THE LATER DEVELOPMENT OF THE FROG 135 



are termed the germinal cells (Fig. 43). Part of the germinal 

 cells become the supporting or_ ylia cells, while the remainder 

 become the functional nerve cells or neuroblasts. The closure of 

 the neural folds is complete and no dorsal fissure is left in the 

 cord. 



The thickening of the walls of the cord begins dorsally and 

 dorso-laterally, so that the central canal is given a ventral loca- 

 tion, its floor formed only by a layer of ependymal cells. Later 

 the ventral wall thickens slightly and the ventro-lateral walls 

 extend below its level forming a shallow groove, the ventral 

 fissure. The central canal becomes compressed laterally, and 

 is later completely surrounded by neuroblasts (gray matter 

 the outgrowths of which form a superficial layer known as 

 white matter of the cord (Fig. 43). 



2. The Peripheral Nervous System 



We should recall in a few words the morphological arrange- 

 ment of the spinal and cranial nerves. The^spinal nerves, of 

 which there are but ten pairs in the adult, although in the tad- 

 pole upward of forty pairs, arise from the cord by a dorsal or 

 afferent root, on which is located the spinal ganglion, and a 

 ventral or efferent root. These roots join to form the trunk of 

 the spinal nerve which is then divided into dorsal and ventral 

 rami and a ramus communicans which passes to the sympa- 

 thetic system (Fig. 46). Of the cranial nerves, connected 

 with the brain, there are commonly described ten pairs, con- 

 siderably varied in morphological, as well as in functional, 

 characteristics. Those regarded as the most typical are pri- 

 marily related with the gill clefts and are therefore known as the 

 branchiomeric nerves; these are the V, VII, IX, and X. Each 

 of these arises by a single, though in some cases compound, 

 root of mixed character, i.e., afferent and efferent, passes into 

 a large ganglion, beyond which it gives off a horizontal branch, 

 and then divides into two branches which pass anteriorly and 

 posteriorly to the gill cleft with which the nerve is associated. 

 The III, IV, and VI cranial nerves are simple, purely efferent, 



