98 



THE EARLY DEVELOPMENT OF MAMMALS. 



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

 two veins unite in a median vessel, the sinus venbsus, S.V, which runs straight 

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

 the veins from the body of the embryo, namely, the anterior cardinals, Jug, and 

 posterior cardinals, card. The two cardinals of each side unite, making a short 

 transverse trunk known as the common cardinal, D.C, which in turn empties 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 



P.O. 



Ora./L 



FIG. 53. 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. 



card, Posterior cardinal vein. D.C, Common cardinal vein. Ht, Heart. Jug, Anterior cardinal vein. 



Om.A, Omphalo-mesaraic or vitelline artery. Om.V, Omphalo-mesaraic or vitelline vein. S.T, Sinus 



terminalis. S.V, Sinus venosus. 



arches and passes back through the main aorta, Ao, and divides at the posterior 

 fork of the aorta, the bulk of the two currents passing out through omphalic" ar- 

 teries, Om.A, and thence to the capillaries of the area vasculosa and so on to the 

 venous trunks again. As shown in the figure, which presents the under side of the 

 area, the left omphalo-mesaraic vein preponderates, and in the latter stages this dif- 

 ference becomes more marked, until finally the right stem is very inconsiderable in 

 comparison with the great left vein. The time at which the disparity commences 

 is extremely variable, as is also the degree of inequality between the two veins. 



The following description probably represents what was the primitive condition 

 of vessels in the mammalian area vasculosa. It applies to an early stage in the 

 rabbit. An essentially similar arrangement of the vessels exists also at a correspond- 



