420 THE PHYSIOLOGY OF REPRODUCTION 



part in the nutrition of the embryo, as evidenced 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. 106). 

 From it the mesoderm reaches out on all sides. As it spreads between 

 the epiblast and hypoblast, the coelom is developed in it. By the 

 thirteenth day one-third of the circumference is surrounded by 

 coelom, 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^). It continues, however, to grow jparz passu with 

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

 enlargement of the coelom. At the twenty-fifth day it is reduced to 

 a solid rod of cells with a few blood-vessels on its outer surface 

 (Fig. 107), and it disappears before the end of pregnancy (Assheton ^). 

 The allantois grows out into the coelom very early and expands with 



Fig. 106. — Elongated blastocyst of sheep at thirteenth day of pregnancy. (From 

 Hertwig's Entwicklungsgeschichte des Mensehen und der Wirhelthiere, by per- 

 mission of Gustav Fischer.) 



bl, Blastocyst ; e, embryonfc shield. 



■extraordinary rapidity, occupying most of the cavity of the blasto- 

 dermic vesicle. Its further developments are described later (p. 430). 

 Hence in the sheep, and in the pig and cow, in which the conditions 

 ,are similar, the yolk-sac is functional only from the first appearance 

 of the vessels in the area vasculosa till about the twentieth day 

 of pregnancy. 



In the horse there is no indication of an allantoic diverticulum 

 at the middle of the third week, but at the end of the third week 

 there is a considerable mass of vascularised allantoic, mesoderm at 

 the caudal end of the embryo into which a small allantoic diverti- 

 culum extends from the hind gut (Ewart^). 



Caenivoea. — The mesoblast and coelom extend completely round 

 the blastocyst, and the vitelhne circulation is active only in the early 



' 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. 



' Ewart, "Studies on the Development of the Horse : I. Development during 

 -the Third Week," Trans. Roy. Soc. Edin., vol. li., 1915. 



