Twi7i Pregnancy 399 



animals. The ovum of the deer undergoes segmentation and 

 then lies in the uterus for four months in an essentially dormant 

 state. About December the distinctive embryonic development 

 begins and birth follows in May or June, or about five months 

 later. If this time of five months, during which the active de- 

 velopment of the fetus has taken place, is compared with the 

 duration of gestation in the sheep, the two periods of gestation 

 become virtually identical. It is suggested that this delay in 

 the development of the young is a provision by which it 

 may be born at a favorable season of the year. 



The Number of Fetuses. 

 In the domestic animals we find great variation in the number 

 of fetuses ordinarily brought forth at a given birth, and we 

 recognize, as a rule, two or three classes of animals in reference 

 to these numbers : the uniparous animals, chiefly the mare and 

 cow, which ordinarily give birth to but a single j^oung at a time ; 

 the biparous, or twin-bearing, animals, among which we find 

 chiefly the goat and the larger breeds of sheep ; and the multi- 

 parous animals, among which we include the carnivora and 

 the sow. 



Twin Pregnancy. 



In all animals, however, there are occasional departures from 

 the rule. Thus, in unipara, we occasionally find twins and 

 sometimes, in our multipara, only one or two young are born at a 

 time. 



Among our uniparous domestic animals, twins are most rare 

 in the mare, although every veterinarian of experience in a 

 horse-breeding district has observed such cases. Among cows, 

 twins are not at all rare. 



The causes of twin pregnancy may vary. Fundamentally, the 

 number of fetuses must depend upon the number of ova which 

 have been discharged into the uterus during a given estrual 

 period. In some cases, two or more eggs are formed in one 

 Graafian follicle and, when discharged and fertilized, may each 

 ■develop into a fetus, but one yellow body remaining in the ovary 

 to indicate their origin. 



Usually twins proceed from the simultaneous rupture of two 

 ripe ovisacs, with the discharge and subsequent fertilization of 

 one ovum from each. 



