FERTILITY 653 



and various diseases of the generative organs, such as endometritis, 

 decidual inflammation, polypoid thickening, etc. It is stated that 

 the excitability of the nerve centres which control the movements 

 of the uterus and the tendency to uterine congestion are greatest 

 at the epochs which would have been menstrual periods if pregnancy 

 had not occurred, and consequently that abortion is especially 

 common at these dates.^ 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 htemorrhage 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 villi, which are only 

 beginning to be numerous. The yolk-sac, which in the marsupial is 

 the organ of fcBtal 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 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 nourishment by means of the yolk-sac is coming to an end, 

 and there is perhaps still 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 allantois, 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 of 

 danger at the end of the third and sixth weeks, but that the most 

 critical period is about the end of the seventh or beginning of the 



> Galabin, Manual of Midwifery, 6th Edition, London, 1904. 



^ Ewart, A Critical Period in the Development of the Horse, London, 1897. 



