Excess of Volume of the Fetus 729 



that the variation in the size of animals because of cross-breed- 

 ing occurs during extra-uterine life, and not during gestation. 



On the other hand, our observations indicate that the male in- 

 fluences the form of the fetus and the comparative volume of 

 certain portions of its body. This variation is noticed chiefly in 

 the size of the head. It has been observed that the crossing of 

 ewes with rams of certain breeds having larger heads may lead 

 to dystokia because of the comparatively large size of the lambs' 

 heads. In one instance we observed that many of the cows bred 

 to a certain bull, which had a very heavy head and neck, re- 

 quired assistance because of the voluminous heads and necks of 

 the calves. 



When the domestic cow is crossed with the buffalo or American 

 bison, with his very heavy head and neck, there is very liable to 

 be difficulty in giving birth to the young, because of the large head 

 and chest of the hybrid fetus. 



The breeding of immature females constantly tends to produce 

 dystokia because of the comparatively large size of the fetus. The 

 fetus itself is actually smaller as a rule than the same female would 

 produce later in life, but the birth canal, and especially the pelvis 

 of the very young female, is not yet developed to a degree which 

 will render birth easy or practicable. It is a common experience, 

 therefore, that young heifers, which give birth to calves when 

 only I y^ years old or even younger, may require some degree of 

 assistance in order to expel the fetus. The same is noted in 

 immature sows, and to some extent in other animals. 



In one instance which we noted, the owner of a number of 

 fillies allowed a stallion colt to run with them at pasture, and 

 several yearlings became pregnant. Before the end of gestation 

 contagious abortion broke out, so that they all aborted, and sev- 

 eral of them required assistance in order to get rid of the very im- 

 mature fetuses. Judging from these observations, it would seem 

 probable that, had they carried their fetuses to the normal close 

 of gestation, there might have been difficulty in expelling them. 



Clinical observations apparently show that the fetuses are larger 

 if the mother has been well fed. But this does not seem to in- 

 terfere greatly with birth, because, while the fetus is compara- 

 tively larger, the expulsive powers of the mother are also greater 

 and the birth as a rule is easier. If, however, an animal is very 



