146 MAMMALIAN SEXUAL CHARACTERS 



mucous membrane in the menstrual cycle is of the 

 same nature as that which takes place in pregnancy. 

 When the ovum or ova are not fertilised the develop- 

 ment comes to an end after a certain time, differing 

 in different species of Mammals, and the membrane 

 sloughs, returns to its original state, and then begins 

 the same process of development again. 



Menstruation, then, must be interpreted as an 

 abortive parturition, both in woman and lower 

 Mammals, though in the latter it is not usually 

 accompanied by hemorrhage, and is called pro-oestrus. 

 The question then to be considered is, what deter- 

 mines parturition and menstruation ? The presence 

 of the fertilised ovum must have been the original 

 cause of the hypertrophy of the uterine mucous 

 membrane, and in its congenital or hereditary de- 

 velopment the chemical substances diffusing from 

 the ova in the uterus or even in the Fallopian tube 

 may well be the stimulus starting the hypertrophy. 

 But what determines the end of the pregnancy ? 

 Is it merely the increasing distension of the uterus 

 by the developing foetus ? This could scarcely be 

 the case in the Marsupials in which the foetus when 

 born is quite minute. Nor can we attribute parturi- 

 tion to renewed ovulation, for this occurs in Dasyurus 

 only once a year. All we can suggest at present is 

 that a certain periodic development takes place by 

 heredity in presence of the hormones exuded by the 

 fertilised ovum and the embryo developed from it. 

 When the ovum or ova, not being fertilised, die, the 

 period of development is (usually) shortened and 

 pro-oestrus or menstruation occurs. In the dog, 

 however, the period of the oestrous cycle is about the 

 same as that of gestation — namely, six months. 



