420 THE EABBIT. 



sides of the heart are perfectly distinct from each other ; and to 

 get from one side to the other the blood must pass through 

 either the pulmonary or the systemic circulation. 



5. The Circulation in the Yolk-sac. 



The circulation in the yolk-sac is definitely established by 

 the tenth day, and presents some points of interest. 



In the rabbit, as in other Vertebrates, the vitelline vessels 

 are developed in the inner, or splanchnopleuric layer of the 

 mesoblast, beyond the embryonal area (Figs. 146 and 147). The 

 mesoblast, as already noticed, only extends over the upper half 

 of the blastodermic vesicle ; the lower half, or hemisphere, having 

 a wall composed of epiblast and hypoblast alone. The boundary 

 between these two halves is a sharp one, and is indicated by an 

 annular vessel, the sinus terminalis (Figs. 146 and 147, si), 

 which runs round the margin of the mesoblast, and marks the 

 outer limit of the vascular area. 



The course of the vitelline vessels in the rabbit differs in 

 some important respects from that of the chick. In the chick 

 the vitelline arteries and veins lie in two layers, the veins being 

 dorsal or superficial to the arteries ; and the sinus terminalis is 

 a vein, which collects the blood from the marginal part of the 

 vascular area and returns it, by branches which form main 

 factors of the vitelline veins, to the heart (c/. Fig. 99). 



In the rabbit, on the other hand, all the vessels of the vascular 

 area lie in one plane. The vitelline arteries run straight back- 

 wards from the embryo, and open at once into the sinus terminalis, 

 which is therefore an artery, and not, as in the chick, a vein. From 

 the vitelline arteries themselves, and from the sinus terminalis, 

 smaller arteries arise, which branch freely and end in capillaries ; 

 the capillaries unite to form veins which open finally into the 

 vitelline veins themselves, a pair of large vessels which run in 

 the vascular area, concentrically with the sinus terminalis, but 

 about midway between this and the embryo. Opposite the 

 anterior or head end of the embryo, the vitelline veins turn 

 sharply backwards, and, entering the embryo along the yolk- 

 stalk, run forwards to the heart. 



There are at first two vitelline arteries, and two vitelline 

 veins. Of the two arteries, the left one soon becomes much the 

 larger, the right one appearing as a mere branch of it. Both 



