have migrated toward the periphery but retain their connec- 

 tion with the internal limiting membrane by means of central processes. 



the neural tube such as are seen in Figures 14 and 15. Figures 

 23, 24, and 25, taken from the brain of the sturgeon, illustrate 

 these relations. 



While the histogenesis is in progress certain changes of form 

 in the neural tube are taking place. The most important of these 

 is the appearance of longitudinal zones. In the region of the spinal 

 cord it is noticeable that the walls of the neural tube do not increase 

 in thickness equally. The lateral walls thicken, while along the 

 mid-dorsal and mid-ventral lines the tube remains thin. These 

 thin bands have been named by His roof -plate and floor -plate, 



