22 INTERNAL SECRETIONS OF THE OVARY 



In the mouse the dedine of the sexual function usually 

 proceeds by the following stages; 



(a) Litter size gradually decreases. 



(b) Coitus becomes infertile owing to ovulation having ceased. 



(c) CEstrous symptoms in the accessory organs become 



infrequent and irregular. 

 {d) Complete menopause anoestrus appears. 



By the last stage the ovary has become entirely devoid of 

 follicles as well as of corpora lutea vera and resembles to a great 

 extent the type produced by exposure to X-rays (see p. 138). 



(6) THE BREEDING SEASON 



The term ' breeding season ' was originally proposed by Heape 

 (287) to cover the time when activity occurs in the reproductive 

 organs. This definition, however, includes the period of preg- 

 nancy and lactation, and since some mammals may spend all 

 their reproductive life in one or other of these states, the term 

 loses some of its force. In discussing here the time of the year 

 at which breeding takes place, the term ' breeding season ' will 

 be used to denote the time at which a species comes into 

 oestrus, namely, in the sense that ' sexual season ' was used by 

 Heape. 



In captive and domesticated mammals, as well as in man, 

 living under conditions of fairly constant food supply (and often 

 of temperature), the occurrence of a restricted breeding season 

 has become rare. The lower mammals in a state of nature, 

 however, have a definite season of the year at which mating 

 takes place, and, in general, this season is so placed that the 

 young are produced at an auspicious time. Copulation takes 

 place, even during the breeding season, only at certain definite 

 periods of oestrus (or ' heat '). Qistrus may occur only once in 

 a breeding season (monoestrous animal) or, in the absence of 

 pregnancy, a regular series of periods may occur (polyoestrous 

 animal). During the non-breeding season (except in pregnant 

 animals) the reproductive organs are in a state of quiescence. 

 This period is known as anoestrus in contrast to oestrus. 



The breeding season is well shown in wild rodents, where a 



