SECOND HALF OF SECOND DAY 



481 



intermediate portion between. The venous portion forms a swelling 

 which later becomes the auricles, while the arterial end also enlarges to 

 form the bulbus arteriosus. The point of the loop becomes the ventri- 

 cles (Fig. 283). 



It is toward the end of the second day that the pair of aortic arches 



Fig. 284. 



Diagrams of the circulation in the chick embryo and area 

 vasculosa. The vascular network of the area vasculosa is omitted 

 for the most part. A. Anterior and central parts of the em- 

 bryo and vascular area at about thirty-eight hours (sixteen pairs 

 of somites). Viewed from beneath. B. Median and anterior parts 

 of vascular area and embryo at about seventy-two hours (twenty- 

 seven pairs of somites). Viewed from beneath. C. The main 

 vascular trunks of the fourth day. 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 ; 6, bulbus 

 arteriosus ; dC, ductus Cuvieri ; dv, ductus venosus ; ec, external 

 cartoid artery ; h, heart ; ic, internal cartoid artery ; la, lateral 

 dorsal aorta ; Iv, left anterior vitelline vein ; p, anterior intestinal 

 portal;, pc, posterior cardinal vein; pv, posterior vitelline vein; 

 TV, right anterior vitelline vein ; s, sinus venosus ; t, sinus ter- 

 minalis ; tr, venous trunks of the area vasculosa ; v, ventricle ; o 

 vitelline artery; vv, vitelline or omphalomesenteric vein. (From 

 Kellicott after Popoff and Lillie.) 



which have bent dorsad (and continue separately as the paired dorsal 

 aortae) unite behind the head to form a single vessel which comes to 

 lie directly beneath the notochord. However, after running but a short 



