84 THE SECOND DAY. [CHAP. 



thus enclose between them a cavity, pp, which rapidly 

 increases behind by reason of the fact that the fold of 

 the splanchnopleure is carried on towards the hinder 

 extremity of the embryo considerably in advance of 

 that of the somatopleure. Both folds, after running a 

 certain distance towards the hind end of the embryo, 

 are turned round again, and then course once more for- 

 wards over the yolk-sac. As they thus return (the 

 somatopleure having meanwhile given off the fold of 

 the amnion, Am), they are united again to form the 

 uncleft blastodermic investment of the yolk-sac. In 

 this way the cavity arising from their separation is 

 closed below. 



It is in this cavity, which from its mode of forma- 

 tion the reader will recognise as a part (and indeed at 

 this epoch it constitutes the greater part) of the general 

 pleuroperitoneal cavity, that the heart is formed. 



This makes its appearance at the under surface and 

 hind end of the foregut, just where the splanchnopleure 

 folds turn round to pursue a forward course (Fig. 29, 

 Ht.) \ and by the end of the first half of the second day 

 (Fig. 28, h) has acquired somewhat the form of a flask 

 with a slight bend to the right. At its anterior end a 

 slight swelling marks the future bulbus arteriosus ; and 

 a bulging behind indicates the position of the auricles. 

 It is hollow, and its cavity opens behind into two 

 vessels called the vitelline veins (Figs. 27, o.f. and 28 sp.), 

 which pass outwards in the folds of the splanchno- 

 pleure at nearly right angles to the axis of the embryo. 

 The anterior extremity of the heart is connected with 

 the two aortae. 



The heart, including both its muscular wall and its 



