THE " SEXUAL SEASON OF MAMMALS. 45 



The Pro-oestrum. 



The pro-oestrum, as I have ah^eady stated, is the forerunner 

 of oestrus. Evidence of it is to be seen in each of the larg-e 

 groups of the Vertebrata, fishes, amphibia, reptiles, birds, and 

 mammals (Wiltshire, 1883), and it must be regarded in all of 

 them as a sign of the preparation of the generative system 

 for the sexual act. 



Pro-oestrum is usually associated, in the minds of breeders, 

 with reproduction, to an extent which entails the supposition 

 that the same stimulus which incites the former also causes 

 the latter; but the fact that pro-oestrum may occur normally 

 without the concurrent production of ova shows that the two 

 functions are not always interdependent, and that what 

 serves as sufficient stimulus for sexual desire is not necessarily 

 sufficient for reproduction. 



A consideration of these relationships belongs rather to the 

 study of ovulation than to the subject-matter of the present 

 paper. I would mei'ely remark here that while the ovary 

 probably does participate to some extent in the excitement 

 evidenced by pro-oestrum, this function in mammals must be 

 considered as evidence mainly of sexual rather than of repro- 

 ductive power. 



Pro-oestrum is first evident in the tissue of the external 

 generative organs and the surrounding parts, and while it 

 increases in intensity there, it extends to the uterus; durino- 

 this time certain changes (to be mentioned below) take place 

 in the uterine tissue, and they are followed by the subsidence 

 of the disturbance, first in the uterus and finally in the 

 external generative organs. 



The length of time during which pro-oestrum lasts is ex- 

 tremely difficult to determine ; there seems to be considerable 

 variation in different animals, and in the same animal at 

 different times ; but that may be due to variation iii the 

 intensity of the external evidence rather than to variation in 

 the duration of the pro-oestrum itself. 



In the rabbit I have observed this period lasts, probably, 



