SEXUAL PERIODICITY IX THE FEMALE MAMMAL 25 



given to this period of post-ovulative activity. The two chief 

 tvpes of cvcle may therefore be illustrated as follows: 



i 



Procestrus 

 (Estrus 



Pregnancy 



I 



Pseudo-Pregnancy 



Anoestrus 



Diagram to Illustrate Moncestrous Cycle. 



One oestrous period per breeding season. A complete cycle occu- 

 pies the whole breeding season, which is separated from the next by 

 anoestrus (as found in dog). 



Y 



CEstrus 



Pregnancy 



Metoestrus 

 Dioestrus 



TD 



seudo-Pregnancy 



Prooestrus 



Single Dioistrous Cycle, which is repeated a number 



OF TIMES during THE BREEDING SEASON. 



In the rat and the mouse pseudo-pregnancy is dependent upon 

 sterile copulation, while in the guinea-pig dioestrus is really pseudo- 

 pregnancy (see p. 35). 



The general account given above has assumed ovulation to be 

 spontaneous, i.e. to occur automatically at oestrus irrespective 

 of copulation. In two common mammals, however, the rabbit 

 and ferret, ovulation occurs only after copulation. In the 

 absence of the male, oestrus, including the presence of mature 

 follicles in the ovar}^ persists right through the breeding season, 

 at the end of w^hich follicular atresia takes place. In these 



