386 THE PHYSIOLOGY OF REPRODUCTION 



by the fact that the vitelline vein is thrice as large as the 

 allantoic vein. 



UNGULATA. In the sheep the blastocyst elongates early, and 

 contains at one part the thickened embryonic area or shield 

 (Fig. 81). From it the mesoderm reaches out on all sides. As 

 it spreads between the epiblast and hypoblast, the ccelom is de- 

 veloped in it. By the thirteenth day one-third of the circum- 

 ference is surrounded by ccelom, and in this area the yolk-sac 

 is separated from the outer wall. At the seventeenth day the 

 separation of . the yolk-sac is complete all round (Bonnet 1 ). 

 It continues, however, to grow pari passu with the blastodermic 

 vesicle, and is gradually pushed to one side by the enlargement 



FIG. 81. Elongated blastocyst of sheep at thirteenth day of pregnancy. 

 (From Hertwig's Entwicklungageschichte des Menschen und der 

 Wirbelthiere, by permission of Gustav Fischer.) 



bl, blastocyst ; E, embryonic shield. 



of the ccelom. At the twenty-fifth day it is reduced to a solid 

 rod of cells with a few blood-vessels on its outer surface (Fig. 82), 

 and it disappears before the end of pregnancy (Assheton 2 ). The 

 allantois grows out into the ccelom very early and expands 

 with extraordinary rapidity, occupying most of the cavity of 

 the blastodermic vesicle. Its further developments are de- 

 scribed later (p. 397). Hence in the sheep, and in the pig and 

 cow, in which the conditions are similar, the yolk-sac is func- 

 tional only from the first appearance of the vessels in the area 

 vasculosa till about the twentieth day of pregnancy. 



CARNIVORA. The mesoblast and ccelom extend completely 

 round the blastocyst, and the vitelline circulation is active 



1 Bonnet, " Beitrage zur Embryologie der Wiederkauer," Arch. f. Anat. 

 u. Physiol., 1889. 



2 Assheton, ''The Morphology of the Ungulate Placenta," Phil. Trans. 

 Roy. Soc., London, Ser. B., vol. cxcviii., 1906. 



