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 



B.C, 



Ht 



AO 



9.T 



0m. A. 



Om 



FIG. 56. DIAGRAM OF THE CIRCULATION OF A CHICK EMBRYO 



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. 



A 0, aorta. A re, aortic arches, card, cardinal vein. DC, duct of Cuvier. 7/2, 

 heart. Jug, jugular vein. Om.A, vitelline artery. Om.V^ vitelline vein. ST. 

 sinus terminalis. S^ 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 



