122 THE BREEDING OF ANIMALS 



The exhaustion of the sire by too frequent service, entail- 

 ing debility of the offspring and disease of the fetus or of its 

 envelopes, must be recognized as a further cause." The 

 symptoms of abortion are similar to those of approach- 

 ing parturiti6n (see p. 75), if the threatened abortion 

 occurs during the later stages of pregnancy. Abortion 

 may occur during the first four weeks of pregnancy with- 

 out any very marked symptoms. The fact of abortion 

 is indicated by the animal again coming in heat. But, 

 as already shown in cases of superfcetation, the occurrence 

 of heat is not absolute evidence that abortion has resulted. 

 Ewart 1 has called attention to the fact that the mare 

 is far more apt to abort at certain stages of gestation 

 than at others. He regards the period from the sixth 

 to the ninth week as one during which the mare is pecul- 

 iarly susceptible to changes in her environment which 

 may have a tendency to cause abortion. This is due 

 to a change in the form of attachment of the foetus to the 

 uterus, from the primitive yolk sac to the more permanent 

 villi-. " At the end of the third week of gestation, when 

 the reproductive system passes through one of its periods 

 of general excitement, about one-fourth of the embryonic 

 sac probably adheres to the uterus ; but at the end of the 

 sixth week, when another wave of disturbance arrives, 

 all the grappling structures are at one pole. Hence, 

 there is probably more chance of the embryo ' slipping ' 

 at the end of the sixth than at the end of the third week. 

 About the end of the seventh week the supply of nourish- 

 ment by means of the yolk sac is coming to an end, and 

 there is, perhaps, still about this time an hereditary tend- 

 ency for the embryo to escape. Unless the new and more 

 permanent nutritive apparatus is provided, unless a 



1 Ewart, quoted by Marshall, loc. cit., p. 615. 



