396 Veterinary Obstetrics 



sheep and swine, which are but little larger in many instances, 

 the duration is more than double; However, in these latter 

 instances, when the young are born they are far more developed 

 than the former. 



It has been claimed that breed exerts some influence upon the 

 duration of pregnancy and Wilhelms has asserted that the 

 Hungarian cow goes some lo days longer than the Dutch cow. 

 Fleming is of the opinion that thoroughbred mares have a longer 

 duration of pregnancy than those of the common breeds. 



Statistics apparently show, also, that a male fetus is carried a 

 trifle longer than a female in those animals where generally but 

 a single young is born at a time, but if any difference exists it 

 is quite unimportant obstetrically. 



It may be that some of the variation in the duration of preg- 

 nancy can be accounted for by the time during estrum at which 

 copulation takes place or the time elapsing between copulation 

 and fertilization. This can scarcely apply to any great extent 

 in such an animal as the cow, in which the estrual period is 

 ordinarily less than 24 hours, while the variation in the duration 

 of pregnancy is very great, and in which fertilization does not 

 follow forced coition at or subsequent to the close of estrum. It 

 has not been determined how long a time is required for fertiliza- 

 tion to take place after impregnation, but it may generally be 

 assumed that the ovulation occurs soon after copulation and that 

 fertilization follows promptly. 



According to Bonnet, the ova of the ewe, sow and bitch have 

 undergone segmentation and passed through the oviducts to the 

 uterus eight to ten days after coition, which would intimate that 

 the fertilization had occurred within a few hours after ovulation 

 and impregnation, and segmentation had promptly begun. Thus, 

 the time elapsing between coition and fertilization must be in- 

 consequential, as related to the span of gestation. 



The chief cause of variation in the span of gestation does not 

 rest upon these considerations. To some extent it may be ex- 

 plained by the fact that the fetus may be expelled in a state of 

 relative immaturity, almost a premature birth ; it may reach 

 that average stage of development which we would designate 

 as normal, or it may remain more or less quiescent in the uterus 

 for a period after the attainment of this normal degree of intra- 



