THE CESTROUS CYCLE IN THE MAMMALIA 37 



this period is followed by gestation ; ' gestation in its turn, after 

 a short puerperium or period of recovery, is followed by nursing 

 or lactation, and the latter is succeeded by another ancestrum 

 at the close of the breeding season.^ 



If, on the other hand, conception does not occur during 

 oestrus, the latter is succeeded by a short Metosstrum, during 

 which the activity of the generative system subsides and the 

 organs gradually resume the normal condition. 



In some animals, such as the dog, the metoestrous period is 

 followed by a prolonged period of rest or anoestrum. In others, 

 such as the rat or the rabbit, the metoestrum may be succeeded 

 by only a short interval of quiescence. This short interval, 

 which sometimes lasts for only a few days, is called the 

 Dicestrum. This in turn is followed by another prooestrous 

 period, and so the cycle is repeated imtil the sexual season is 

 over. Such a cycle (consisting of a succession of the four periods, 

 prooestrum, oestrus, metoestrum, and dioestrum) is known as 

 the Dioestrous cycle. The number of dioestrous cycles in one 

 sexual season depends upon the occurrence or non-occurrence of 

 successful coition during oestrus. Thus, if conception takes 

 place during the first oestrous period of the season, there can be 

 no repetition of the cycle, at any rate imtil after parturition. 

 The cycle may then be repeated. If conception does not occur 

 at any oestrus during the sexual season, the final metoestrous 

 period is succeeded by a prolonged anoestrous or non-breeding 

 period. This is eventually followed by another prooestrum, 

 marking the commencement of a new sexual season. The 

 complete cycle of events is called the (Estrous cycle. 



The number of dioestrous cycles which can occur in a female 

 Mammal in the absence of the male, or in the absence of successful 

 coition, depends upon specific and individual differences. Thus 

 in some animals, such as the Scotch black-faced sheep in the 



' There is evidence that "heat" may occur abnormally during gestation. 

 This phenomenon has been observed in dogs, cows, horses, and other 

 animals (see p. 51). Coition during pregnancy may result in superfcetation 

 (see p. 159), and may tend to occur periodically at times corresponding to 

 what would have been the regular heat periods if the animal had remained 

 non-pregnant. 



2 In some animals parturition is followed almost immediately by another 

 prooestrum and oestrus, in spite of lactation, 



