X.] GENERAL DEVELOPMENT. 331 



amnion (of) 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. 107. 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 1 . 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 (ihh), and is placed in a special section 

 of the body cavity (ph), 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 sepa- 

 rated, 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 ad- 

 vance the allantois is formed in the same way as in 

 Birds. The somites increase in number to about twelve. 

 The amniotic folds nearly meet above the embryo. 



1 The details of the development of the heart are described below 

 (ch. xii.). 



