THE NEURAL GROOVE AND TUBE 



51 



expanded portion the term sinus rhomboidalis has been applied (Fig. 18). Before 

 the neural groove is closed a ridge of ectodermal cells appears along the prominent 

 margin of each neural fold; this is termed the neural crest or ganglion ridge, and from 

 it the spinal and cranial nerve ganglia and the ganglia of the sympathetic nervous 

 system are developed. By the upward growth of the mesoderm the neural tube 

 is ultimately separated from the overlying ectoderm. 



.Head fold of amnion partly 

 covering the fore-brain 



Mid-brain 



Hind-brain 



Nerve ganglion J 

 Auditory vesicle -W-= 



Vilelline vein 



Fourteenth primitive , 



Paraxial mesoderm 



Neural fold -, 



Sinus rhomboidalis -* 



Remains of primitive streak _ 



Heart 



KIG. 18. Chick embryo of thirty-three hours' incubation, viewed from the dorsal aspect. X 30. 

 (From Duval'a "Atlas d'Embryologie.") 



The cephalic end of the neural groove exhibits several dilatations, which, when 

 the tube is closed, assume the form of three vesicles; these constitute the three 

 primary cerebral vesicles, and correspond respectively to the future fore-brain (pros- 

 encephalon), mid-brain (mesence photon), and hind-brain (rhombe?icephalon) (Fig. 

 18). The walls of the vesicles are developed into the nervous tissue and neuroglia 

 of the brain, and their cavities are modified to form its ventricles. The remainder 



