6 WALTER HE APE. 



organs and tlie form which that activity takes is modified by 

 conception, and it is necessary to consider the subject under 

 two heads : (1) when reproduction does not take place, that 

 is in the absence of the male, or when coition does not 

 result in conception ; and (2) when reproduction does take 

 place. 



Under either of these circumstances the changes which 

 take place in the generative system are both complex and 

 variable, and for purposes of comparative study must be 

 identified. 



(1) When Reproduction does not take place. — In 

 the first place we will consider the changes which take place 

 in the simplest form of the female sexual season, and after- 

 wards indicate the nature of the more complicated processes. 



Pro-oestrum, or the Pro-oestrous Period, is the term 

 I have adopted to describe the first phases of generative 

 activity in the female mammal at the beginning of a sexual 

 season ; it is identical with the period spoken of by the moi-e 

 accurate breeders as the time during which an animal is 

 '^ coming on heat," or "coming in season.'' During this 

 period certain changes take place in the generative organs 

 Avhich, while in some animals they are more drastic, in some 

 more complete than in others, are based ou the same plan, 

 have the same object, and the same effect in all. They result 

 in a condition which I have called — 



CPjstrus. — This is the climax of the process; it is the special 

 period of sexual desire of the female; it is during oestrus, 

 and only at that time, the female is willing to receive the 

 male and fruitful coition rendered possible in most, if not in 

 all, mammals. 



CEstrus may be a brief period and exist for only a few 

 hours, or it may extend for days, or apparently even for weeks; 

 it is possible, however, normally, only as a result of the active 

 changes which take place in the generative organs during 

 pro- oestrum. 



The period of oestrus is referred to by various writers 

 as ''Brnnst/' "rut," "heat," "season," "brim," or 



