FCETAL NUTRITION: THE PLACENTA 379 



series of lateral vitelline branches. These arteries break up into 

 a deeper arterial network, from which the blood is collected into 

 the sinus terminalis and the superficial venous network, and in 

 this way reaches the vitelline veins and so passes to the heart. 



During the spread of the mesoblast, it splits into an external" 

 layer or somatopleur, and an internal layer or splanchnopleur. 

 The former is non-vascular and adheres to the inner aspect of 

 the trophoblast, forming with it the diplo-trophoblast, and the 

 splanchnopleur is applied externally to the hypoblastic wall 

 of the yolk-sac. By the splitting a space is formed between the 

 two layers. This is the extra-embryonic ccelom, which thus 

 intervenes over a larger or smaller area between the diplo-tropho- 

 blast and the yolk-sac. 



While the above changes are taking place, the allantois 

 grows out (on the tenth day in the rabbit) from the hind-gut 

 as a vesicle lined by hypoblast, and covered externally by a 

 layer of splanchnopleur. In some Mammals the cavity of 

 the allantois is not continued beyond the body-wall of the 

 embryo, the extra-embryonic portion consisting of a solid rod 

 of mesoblast. In all orders below the Primates, however, 

 it projects free for a time into the ccelom, and later fuses, except 

 in the Marsupials, with the whole or part of the outer wall of 

 the blastocyst. In the allantoic mesoblast many vessels are 

 developed, and branches extend into the projections which form 

 the cores of the villi. The blood is brought by two allantoic 

 arteries continued from the terminal bifurcation of the dorsal 

 aorta, and returned by one, or more rarely two, allantoic veins. 

 " While the placenta is being developed, the folding off of the 

 embryo from the yolk-sac becomes more complete, and the 

 yolk-sac remains connected with the ileal region of the in- 

 testine by a narrow stalk, the vitelline duct. While the true 

 splanchnic stalk of the yolk-sac is becoming narrow, a somatic 

 stalk connecting the amnion with the walls of the embryo is 

 also formed, and closely envelops the stalk both of the 

 allantois and yolk-sac. The somatic stalk, together with its 

 contents, is known as the umbilical cord " (Balfour *). The 

 yolk-sac atrophies completely in some, but in others it is only 

 removed at birth. 



1 Balfour, Comparative Embryology, London, 1881. 



