THE CLOACA, ASD THE BLOOD-VESSELS. 413 



utmost importance, as it affords the means through which the 

 embryo receives its supply of nutriment from the mother, and 

 is enabled to effect the necessary respiratory and excretory 

 interchanges. 



The vitelline circulation will be dealt with in the present 

 section ; the relations of the allantoic vessels in the placenta 

 will be treated separately, in the concluding section of this 

 chapter. 



1 . The Heart. 



The heart of the rabbit, like that of the chick, is formed by 

 the union of the two vitelline veins, which return to the embryo 

 the blood from the vascular area. 



The vitelline veins are formed in the mesoblast of the 

 splanchnopleure, and appear at an early stage of development, 

 when the folding-off of the embryo from the yolk-sac, by the side 

 folds, has only just commenced. The right and left vitelline 

 veins, and consequently the two halves of the heart as well, are 

 therefore at first a considerable distance apart ; and in rabbit 

 embryos of the ninth day (Fig. 145, R) they appear as a pair of 

 tubes, lying along the sides of the head, opposite the hind-brain. 



As the side-folds deepen, constricting off the embryo from 

 the yolk-sac, the two tubes get carried round to the ventral 

 surface of the embryo, where they lie close together, side by 

 side. About the middle of the tenth day they fuse together to 

 form a single tubular heart, lying in the floor of the pharyngeal 

 region of the mesenteron, and having relations very similar to 

 those of the heart in a chick embryo of about the thirtieth 

 hour. 



In the latter part of the tenth day, the heart, while it remains 

 attached to the floor of the pharynx at both its ends, becomes 

 free in the middle portion of its length ; and, growing rapidly, 

 hangs down into the body cavity as a loop, which soon becomes 

 twisted on itself like a letter S, and partially divided by con- 

 strictions into chambers (Fig. 147, R). The hinder or proximal 

 limb of the heart, which receives the great veins, is the sinus 

 venosus ; the first loop of the S is the auricular portion ; the 

 second loop is the ventricular portion ; and the distal or anterior 

 limb is the truncus arteriosus, from which the aortic arches arise 

 as right and left branches. 



