44 BASIS OF THE PKOBLEM 



to receive the male. This is so among mammals whether the 

 male experiences a sexual season known as the ' rut ', as among 

 stags, or whether he does not, as among dogs ; for the period 

 of * rut ' lasts longer than the period during which the female is 

 ready to receive the male. The facts, however, regarding the 

 sexual season in mammals are somewhat comphcated owing to 

 the nature of the sexual season experienced by the female. A 

 number of different types of mammalian sexual season have been 

 distinguished ; it is not necessary, however, to go into these 

 distinctions. It is sufficient to say that each sexual season consists 

 of one or more sexual cycles, known as oestrous cycles. An 

 oestrous cycle may be divided into four periods ; the first, known 

 as the pro-oestrous, is the period of preparation which ends in 

 the rupture of the blood-vessels in the mucous membrane of the 

 uterus. When the blood passes to the exterior, it is known as the 

 menstrual flow. The second period or oestrous is the period of 

 desire. This period is always short ; in the sheep it lasts about 

 twelve hours and often in other species does not last as long. 

 Only during this latter period will the female receive the male ; ^ 

 copulation never takes place at any other period, and yet in spite 

 of the fact that the opportunity for copulation is narrowly 

 restricted, so strong is the sexual instinct in the male, that it is 

 very rare for a female, so far as observation goes, not to engage 

 in copulation at each oestrous. The period of oestrous is followed 

 by the period of metoestrous during which the activity of genera- 

 tion subsides, and the metoestrous is followed by the anoestrous 

 or period of rest, after which another pro-oestrous period begins 

 a new cycle. 



The period of oestrous, during which copulation takes place, 

 is usually marked by the presence of ripe female gametes which 

 are therefore at once fertilized. Eipe ova are, however, not always 

 present at oestrous ; in the rabbit ovulation takes place an hour 

 and a half after copulation. In the bat there is a very marked 

 want of coincidence between oestrous and ovulation ; copulation 

 takes place in the autumn, but ovulation does not take place until 

 the following spring. In such cases the spermatozoa remain alive 

 in the uterus until the female gametes are ripe, when fertilization 

 is achieved ; thus in the bat the spermatozoa remain alive for 

 several months. When such cases occur, therefore, it does not 



' Except in man — the importance of which fact is dealt with in Chapter IV. 



