ACTIVITIES OF THE OVARY 81 



7. Internal Conditions of the Ovary as an Ovulatory Factor 



Internal conditions of the ovary undoubtedly are important in controlling 

 follicular growth and ovulation. For example, in the Northern fur seal, 

 Callorhinus ur sinus, the female begins to breed at the age of two years. These 

 seals travel north once a year to the Pribilof Islands in the Bering Sea where 

 they go on land to give birth to the single young and also to breed. Most of 

 the cows arrive between the middle of June and the middle of July. Heavy 

 with young, the females give birth to their offspring within a few hours or 

 days after their arrival. Breeding again takes place about six days after par- 

 turition. However, lactation continues, and the young are taken care of during 

 the summer months. 



Accordingly, these seals mate each year and it appears that for any par- 

 ticular year the mating behavior and ovulation of the egg are controlled by 

 the ovary, which does not have a corpus luteum. As the corpus luteum, which 

 forms after ovulation in the site of the Graafian follicle, from which the egg 

 is discharged, remains intact for a considerable portion of the year, the ovary 

 which does not have the corpus luteum develops the Graafian follicle for the 

 next summer period. The following year the other ovary will function, and 

 so on, alternating each year (Enders, et al., '46). Thus, the corpus luteum 

 appears to function as a suppressor of follicular growth within the ovary in 

 which it lies. In the human female, one ovary functions to produce an egg 

 one month, while the following month the other ovary ovulates its single egg. 

 It is possible that here also the large corpus luteum suppresses follicular growth 

 within the particular ovary concerned. 



During gestation, the presence of the corpus luteum and its hormone, 

 progesterone, suppresses follicle growth and ovulation in most of the mam- 

 malian group. (The placenta may be the source of progesterone during the 

 later phases of pregnancy in forms such as the human.) On the other hand, 

 in the mare, according to Cole, Howell, and Hart (31), ovulation may occur 

 during pregnancy. Species differences, therefore, exist relative to the control 

 of ovulation by the corpus luteum and its hormone, progesterone. 



8. Number of Eggs Produced by Different Vertebrate Ovaries 



The number of eggs produced during the lifetime of the female varies with 

 the species and is correlated generally with the amount of care given to the 

 young. In many fishes which experience little or no parental care, enormous 

 numbers of eggs may be produced, as for example, in the cod where several 

 millions of eggs are spawned in one season. However, in many of the elas- 

 mobranch fishes (i.e., the shark group) the eggs develop within the oviduct, 

 and the young are born alive. Therefore, only six to a dozen eggs produced 

 each reproductive period is sufficient to keep the shark species plentiful. In 

 the hen, where careful breeding and selection have been carried out with a view 

 to egg production, a good layer will lay from 250 to 300 eggs a year. The 



