252 



PRINCIPLES OF EMBRYOLOGY 



Bellairs (1953, 1954) has recently studied its development in detail, with 

 the aid of vital stained marks. The foregut opens posteriorly on to the 

 space betw^een the blastoderm and the yolk, and its posterior edge is a 

 clearly defmed landmark in early chick embryos. As the head fold is 

 pushed further and further backwards under the embryo, so the opening 

 of the foregut moves posteriorly; in fact it does so even faster than the 

 head fold progresses, and it is probable that its movement is not as simply 

 dependent on that of the head fold as appearances might at first suggest 

 (Waddington 1952, p. 149). After the opening to the foregut has pro- 

 gressed some distance towards the posterior, the mesoderm in the edge 

 of the fold develops into a hollow tube, which at first extends some dis- 

 tance along each side of the crescent-shaped opening. This tube is the 



Foregut 



Figure 12.1 



Hind^ut 



Diagrammatic longitudinal section through a chick embryo, to show the 

 relation of the embryo to the yolk. 



rudiment of the heart. It is really a double structure, since the mesoderm 

 which forms the tube on the left side originally lay well out at the left of 

 the blastoderm, and a considerable distance away from that which forms 

 the tube on the right side; it is only the backward progress of the edge of 

 the foregut, which takes place by a sort of zip-fastener process along the 

 midline, which has brought them together. 



This obvious doubleness in the early rudiment of the heart is one of the 

 points in which the cliick embryo seems to differ most radically from the 

 amphibian type. Even so, the two systems are not completely different. 

 We can relate them by the imagining what would happen if the central 

 part of the chick blastoderm were wrapped round a spherical core of 

 yolky endoderm; if the two presumptive heart regions were at the edges 

 of this part of the blastoderm, they would come into contact on the ventral 



