IV.] THE HEART. 63 



fold has not proceeded very far backwards, and its limits 

 can easily be seen in the fresh embryo both from above and 

 from below. 



5. It is in the head-fold that the formation of the heart 

 takes place, its mode of origin being connected with that 

 cleavage of the mesoblast and consequent formation of splanch- 

 nopleure and somatopleure of which we have already spoken. 



At the extreme end of the embryo (Fig. 16), 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 very little further back, 

 close under the blind end of the foregut, the cleavage (at 

 the stage of which we are speaking) begins, and the somato- 

 pleure, F. So, and splanchnopleure, F. Sp. diverge from 

 each other. They 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 forwards over the 

 yolk-sac. As they thus return (the somatopleure having 

 meanwhile given off the fold of the ammion, Am.), they are 

 united again to form the uncleft blastodermic investment of 

 the yolk-sac. In this way the cavity arising from their sepa- 

 ration is closed below. 



It is in this cavity, which from its mode of formation 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. 



It 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. 16, Ht)\ and by the 

 end of the first half of the second day (Fig. 14, h) has ac- 

 quired somewhat the form of a flask with a slight bend to 

 the right. At its anterior end a slight swelling marks the 

 future bulbus arUriosus ; and a bulging behind indicates the 

 position of the auricles. It is hollow, and its cavity opens 

 below and behind into two vessels called the omphalo-mesaraic 

 veins (Figs. 14, 15, o.f), which pass outwards in the folds of the 

 splanchnopleure at nearly right angles to the axis of the 



