so IN TROnUC^TION TO SIOXUAL PHYSIOLOGY 



the opossum lias been shown to be polyoestrous with a short 

 ryi'lo of about a month (Hartman). In this animal the Graafian 

 tollieles inerease to a maximum at oestrus when ovulation takes 

 place and the corpora lutea are formed. The duration of these 

 organs is nearly the same whether pregnancy occurs or not, and 

 the uterine and mammary changes are indistinguishable, facts 

 which were }n'eviously noted for the marsupial cat (Hill and 

 O'Donoghue). Which condition obtains can only be decided by 

 an examination of the contents of the uterus. In the marsupial, 

 therefore, a pronounced pseudo-pregnancy associated with the 

 development of the^corpus luteum is a normal condition in the 

 absence of true gestation. It is noteworthy that the corpora 

 lutea degenerate completely prior to the approach of a new heat 

 period. It has been established that the activity of the uterus 

 and mammary glands begins some days before ovulation in the 

 opossum in regard to both the hypertrophy and the congestion. 

 The earlier changes are clearly comparable to the prooestrum of 

 the higher mammals. 0^-ldation takes place shortly after 

 oestrus is over, having much the same time relation as other 

 polyoestrous animals so widely divergent as the cow and the 

 rat. In regard to the oestrous cycle generally, it is apparent 

 that the marsupial may be brought into line with other animals. 

 It differs mainly in the fact that owing to the brevity of 

 gestation (eight to thirteen days) this period is actually 

 much shorter than — indeed, less than half of — the dioestrous 

 cycle. 



In all the animals above mentioned ovulation takes place 

 spontaneously at or very shortly after oestrus. In the rabbit, 

 however, ovulation only occurs after sexual intercourse (Heape). 

 The maturation processes in the ovum also depend upon coition, 

 and begin shortly afterwards. The actual discharge of the 

 follicle supervenes about ten hours after coition. The matura- 

 tion and ovulation processes })resumably depend upon a nerve 

 reflex set up by the female orgasm in copulation. In the absence 

 of the buck the doe rabbit remains " on heat " for a prolonged 

 period, and there is no " dioestrous cycle " (Hammond). Eventu- 

 ally, however, the enlarged follicles undergo atrophy, their 

 cavities becoming filled or partly filled with blood which is 

 eventually absorbed as the follicle grows smaller. The ferret 

 resembles the rabbit in not ovulating without coition, and in 



