Duration of Pregnancy 395 



In either hydronephrosis or ascites, along with general 

 symptoms of those maladies, the trocar may be used as an aid in 

 diagnosis. 



The Duration of Pregnancy. 



The duration of pregnancy varies greatly according to the 

 species of animal, in the individuals of the same species and even 

 in the same individual during different pregnancies. In our 

 domestic animals we find the extremes of variations in normal 

 duration in the rabbit, where the young are carried for 28 to 30 

 days, and in the elephant, where the duration is i^ years, or 

 about 21 months. The duration of pregnancy is not definite in 

 the individuals of any species and the limits in variation increase 

 somewhat according to the increase in the average duration. 

 In those animals having a short period of gestation, like the 

 rabbit, the variation is very slight, scarcely exceeding two days, 

 while in the mare, with an average duration of a trifle more 

 than II months, the variability is increased to two or three 

 months. The variations in the duration of normal pregnancy in 

 the mare are more than twice the normal duration of pregnancy 

 in the rabbit. 



The duration of pregnancy bears a somewhat inconstant ratio 

 to the size of the animal and, as a general rule, the larger the 

 animal, the greater the duration of pregnancy. This law, how- 

 ever, is not wholly applicable and in some cases would seem to be 

 well-nigh overthrown. In a given species where there are great 

 variations in size, as in the dog, there is no variation in the 

 duration of pregnancy because of size, so that the great Mastiff, 

 weighing more than 100 pounds, has no greater gestation period 

 than the Toy Terrier weighing 2 or 3 pounds. In some cases, 

 also, species which are somewhat closely allied have very uniform 

 periods of pregnancy, which do not accord with the differences in 

 size. Thus, the duration of gestation in the dog is only a trifle 

 greater (about 12%) than in the cat. In those species which 

 cross to produce hybrids, such as the horse and ass, the duration 

 of pregnancy is naturally nearly the same. 



The duration of pregnancy is also somewhat dependent upon 

 the state of development in which the young are, born. Thus, 

 in the carnivora we find a duration of 8 or 9 weeks, while in 



