1 66 Vertebrate Embryology 



day, the embryo has been lying " face down- 

 wards " upon the yolk. During the third day, 

 the embryo turns over until it lies on its left 



AO 



Om.A. 



Om. 



Fig. 56.— Diagram of the circulation of a chick f.mbryo 

 at the end of the third day of incubation, as seen from the 

 UNDER SIDE. (After Minot.) The embryo, with the exception of 

 the heart, is dotted ; the veins are black. 



./I £7, aorta. ./4rr, aortic arches, rar^/, cardinal vein. /)C, duct of Cuvier. Hiy 

 heart. 5^w^, jugular vein. (97«.^ , vitelline artery. O^h.F, vitelline vein. ST. 

 sinus terminalis. SV^ sinus venosus. 



side (Figs. 57 and 80). The head, which is 

 the first part to be affected by this change, 

 begins to turn early in this day, and by the 



