98 



THE EARLY DEVELOPMENT OF MAMMALS. 



should be remembered, the embryo and the capillary network are drawn many 

 times too large in proportion to the area vasculosa. The area is bounded by a 

 broad circular vessel, the sinus terminalis, 5. T., which constitutes a portion of 

 the venous system in birds, for in front of the head of the embryo the sinus leaves 

 a gap, and is reflected back along the sides of the body of the embryo to make 

 two large veins, which, after uniting with the other venous channels coming 

 from various parts of the area vasculosa on each side, enter the embryo as two 

 large trunks, Om. V., known as the omphalo-mesaraic veins; these two veins 

 unite in a median vessel, the sinus venosus, S. V., which runs straight forward 



AO 



Om.A. 



Fig. 44. — Diagram of the Circulation in a Chick at the End of the Third Day, as seen from 



the Under (Entodermal) Side. 

 The embryo, with the exception of the heart, is dotted; the veins are black. Ao, Aorta. Arc, Aortic arches. 

 1 Cardinal vein. D. C, Duct of Cuvier. Hi, Heart. Jug, Jugular vein. Om.A, Oiuphalo-mesaraic or 

 vitelline artery. Om, V, ( Imphalo mesaraic or vitelline vein. .V. 7\ Sinus terminalis. S. V, Sinus venosus. 



and enters the posterior end of the heart. The sinus venosus also receives the 

 veins from the body of the embryo, namely, the jugulars, /w<7.,and cardinals, 

 card.; the former from in front unite each with the cardinal of the same side, 

 making a short transverse trunk ktiown as the ductus Cuvicri, D. C; the two 

 ducts empty into the sinus venosus. The entire venous current is thus brought 

 to the heart in a united stream; it passes out through the aorta, the greater part 

 ascends the aortic arches and passes back as shown in the figure, Ao., and divides 

 at the posterior fork of the aorta, the bulk of the two currents passing out 



