SEXUAL PERIODICITY IN THE FEMALE MAMMAL 23 



series of cestrous periods, probably separated by pregnancy, 

 occur during the spring and summer, while during the winter 

 months the reproductive organs enter into a prolonged state of 

 rest. In animals with a long period of gestation, what would 

 be the anoestrous period may be occupied by pregnancy. 

 Mares, for instance, having a period of gestation of 11 months, 

 foal about the beginning of the next breeding season. 



The wild prototypes of the cow, pig, sheep and goat all 

 probably exhibit a restricted mating season, which in the case of 

 the last two appears to be in the autumn, but domestication has 

 resulted in the gradual expansion of the season until the 

 domestic strains of these animals will breed at almost any 

 period of the year. The greatest readiness to breed, however, 

 is still found at a time which probably corresponds to the 

 primitive breeding season, especially in the less highly domes- 

 ticated varieties. 



The dog has two breeding seasons a year, in spring and 

 autumn, and only one cestrous period occurs in each season. 

 Whatever the primitive state of affairs may have been, the 

 monoestrous cycle, with its limited opportunities for the animal 

 to become pregnant, is the rarer condition at the present time in 

 such animals as have been studied. The cause of the onset of 

 the breeding season of mammals is obscure, but, in so far as 

 it occurs in spring and summer, it is probably a combined effect 

 of raised temperature and increased food supply (359, 509). 



Civilized man retains practically no vestiges of a breeding 

 season beyond a slight seasonal variation in the birth rate 

 (especially the illegitimate birth rate), and therefore in the 

 conception rate. Many primitive tribes, however, show a 

 marked lumping of the births in one season of the year (444), and 

 it is possible that man originally had a definite, if not entirely 

 restricted, breeding season. 



[c) ESSENTIAL FEATURES OF THE CESTROUS CYCLE 



The essential feature of ovarian activity is the maturation 

 of the Graafian follicle and the discharge of the ovum. This 

 periodic occurrence, together with the intervening growth of 

 the corpora lutea, constitutes the ovarian cycle. While these 



