THE NERVOUS SYSTEM. 



475 



roof of the diencephalon by another slight constriction is another expansion of 

 the dorsal wall, the roof of the mid-brain or of the mesencephalon which arches 

 over the cephalic flexure. It is separated by another constriction (plica 

 rhombo-mesencephalica) from the rhombic brain or rhombencephalon, which latter 

 tapers into the cord. A ventral bulging of the rhombencephalon indicates the 

 future pons region (Figs. 414 and 415). 



Heart 



Ant. entrance to 

 prim, gut (Ant. 

 "Darmpforte") 



Post, entrance to 

 prim, gut (Post. 

 "Darmpforte") 



Cerebral plate 



Amnion 



Yolk 

 (cut edge) 



Yolk sac 



Belly stalk 



Neural tube 



Primitive 

 segment 



Neural fold 

 Neural groove 



Neural fold 



FIG. 413. (a) Ventral view; (6) dorsal view of human embryo with 8 pairs of primitive 



segments (2.11 mm.). Eternod. From models by Ziegler. 



In b the amnion has been removed, merely the cut edge showing; in a the yolk sac has 



been removed. 



Even at this early stage the cavity of the caudal part of the rhombencephalon 

 is expanded dorsally due to an expansion of the roof plate, which forms only the 

 narrow dorsal median part of the rest of the tube. This expansion reaches its 

 maximum about opposite the auditory vesicle. 



The principal changes in form during the next two weeks are the following 

 (Figs. 416 and 472): The cephalic flexure becomes still more pronounced so 

 that the anterior end of the neural tube is folded back upon the ventral side of 

 the rest of the brain, an effect probably enhanced by the expansion of the 



