402 Veterinary Obstetrics 



then, as a rule, it is not suspected except in those cases where 

 the difficult labor is produced by simultaneous presentation of 

 parts of the two fetuses. Sometimes a twin pregnancy may be 

 suspected on account of the very great size of the abdomen, and 

 it might be diagnosed by an exploration per rectum, though not 

 with great certainty. When two young present simultaneously 

 at the pelvic inlet and cause dystokia, the condition can usually 

 be readily diagnosed by tracing the presenting parts, limbs or 

 head, to the separate bodies of the fetuses. The only difficulty 

 occurring, as a rule, is when a single fetus undergoes that de- 

 formity which we know as campylorrhachis or schistocormus re- 

 flexus. Figs. 71 and 72, in which case the spine of the fetus 

 is abruptly bent upon itself, so that the head and all four feet 

 present simultaneously at the pelvic inlet, thus closely simulat- 

 ing twins. This abnormality is to be differentiated from twins 

 by the fact that, when one portion is repelled or advanced, the 

 other part moves in harmony with it, which is not true of cases 

 of twin pregnancy, where the one fetus can be repelled while the 

 other advances. 



Position of Fetuses in Multiple Pregnancy. In animals 

 which are normally uniparous, as the mare and cow, it is inter- 

 esting to study the position of fetuses in case of twins or triplets. 

 In general it may be stated that, where twins occur as a result of 

 the simultaneous rupture of two ovisacs and the fertilization of 

 the eggs, they develop two complete sets of membranes, each 

 fetus having its own amnion and chorion. In those cases where 

 twin pregnancy is due to the fission of a single ovum, the two 

 embryos are developed in the same chorion, but each has its own 

 amnion. 



When the two fetuses each have separate membranes, it is pos- 

 sible for one of them to be expelled at one time and the other to 

 remain in the uterus for a considerable period afterward and 

 finally be born alive. In this way we find recorded, in our liter- 

 ature, occasional instances where an animal has aborted one fetus 

 at a comparatively early period in gestation and later, after the 

 normal duration of pregnancy, has given birth to a living twin. 

 We find quite commonly, in the mare, that one of a pair of twins 

 perishes and remains in the uterus for a considerable time, while 

 the other continues to live and develop. Finally they are aborted, 



