THE EMBRYO. 



1179 



from epiblast. The spinal nerves are developed as follows : close to the point of 

 involution of the epiblast in the median line, that is to say, in the angle of junc- 

 tion of the neural and general epiblast, a cellular swelling, the neural crest, 



..Rhomboid 

 Up. 



\ 



'li/ixxjlossal 

 nerve. 



FIG. 727. Transverse section of medulla oblongata of human embryo. (After His.) 



appears and forms a continuous ridge of epiblast on the dorsal aspect of the neural 

 canal (Fig. 728). On this crest enlargements occur corresponding with the middle 

 of each protovertebral segment. These enlargements grow downward between 



Neural canal. 

 Neural crest. 



Somatopleure. 



Plrnroperitoneal cavity. 



ftplanchnopleure. 

 Omphalo-mesenteric vein. 



FIG. 728. Transverse section of a portion of a chick embryo of twenty-nine hours' incubation. (From 

 Duval's Atlas d'Embryologie.) 



the neural canal and the protovertebrse, and occupy a position on the lateral wall 

 of the canal. They are the rudiments of the ganglia of the posterior roots, and 

 at first are attached to the neural crest from which they spring, but subsequently 

 this attachment becomes lost, and they then form isolated masses on either side of 

 the neural canal. They consist of oval cells, from either end of which a process 

 eventually springs ; one, growing centrally, passes into the embryonic cord and 

 constitutes the posterior root of the nerve; the other, growing peripherally, joins 

 the fibres of the anterior root to form the spinal nerve. 



The anterior root is, according to the researches of His, a direct outgrowth of 

 the neuroblasta which are found in the rudimentary cord (Fig. 722). These cells, 

 at first rounded or oval, become pear-shaped, with their tapering prolongations 

 directed outward toward the surface of the cord. These prolongations are the 

 future axis-cylinders of the anterior nerve-roots ; they pass out of the cord 

 in bundles and penetrate the mesoblast to join with the fibres of the pos- 

 terior root, and from the point of union the nerve grows toward its peripheral 

 termination. 



Cranial Nerves. With the exception of the olfactory and optic nerves, which 

 will be specially referred to, the cranial nerves may be developmentally considered 



