FERTILITY 615 



dates.i The membranes are usually cast off with the foetus, 

 but the decidua is said in some cases to remain, and to regenerate 

 a normal uterine mucosa. The expulsion of the foetus and 

 membranes is accompanied by " pains " comparable to those 

 occurring in normal parturition, the two processes having a 

 general similarity, which is closer if abortion takes place in the 

 later part of pregnancy. There is generally also a considerable 

 loss of blood. After the expulsion the haemorrhage and pains 

 cease, and a process of puerperal involution sets in. 



In horses abortion is probably most frequent during the 

 period from the sixth to the ninth week of pregnancy. This is 

 explained by Ewart ^ as being due to the fact that about this 

 time the embryo loses its primitive attachment to the uterus 

 before acquiring its more permanent connection by means of 

 the allantoic viUi, which are only beginning to be numerous. 

 The yolk sac, which in the marsupial is the organ of foetal 

 nourishment throughout the whole of pregnancy, in the case of 

 the horse ceases to provide a sufficient supply at about the end 

 of the seventh week ; but the horse embryo, instead of being 

 born at this period, like the marsupial, acquires new and more 

 efficient structures in the allantoic vilh. " 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 dis- 

 turbance arrives, all the grappling structures are at one pole. 

 Hence there is probably more chance of the embryo ' shpping ' 

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

 the end of the seventh week the supply of nourishment by means 

 of the yolk sac is coming to an end, and there is perhaps stiU 

 about this time an hereditary tendency for the embryo to escape. 

 Unless the new and more permanent nutritive apparatus is 

 provided, unless a countless number of villi rapidly sprout out 

 from the aUantois, the embryo, will die from starvation during 

 the eighth week, and in a few days be discharged. It may 

 therefore be taken for granted that there is a certain amount 



' Galabin, Maniuil of Midimfery, 6th Edition, Loudon, 1904. 

 ' Ewart, A Critical Period in the Development of the Horse, London, 

 1897. 



