58 EARLY EMBRYOLOGY OF THE CHICK 



tubular structure lying in the mid-line ventral to the fore-gut. 

 Its mid-region has noticeably thickened walls and is somewhat 

 dilated. Anteriorly the heart is continuous with the large 

 median vessel, the ventral aorta, posteriorly it is continuous 

 with the paired omphalomesenteric veins. The fork formed 

 by the union of the omphalomesenteric veins in the posterior 

 part of the heart lies immediately cephalic to the crescentic 

 margin of the anterior intestinal portal, the veins lying within 

 the fold of entoderm which constitutes its margin. 



Organization in the Area Vasculosa. The extra-embryonic 

 vascular area at this stage is undergoing rapid enlargement 

 and presents a netted appearance instead of being mottled as 

 in the earlier embryos. The peripheral boundary of the area 

 vasculosa is definitely marked by a dark band, the precursor 

 of the sinus terminalis (marginal sinus) . Its netted appearance 

 is due to the extension and anastomosing of blood islands. 

 The formation of the network is a step in the organization of a 

 plexus of blood vessels on the yolk surface which will later be 

 the means of absorbing and transferring food material to the 

 embryo. The afferent yolk-sac or vitelline circulation is estab- 

 lished in the next few hours of incubation when this plexus of 

 vessels developing on the yolk surface comes into communica- 

 tion with the omphalomesenteric veins already developing 

 within the embryo and extending laterad. The efferent vitelline 

 circulation is established somewhat later when the omphalo- 

 mesenteric arteries arise from the aorta of the embryo and 

 become connected with the yolk-sac plexus. (Cf. Figs. 15, 18, 



21). 



