DEVELOPMENT OF THE NERVOUS SYSTEM. 47 



as they are formed and in general the fibers run to the outer 

 surface of the cord or brain, forming a peripheral fiber layer. 

 Some time after the formation of the nerve fiber the neuroblast 

 gives off other processes which branch more or less richly in the 

 substance of the brain wall. The nerve fiber first formed remains 

 always a relatively slender fiber of nearly uniform diameter. 

 It is called the neurite. The processes later formed become more 

 or less bush-like and are known as dendrites. 



In most parts of the central nerrous system the greater number 

 of nerve cells remain near the central canal and there form the 

 central gray matter. But many cells migrate toward the surface and 

 in some parts of the brain very rich layers of nerve cells are found 



Tr. lobo-bulbaris 



FIG. 23. A part of a transverse section of the inferior lobe of the sturgeon, 

 stained by the Golgi method. The cells all retain their primitive position adjacent 

 to the cavity. 



at the outer surface. In fishes it is observed that in any given 

 part of the brain those cells which are farthest removed from the 

 central canal have undergone the greatest modifications of form 

 as compared with the simple structure of the neuroblasts. 

 The dendrites may become more richly branched, more nu- 

 merous and more widely extended; they may be disposed par- 

 allel with the surface or otherwise modified to increase their 



