THE NERVOUS SYSTEM. 



481 



trally at about a right angle to the longitudinal axis of the remaining portion of 

 the tube. This bending begins before the closure of the cephalic part of the 

 neural tube (Fig. 120). From each side of the brain near the cephalic ex- 

 tremity is an evaginatiori of the brain wall, the beginning of the optic vesicles. 



Neural 



fold 



Ectoderm 



Mesoderm 



x Chorda anlage Entoderm 



FIG. 411. Transverse section through dorsal part of embryo of frog (Rana fusca). 

 x, Groove indicating evagination to form mesoderm. 



Ziegler 



The process of evagination and consequently the location of the vesicle begins 

 before the closure of the tube. 



Dorsal and anterior to the optic vesicles can be seen a slight unpaired pro- 

 trusion of the dorsal wall, the beginning of the pallium. The area basal to it and 



Prim. Intermed. 

 cell mass 



Parietal and 

 visceral mesoderm 



Chordal Prim, 

 plate aorta 



^^^ 

 Coelom Entoderm Blood vessels 



FIG. 412. Transverse section of dog embryo with ten pairs of primitive segments. Bonnet. 



extending a short distance into the anterior wall of the optic vesicle is the site of 

 the future corpus striatum (Figs. 414 and 415). 



Caudal to the pallium and separated from it by a slight constriction (in- 

 dicated best by the ridge on the inner wall) is another protrusion of the dorsal 

 wall, the roof of the diencephalon. Still further caudally and separated from the 



