THE CESTROUS CYCLE IN THE MAMMALIA 33 



part of the year. Its close marks the beginning of the sexual 

 season. 



The first part of the sexual season is occupied by the Procexlriim. 

 This period is characterised by marked changes in the generative 

 organs, the uterus becoming congested, while in the later stages 

 there is often a How of blood from the external opening of the 

 vagina. The prooestrum is the period often referred to by breeders 

 as the time when an animal is "coming on heat," or "coining in 

 season." 



The next period, (Exti-nx, or (Entrant (as it is sometimes called), 

 " marks the climax of the process ; it is the special period of desire 

 in the female ; it is during oestrus, and only at that time, that the 

 female is willing to receive the male, and fruitful coition rendered 

 possible in most, if not in all, mammals." 1 



The periods of prooestnun and oestrus are commonly referred to 

 together as the " heat " or " brunst " period, and sometimes as the 

 period of "rut," 2 and no attempt is then made to distinguish 

 the time occupied by " coming in season," and the time at which the 

 female is ready to receive the male. This failure to distinguish 

 the two periods (prooestrum and oestrus) has led to much confusion, 

 especially in regard to the nature of the relation between "heat" 

 in the lower Mammals and menstruation in the human female. 

 As was first pointed out by Heape, it is the prooestrum and not 

 the entire "heat" period which is the physiological homologue of 

 menstruation. This is a point which will be dealt with more fully 

 in the next chapter of this book. 



If conception takes place as a result of coition during oestrus, 

 this period is followed by gestation ; 3 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 anipstrum at the 

 close of the breeding season. 4 



If, on the other hand, conception does not occur during u>strus, 

 the latter is succeeded, either by a short Mrtu-xirinn, during which 

 the activity of the generative system subsides and the organs 

 gradually resume the normal condition (cat, rabbit), or by a 

 period which may be called pseudo-pregnancy, in which the changes 



1 Heape, lo<: <-it. 



2 The term "rut" is used l>v lleape in the case of the male only, the 

 "rutting season," as stated above, being the malt- sexual .season. 



3 There is evidence that "heat" may occur abnormally dining gestation. 

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

 (see p. 46). Coition during pregnancy may result in sujK-rfu-tation (see p. 1.~>1), 

 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. 



4 In some animals parturition is followed almost immediately ly another 

 prou-strum and <rstrus. in spite of lactation. 



