68 EARLY EMBRYOLOGY OF THE CHICK 



have grown outward. Thus are established the afferent vitel- 

 line channels (Fig. 21). 



The efferent vitelline channels have not yet appeared and 

 there is no circulation of the blood corpuscles which are being 

 formed in the area vasculosa. Thp intra-embryonic blood 

 vessels remain empty until the extra-embryonic circuit is com- 

 pleted. The embryo meanwhile draws its nutrition from the 

 yolk by direct absorption. 



The Formation of the Heart. The structural relations of 

 the heart and the way in which it is derived from the mesoderm 

 can be grasped only by the careful study of sections through 

 the heart region in several stages of development (Fig. 26). 

 The fact that the heart, itself an unpaired structure, arises 

 from paired primordia which at first lie widely separated on 

 either side of the mid-line, is likely to be troublesome unless its 

 significance is understood at the outset. The paired condition 

 of the heart at the time of its origin is due to the fact that the 

 early embryo lies open ventrally, spread out on the yolk sur- 

 face. The rudiments of all ventral structures which appear at 

 an early age are thus at first separated, and lie on either side 

 of the mid-line. 



As the embryo develops, a series of foldings undercut it and 

 separate it from the yolk. This folding off process at the same 

 time establishes the ventral wall of the gut and the ventral body 

 wall of the embryo by bringing together in the mid-line the 

 structures formerly spread out to right and left. The primordia 

 of the heart arise in connection with layers which are destined 

 to form ventral parts of the embryo, but at a time when these 

 layers are still spread out on the yolk. As the embryo is com- 

 pleted ventrally the paired primordia of the heart are brought 

 together in the mid-line and become fused (Fig. 27). 



The first indication of heart formation is to be seen in trans- 

 verse sections passing through a 25-hour chick immediately 

 caudal to the anterior intestinal portal. Where the splanchno- 

 pleure of either side bends toward the mid-line along the lateral 

 margin of the intestinal portal there is a marked regional thick- 

 ening in the splanchnic mesoderm of either side (Figs. 26, A 

 and 27, A). This pair of thickenings indicates where there has 

 been rapid cell proliferation preliminary to the differentiation 

 of the heart. Loosely associated a-lls can already be seen 



