GENERATION AND DEVELOPMENT 



717 



The cause of mares ' breaking service ' from the sixth to the 

 ninth week is explained by Ewart in the light of his inquiries. 

 At the third, sixth, and ninth week the physiological changes 

 associated with oestrus are likely to supervene and shake the 

 reproductive system. At the third week the risk of casting off 

 the embryo is not so great, as the area by which it is attached 

 to the uterine wall is sufficiently large to render it moderately 

 secure, but at the sixth week a change from yolk sac to placental 

 nourishment is being effected and the yolk-sac area is less than 

 it was at the third week. At such a time a contraction of the 

 uterine horn will be followed by expulsion of the embryo. At 

 the ninth week the question of not ' breaking service ' depends 



Fig. 252. — A Seven Weeks Horse Embryo, Half Natural Size (Ewart). 



all., Allantois ; am., amnion ; c.v., non-vascular villi between the allantois and 

 the yolk sac, not hitherto found in any mammal, and function unknown ; 

 y.s„ yolk sac ; a-c, absorbing area of the yolk placenta ; v 1 , vascular villi of 

 allantois ; t.t., external vascular villi over the surface of the embryonic sac. 



on whether the villi have appeared in time, and obtained a suffi- 

 ciently intimate relation with the uterine vessels to supply 

 the embryo with the additional nourishment its development 

 requires, that through the yolk sac being insufficient. 



Ewart says that the embryo of the mare usually occupies 

 the right horn of the uterus, and in the early days is suspended 

 by the yolk sac from the upper wall of the organ, the head being 

 towards the body of the womb. Later the foetus may lie in the 

 body of the uterus, but the hind-limbs remain to the last in the 

 right horn. 



<" Foetal Circulation. — With the formation of the foetal envelopes 

 and the development of the heart, the circulation takes on a 

 course altogether different from that in the vascular area in 



