SEXUAL PERIODICITY IN THE FEMALE MAMMAL 27 



The diversity in the details of the cyclic changes in the female 

 genitalia makes any sort of classification of the types of cycle 

 extremely difficult. Scarcely two mammals, except the rat and 

 mouse, are entirely comparable, and species which are closely 

 related zoologically may show entirely different features in the 

 sexual cycle. The rat and the rabbit, for instance, are extra- 

 ordinarily different in their reproductive phases. The division 

 of animals into monoestrous and polyoestrous is unsatisfactory 

 from a descriptive point of view because {a) the condition varies 

 under domestication, and (b) the only common true monoestrous 

 Eutherian is the dog. 



Certain animals, however, have various salient points in 

 common. The rabbit and ferret, for instance, both of which 

 remain on oestrus throughout the breeding season in the absence 

 of copulation, and do not ovulate spontaneously, are comparable 

 to some extent. In the description of the oestrous cycles of the 

 chief mammals, comparable species are as far as possible 

 discussed together. 



