THE " SEXUAL SEASON " OF MAMMALS. 59 



disorganise the sexual season^ and this depends on whether 

 or not they interfere with its recurrence or with that of 

 oestrus. 



The above is true for both moncestrous and polyoestrous 

 mammals^ for both gestation and nursing ; but whereas 

 gestation interferes with the recurrence of the oestrus, only 

 if it extends over the time which would otherwise be a sexual 

 season, the interference of nursing depends upon the vigour 

 of the mother and her powers of recuperation. 



The Pro -oestrum, — Evidence of pro-oestrum is to be seen 

 in all Vertebrata, and is the forerunner of oestrus. It is 

 first noticeable in mammals in the external generative 

 organs, and extends thence to the uterus. 



The essential manifestations thereof are first hypertrophy, 

 and secondly congestion of the tissues affected, and this is 

 very usually, indeed probably always, followed by a 

 discharge. 



The discharge always consists partly of mucus from the 

 uterus, and partly of desquamated vaginal epithelium and 

 the products of broken-down epithelial tissue. 



In some animals always, and in others sometimes, blood is 

 also evacuated, which has its origin from the uterine mucosa, 

 in which case there is always more or less of uterine tissue 

 also contained in the discharge. 



There is very considerable variation in the extent of both 

 hypertrophy and congestion of the tissue in various mammals, 

 but it is essential to note that these phenomena are to some 

 extent always present, and are frequently combined with 

 the rupture of the congested vessels in the mucosa, and also, 

 more rarely, with a discharge of blood from, and still more 

 rai'ely a denudation of, the superficial uterine mucosa. 



The evolution of the pro-oestrum in its most advanced form, 

 that is to say the menstruation of the Primates, from the 

 simplest form, as it appears in such animals as the mole, is 

 traced, and menstruation is shown to be identical with 

 "heat." 



The Period of CEstrus. — This is possible only after 



