X.J THE CEREBRAL VESICLES. 329 



Birds, which does not become converted into the body 

 of the embryo. Outside the area pellucida, an area 

 vasculosa has become very well defined. In the em- 

 bryo itself (Fig. 106 A) the disproportion between head 

 and trunk is less marked than before ; the medullary 

 plate dilates anteriorly to form a spatula-shaped ce- 

 phalic enlargement; and three or four somites are 

 established. In the lateral parts of the mesoblast of 

 the head there may be seen on each side a tube-like 

 structure (hz). Each of these is part of the heart, which 

 arises as two independent tubes. The remains of the 

 primitive streak {pr) are still present behind the me- 

 dullary groove. 



In somewhat older embryos (Fig. 106 B) with about 

 eight somites, in which the trunk considerably exceeds 

 the head in length, the first distinct traces of the 

 folding off of the head end of the embryo become ap- 

 parent, and somewhat later a fold also appears at the 

 hind end. In the formation of the hind end of the 

 embryo the primitive streak gives rise to a tail swelling 

 and to part of the ventral wall of the post-anal gut. In 

 the region of the head the rudiments of the heart (h) 

 are far more definite. The medullary groove is still 

 open for its whole length, but in the head it exhibits a 

 series of well-marked dilatations. The foremost of 

 these [vh) is the rudiment of the fore-brain from the 

 sides of which there project the two optic vesicles {ah) ; 

 the next is the mid-brain [mh) and the last is the hind- 

 brain [hh), which is again divided into smaller lobes by 

 successive constrictions. The medullary groove behind 

 the region of the somites dilates into an embryonic 

 sinus rhomboidalis like that of the bird. Traces of the 



