96 



THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION 



Length of Estrus or Period 

 of Heat 



Time of Ovulation 



Rabbit (tame) 



Shrew 

 Ferret 



Estrus prolonged indefinitely 

 during the breeding season 

 from spring to summer; a 

 series of different sets of 

 egg follicles matured; each 

 series lasts about a week, 

 then becomes atretic 



Estrus prolonged 



Estrus prolonged 



Ovulation 10-14 hrs. after mating 



About 55-70 hrs. after mating 

 About 30 hrs. after mating 



If ovulation and subsequent pregnancy are not permitted by mating, ovarian 

 involution occurs, and an anestrous interlude is established. Anestrus in the 

 common rabbit, Oryctolagus cuniculus, occurs from October to March, but 

 is not absolute. 



c. Non-ovulatory (Anovulatory) Sexual Cycles 

 Not all of the cyclic changes referred to above in those species which nor- 

 mally experience spontaneous ovulation are related to definite egg discharge. 

 Some cycles occur, more or less abortively, without ovulation of the egg. This 

 may happen in the human or in other mammals, such as the dog and monkey. 

 Cycles without ovulations are called non-ovulatory cycles. Menstruation may 

 follow non-ovulatory cycles in the human female. 



d. Control of the Estrous Cycle in the Female Mammal 

 In the control of a reproductive cycle in the vertebrate animal, three main 

 categories of factors appear to influence its appearance and course. These are: 



(1) external environmental factors, such as light and temperature, 



(2) external factors governing food supply, and 



(3) internal factors resulting from an interplay of the activities of the pi- 

 tuitary gland, the ovary, general body health, and of the particular 

 hereditary constitution of the animal. 



These factors should be considered not alone in terms of the immediate 

 production of fertile conditions in the parent, but rather, in view of the total 

 end to be achieved, namely, the production of a new individual of the species. 

 For example, the reproductive cycle in the deer reaches its climax or estrus 

 in the autumn after a long period of lush feeding for the mother. The young 

 are born the next spring amid favorable temperatures, followed by another 

 period of bountiful food supply for the mother during lactation and for the 

 fawn as it is weaned. A receding light factor in the late summer and early 

 fall thus may be correlated with the period of heat, which in turn proves to 

 be an optimum time of the year for conception with the resulting birth the 

 following spring. Similarly, light ascendency is a factor in producing fertility 



