74 EARLY EMBRYOLOGY OF THE CHICK 



the embryonic circulation. Both the ventral aortic roots and 

 the omphalomesenteric veins are direct continuations of the 

 paired endocardial primordia of the heart. The epi-myocardial 

 coat is formed about the original endothelial tubes only where 

 they are fused in the region destined to become the heart. The 

 development of the heart at this stage is an epitome of its 

 phylogenetic origin. The local investment of the endocardial 

 tubes by the epi-myocardium, as seen in the formation of the 

 chick heart, is a recapitulation of the evolutionary origin of 

 the heart by the local addition of a heavy muscular coat about 

 the walls of a blood vessel. 



During early embryonic life the cardinal veins are the main 

 afferent vessels of the intra-embryonic circulation. The main 

 cardinal trunks are paired vessels symmetrically placed on 

 either side of the mid-line. There are two pairs, the anterior 

 cardinals which return the blood to the heart from the cephalic 

 region of the embryo, and the posterior cardinals which return 

 the blood from the caudal region. The anterior and posterior 

 cardinal veins of the same side of the body become confluent 

 dorsal to the level of the heart. The vessels formed by the 

 junction of the anterior and posterior cardinals are the ducts of 

 Cuvier or common cardinal veins. The right and left ducts of 

 Cuvier turn ventrad, one on either side of the fore-gut, and enter 

 the sinus- venosus along with the right and left omphalomesen- 

 teric veins, respectively (Fig. 24). 



In chicks of 33 hours the anterior cardinal veins can usually 

 be made out in sections (Fig. 28, B, C). By 38 hours the an- 

 terior cardinals and the ducts of Cuvier are readily recognized. 

 The posterior cardinals appear somewhat later than the an- 

 terior cardinals but are ordinarily discernible in the region of the 

 duct of Cuvier by 33 to 35 hours and well established by 38 

 hours. For the sake of simplicity and clearness the cardinal 

 veins have been represented in Figure 24 larger and more 

 regularly formed than they are in actual specimens. Like all 

 the other blood vessels of the embryo they arise as irregular 

 anastomosing endothelial tubes, only gradually taking on the 

 regularity of shape characteristic of fully formed vessels. 



