82 THE SECOND DAY. [CHAP. 



or tucking in of a limited portion of the blastoderm, 

 first at the anterior extremity, and afterwards at the 

 posterior extremity and at the sides. One of the results 

 of this doubling up of the blastoderm to form the head 

 is the appearance, below the anterior extremity of the 

 medullary tube, of a short canal, ending blindly in 

 front, but open widely behind (Fig. 29, D), a cul de 

 sac, in fact, lined with hypoblast and reaching from the 

 extreme front of the embryo to the point where the 

 splanchnopleuric leaf of the head-fold (Fig. 29, F. Sp) 

 turns back on itself This cul de sac, which of course be- 

 comes longer and longer the farther back the head-fold is 

 carried, is the rudiment of the front end of the alimen- 

 tary canal, the fore-gut, as it might be called. In trans- 

 verse section it appears to be flattened horizontally, 

 and also bent, so as to have its convex surface looking 

 downwards (Fig. 30, al). At first the anterior end is 

 quite blind, there being no mouth as yet; the formation 

 of this at a subsequent date will be described later on. 



At the end of the first half of the second day the 

 head-fold has not proceeded very far backwards, and 

 its limits can easily be seen in the fresh embryo both 

 from above and from below (Fig. 28). 



The heart. It is in the head-fold that the forma- 

 tion of the hea7^t takes place, its mode of origin being 

 connected with that cleavage of the mesoblast and con- 

 sequent formation of splanchnopleure and somatopleure 

 of which we have already spoken. 



At the extreme end of the embr}^ (Fig. 29), where 

 the blastoderm begins to be folded back, the mesoblast 

 is never cleft, and here consequently there is neither 

 somatopleure nor splanchnopleure ; but at a point a 



