228 GENERAL GROWTH OF THE EMBRYO. 



nearly meet behind and so cut off the front portion of the primi- 

 tive streak, which then appears as a projection in the hind end 

 of the medullary groove. In an embryo rabbit, eight days after 

 impregnation, the medullary groove is about r8o mm. in length. 

 At this stage a division may be clearly seen in the lateral plates 

 of mesoblast into a vertebral zone adjoining the embryo and 

 a more peripheral lateral zone ; and in the vertebral zone indi- 

 cations of two somites, about 0*37 mm. from the hinder end of 

 the embryo, become apparent. The foremost of these somites 

 marks the junction, or very nearly so, of the cephalic region and 

 trunk. The small size of the latter as compared with the former 

 is very striking, but is characteristic of Vertebrates generally. 

 The trunk gradually elongates relatively to the head, by the 

 addition behind of fresh somites. The embryo has not yet 

 begun to be folded off from the yolk-sack. In a slightly older 

 embryo of nine days there appears (Hensen, Kolliker) round the 

 embryonic area a delicate clear ring which is narrower in front 

 than behind (fig. 144 A, ap}. This ring is regarded by these 

 authors as representing the peripheral part of the area pellucida 

 of 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 embryo itself (fig. 144 A) the 

 disproportion between head and trunk is less marked than be- 

 fore ; the medullary plate dilates anteriorly to form a spatula- 

 shaped cephalic 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 (Jiz\ 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 medullary groove. 



In somewhat older embryos (fig. 144 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 apparent, 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 (Ji) are far more definite. 

 The medullary groove is still open for its whole length, but in 



