60 THE BREEDING OF ANIMALS 



pregnancy is common while the mothers are still suckling. 

 This is true of the domestic rabbit, guinea pig and rat. 1 



59. Heat during pregnancy. Animals do not nor- 

 mally come in heat during pregnancy. The fertilization 

 of the egg of the female by the sperm-cell of the male 

 sets in motion a series of physiological phenomena which 

 react upon the ovaries in such a way as to cause a cessa- 

 tion of the heat periods. The ripening of eggs also does 

 not normally occur during pregnancy. There are excep- 

 tions to this rule among certain mammals which seem to 

 be otherwise entirely normal. 



60. Super! oetation. It sometimes happens that a 

 pregnant mammal will not only come in heat and exhibit 

 the various phases of the oestrum, but will ripen an egg 

 and if bred to the male will conceive. Such an occurrence 

 is given the name of superfoetation. This condition is 

 somewhat rare, but has been observed more frequently 

 among mares than other domestic animals. The reason 

 for this is probably due to the fact that the records of 

 breeding are more carefully kept for mares than for cows, 

 sows or ewes. It is difficult even among mares to deter- 

 mine, when twins are born, whether these are the result 

 of one mating or whether they may actually be of different 

 ages. Such authentic cases as are known have been 

 those observed in the mule-breeding districts of the United 

 States. If the mare is first bred to a stallion and three 

 or six weeks later to a jack, and twins are born, one a 

 mule and the other a horse, there can be no doubt that 

 these colts are of different ages. Such a result could 

 only happen in mares which come in heat and ripen an 

 egg during pregnancy. 



1 Heape, Quarterly Journal of Microscopic Science, vol. 44, 

 p. 43. 



