THE EARLY DEVELOPMENT OF THE CHICK 291 



FIG. 118. Diagrams of the circulation in the chick embryo and area vas- 

 culosa. The vascular network of the area vasculosa is omitted for the most part. 



A. Anterior and central parts of the embryo and vascular area at about thirty- 

 eight hours (sixteen pairs of somites). Viewed from beneath. After Popoff. 



B. Median and anterior parts of vascular area and embryo at about seventy-two 

 hours (twenty-seven pairs of somites). Viewed from beneath. After Popoff. 



C. The main vascular trunks of the fourth day. After Lillie (modified), a, 

 Dorsal aorta; aa, aortic arches (first and second in A, second, third and fourth 

 in C) ; ac, anterior cardinal vein; al, allantois; au, auricle; b, bulbus arteriosus; 

 dC, ductus Cuvieri; dv, ductus venosus; ec, external carotid artery; h, heart; 



