34 THE PHYSIOLOGY OF REPRODUCTION 



occurring in the sexual organs are in a general way similar to those 

 whicli take place during true pregnancy but without being so 

 pronounced (dog). 1 The pseudo-pregnant period is then followed 

 by another anoestrum or period of prolonged rest. 



In some animals, such as the rat or the rabbit, the meto'stnim 

 may be succeeded by only a short interval of quiescence. This short 

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

 Di(es1i-n.in. This in turn is followed by another procestrous period, 

 and so the cycle is repeated until the sexual season is over. Such 

 a cycle (consisting of a succession of the four periods, proo?strmn, 

 ii -strus, metcestrum, and diceetrum) is known as the J)iu>strous 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 cestrous period of 

 the season, there can be no repetition of the cycle, at any rate until 

 after parturition. The cycle may then be repeated. If conception 

 does not occur at any oestrus during the sexual season, the final 

 metopstrons period is succeeded by a prolonged anoestrous or non- 

 breeding period. This is eventually followed by another prou-strum, 

 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 blackfaced sheep in the Highlands, 

 this number is usually limited to two. In many Uodents, un the 

 other hand, there may be six or seven, or even more recurrences of 

 the cycle within the limits of a single sexual season. 



Animals in which the u-strus does not recur during the sexual 

 season, Heape has called Moim-stroiis animals. Those in which 

 there is a recurrence of the dicestrous cycle during a single season, 

 have been designated Polyo-stroiis animals. The polyoestrous condition 

 may be regarded as a device (using ideological language; to increase 

 the reproductive powers by providing more frequent opportunities 

 for successful coition. Hut as to what factors are actually involved 

 in bringing about the rhythmic recurrence of the cycle is a question 

 which must at present be left open. (Cf. p. 387.) 



The differences in sexual periodicity in both mono'strous and 

 polyoestrous .Mammals, the differences in the duration of the sexual 

 season in polyoestrous Mammals, the great variation which occurs 

 even in closely allied forms or even within the limits of a single 



1 Marshall and Hainan, "On the l'<t -< Ksti CMS ( 'li.-m^cs occiii i inic jn tin- 

 ( i.-nt-iativc Organs and Maimuarv (Jlands of tin- Nnn-l'iv^nant I ><>L:,' !',<. /{<,y. 

 Soc., B., vol. Ixxxix., 1917. 



