STRUCTURE OF FOUR-DAY CHICKS I39 



the superior mesenteric of the adult. The coeliac and the 

 inferior mesenteric arteries arise from the aorta independently 

 at a later stage. 



The cardinal veins are the principal afferent systemic vessels 

 of the early embryo. They appear toward the end of the second 

 day as paired vessels extending anteriorly and posteriorly on 

 either side of the mid-line. At the level of the heart the anterior 

 and posterior cardinal veins of the same side of the body become 

 confluent in the ducts of Cuvier and turn ventrad to enter the 

 sinus venosus (Figs. 24 and 35) . Chicks of four days show little 

 change in the relationships of the cardinal veins (Fig. 47). 

 Later in development the proximal ends of the anterior cardinals 

 become connected by the formation of a new transverse vessel 

 and empty together into the venous atrium of the heart. Their 

 distal portions remain in the adult as the principal afferent 

 vessels (jugular veins) of the cephalic region. 



The posterior cardinals lie in the angle between the somites 

 and the lateral mesoderm (Fig. 36, D, E). When the mesone- 

 phroi develop from the intermediate mesoderm, the cardinal 

 veins lie just dorsal to them throughout their length (Figs. 52, C 

 and 46, E to H). In young embryos the posterior cardinals 

 are the main afferent vessels of the posterior part of the body. 

 Later in development they are replaced by a new vesssel, the 

 inferior vena cava. The changes by which posterior cardinals 

 become reduced and broken up to form small vessels with new 

 associations, belong to stages of development beyond the scope 

 of this book. 



The Heart. — The heart in adult vertebrates is a ventral 

 unpaired structure. Its origin in the chick from paired primor- 

 dia is correlated with the way the young embryo lies spread out 

 on the yolk surface. When the ventral body wall is completed 

 by the folding together of layers which formerly extended to 

 right and left over the yolk, the paired primordia of the heart 

 are brought together in the mid-line. Their fusion establishes 

 the heart as an unpaired structure lying in the characteristic 

 ventral position (see Chap. IX and Figs. 26 and 27). 



After the fusion of its paired primordia the heart is a nearly 

 straight, double-walled tube (Figs. 49, A and 19). The primor- 

 dial endocardium of the heart has the same structure and arises 

 in the same manner as the endothehal walls of the primitive 



