190 GENERAL GROWTH OF THE EMBRYO. 



become converted into the body of the embryo. Outside the area 

 pellucida, an area vasculosa has become very well denned. In the 

 embryo itself (rig. 144 A) the disproportion between head and trunk 

 is less marked than before ; 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 (hz). 

 Each of these is part of the heart, which arises as two independent 

 tubes. The remains of the primitive streak (_) 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 (It) 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 (ab) ; the next is the mid-brain (w/*), and the last 

 is the hind-brain (hfi), 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 amnion (af) are now apparent both in 

 front of and behind the embryo. 



The structure of the head and the formation of the heart at this 

 age are illustrated in fig. 145. The widely-open medullary groove 

 (rf) is shewn in the centre. Below it the hypoblast is thickened to 

 form the notochord dd' ; and at the sides are seen the two tubes, 

 which, on the folding-in of the fore-gut, give rise to the unpaired heart. 

 Each of these is formed of an outer muscular tube of splanchnic 

 mesoblast (ahh), not quite closed towards the hypoblast, and an inner 

 epithelioid layer (ihli) ; and is placed in a special section of the body- 

 cavity (pli), which afterwards forms the pericardial cavity. 



Before the ninth day is completed great external changes are 

 usually effected. The medullary groove becomes closed for its whole 

 length with the exception of a small posterior portion. The closure 

 commences, as in Birds, in the region of the mid-brain. Anteriorly 

 the folding-off of the embryo proceeds so far that the head becomes 

 quite free, and a considerable portion of the throat, ending blindly 

 in front, becomes established. In the course of this folding the, 

 at first widely separated, halves of the heart are brought together, 

 coalesce on the ventral side of the throat, and so give rise to a 

 median undivided heart. The fold at the tail end of the embryo 

 progresses considerably, and during its advance the allantois is 

 formed in the same way as in Birds. The somites increase in 



