400 Veterinary Obstetrics 



In other cases, twin pregnancy may be caused by complete 

 division of the ovum or blastoderm to form two separate embryos, 

 in which instance they become identical twins, being of like sex 

 and enclosed in a common chorion each having a separate 

 amnion. 



Twin pregnancy in the mare nearly always results in abortion 

 at about the eighth to tenth month and in a large proportion of 

 cases it is found, when the two fetuses are expelled, that one is 

 much more developed than the other, indicating that the least 

 developed one has been dead in the uterus for a considerable 

 period of time, without having undergone decomposition. In 

 the mare, this twin abortion occurs in probably 90% of the cases 

 and, among those which are born alive, it not infrequently 

 occurs that one of the pair is liliputian in size. 



Rueff records one case of twin gestation in the mare in each 

 250 cases and this seems to be somewhat near to the average. 

 While we have personally observed several instances where both 

 twins have been born alive, they are Usually more or less im- 

 perfect and tend to perish soon after birth. In other cases, 

 however, we have seen them quite normal and vigorous, very 

 much alike in size and form and partaking of that striking 

 likeness common in the twins of other animals and of man. 

 Triplets and quadruplets have also been recorded very rarely 

 in the mare. As in twins, abortion usually brings the preg- 

 nancy to a premature close or, if born alive, the foals are 

 generally weak and tend to perish. Saint-Cyr cites one case in 

 which a mare, from a single stallion service, aborted two fetuses 

 in one chorion and, some months later, gave birth to a living, 

 and vigorous foal. 



The most interesting cases of twin pregnancy recorded are 

 those of a series quoted by Saint-Cyr, in which the twins were 

 due to successive copulations. The mare, having been put 

 first to a stallion and then to a male ass, brought forth a 

 foal and a mule at the same time. In these cases the two copula- 

 tions were not very widely separated, usually during the same 

 day or, in one case, after an interval of 15 days, probably one 

 prolonged estrum. 



In the cow, twin and triplet pregnancies are not so uncommon 

 and the tendency to twin abortion, which is seen in the mare. 



