136 OUTLINES OF CHORDATE DEVELOPMENT 



supplying the muscles of the eye- ball. The development of the 

 nerves commonly described as I and II will be considered in 

 connection with the development of the olfactory and optic 

 sense organs. 



Embryologically the rudiments of the cranial nerves are com- 

 posite structures, three elements entering into their formation. 

 These are, (a) cell masses derived from the neural crests, (&) 

 cells from ectodermal patches on the surface of the head, (c) 

 cell processes extending out from neuroblasts in the ventro- 

 lateral walls of the spinal cord. In the spinal nerves the ele- 

 ments from the surface ectoderm are lacking. 



A. THE CRANIAL NERVES 



The rudiments of certain of the cranial nerves appear very 

 early, and we may therefore describe them first although they 

 are more complicated than the later appearing spinal nerves. 

 While the central nervous system is still in the form of a flat plate 

 we have already seen that its margin is considerably thickened, 

 on account of the proliferation of the cells of the inner or ner- 

 vous layer there. These thickened margins are visible on the 

 surface of the embryo as the medullary ridges. Transverse 

 sections of this stage (Fig. 44) show that these thickened masses 

 of inner ectoderm become delaminated, both from the outer 

 stratum of ectoderm and from the medial portion of the medul- 

 lary plate which then goes to form the neural tube proper. 

 These lateral cell masses are the beginnings of the neural crests. 

 In the head region these masses become very large, and as the 

 neural plate begins to close each becomes transversely divided 

 into three masses. Posteriorly to the brain region the neural 

 crests are much smaller, but are typically formed. As the 

 neural tube closes these cell masses are left in situ along the 

 sides of the brain and cord (Corning, Brachet). 



The three divisions of the head portion of each crest soon 

 become quite distinct. The anterior section, which begins in 

 the mid-brain region, is to be recognized as the rudiment of the 

 trigeminal ganglion (V nerve), the middle section as the rudi- 



