526 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



completely fused. Posteriorly the neural folds seem to be continuous 

 with the lips of the primitive groove. A faint continuation forwards 

 of the primitive groove may be seen in the floor of the rhomboidal 

 sinus. 



P a 



m.s. 



Vv>. 231. — Blastoderm with Fowl embryo with about 10 or 11 mesoderm segments. 



a.v, vascular area ; /.(/, foregnt ; k, head ; m.f, medullary fold ;*m.s. mesoderm segment ; 

 o.r, optic rudiment ; pa, proamnion. 



Important mesoderm features are to be noticed. The mottled 

 appearance of the vascular area produced by the rudiments of blood- 

 vessels developing , in the splanchnic mesoderm is conspicuous. The 

 formerly isolated vascular rudiments (white in the figure) are now 

 becoming joined up to form a network and the network can be traced 

 — less distinct and on a smaller scale — across the pellucid area. At 



