OESTRUS CYCLE. 517 



the sexual season.* In males this season is characterized by 

 testicular activity and sexual excitement, and is generally called 

 rut. In females the matter is more complicated. In them the 

 sexual season comprises a series of phenomena which constitute 

 the " oestrus cycle.'" The typical course of the oestrus cycle is 

 as follows. It begins (1) with a congestion cf the external 

 generative organs which spreads to the uterus and leads to (2) a 

 growth which causes a thickening of the mucosa. This is fol- 

 lowed by (3) a rupture of the capillaries and extravasation of 

 blood into the uterine mucous membrane, which extravasated 

 blood in some cases (Primates, some Rodentia, Ungulata and 

 Garni vora, etc.), in consequence of tissue degeneration, finds its 

 way into the uterus and thence to the exterior. This pheno- 

 menon is in the Primates known as the menstrual flow.f The 

 uterus then rapidly heals and the last stage (4) of the cycle is 

 reached. This is known as the oestrus or period of desire. 

 During it copulation takes place and the cycle comes to an end. 



The different stages of this cycle, the first three of which con- 

 stitute the prooestrum, vary in their severity in different animals, 

 and in some of them,. as stated above, the extravasated blood 

 does not break through the mucosa and the menstrual flow of 

 blood does not occur, but the homology of the phenomena 

 throughout the series is, undoubted. 



Ovulation is supposed to take place at some period during the 

 oestrus cycle, but the course of this phenomenon is not fully 

 understood and there is probably considerable variation in it, 

 in different animals and even in the same animal, at different 

 periods of life. In some cases (e.g. ferrets, domestic rabbits) 

 the stimulus of copulation seems necessary to bring about ovu- 

 Jation ; e.g. in the domestic rabbit ovulation occurs 9J hours 

 after copulation. In other cases, probably the majority, ovu- 

 lation is independent of copulation and takes place in its absence 

 during some period of the oestrus cycle (in bitches it takes place 

 during the external bleeding or later). The case of the bat 

 is peculiar. In this animal the sexual season occurs in the 



* W. Heape, " The sexual season of Mammals and the relation of the 

 prooestrum to menstruation, Q.J.M.S., vol. 44, 1901, p. 1. F. H. A. 

 Marshall, The oestrus cycle and the formation of the corpus luteum in, 

 the sheep, Phil. Trans., 1903, p. 47. 



f W. Heape, The menstruation of Semnopithecus entellus, Phil. Trans., 

 1894. Id., " The menstruation and ovulation of Macacus rhesus," Phil. 

 Trans., 1897. 



