128 A. V. Nalbandov 



6 hr after operation. One of these groups received, simultaneously with LH, 

 an intramuscular injection of six Cartland-Nelson units of pregnant mare's 

 serum (PMS). Both groups responded to the LH injection by ovulations 

 (upper part of Table 4). Thirty hours after operation, a dose of LH was 

 injected which in previous experiments had been found maximally effective 

 in inducing double ovulations (Table 3). It will be noted that, while 8 of the 

 10 hens injected with LH alone ovulated (7 of them twice), only one double 

 and no single ovulations were obtained from the 6 hens supported with PMS. 

 If the initial ovulating dose of LH is high, no subsequent ovulations can 

 be obtained when the second dose of LH is given 12, 18 or 30 hr after 

 operation (bottom part of Table 4). While these results should be considered 

 tentative they may mean that enough FSH was contained in the initial dose 

 of LH to provide support and to prevent the ovulation of otherwise ovulable 

 follicles which should have ruptured after the second LH injection. 



DISCUSSION 



The data presented lead to the conclusions that ovulability of follicles is 

 greater in hypophysectomized hens than it is in intact control animals; that 

 ovulability increases progressively with time after hypophysectomy until 

 follicles become physically unable to ovulate; and that less LH is required to 

 ovulate follicles which have not had gonadotropic hormone support for a 

 longer time than those follicles which had been under the influence of 

 gonadotropin more recently. 



In view of these findings the theory is proposed that ovulation is normally 

 a two-stage phenomenon. During the first stage follicles reach ultimate 

 ovulatory size, which is determined by the total available circulating tropic 

 hormones, distributed in the follicular circulatory system in accordance 

 with the vascular capacity of individual follicles. One indication that the 

 amount of circulating hormone limits follicular size is the fact that, in both 

 mammals and birds, follicles can be caused to grow beyond their normal 

 ovulatory size if exogenous gonadotropic hormone is injected. In the chicken 

 the normal follicular size hierarchy can be easily obliterated by the injection 

 of exogenous gonadotropins (Nalbandov, 1959). The individual vascular 

 system of avian follicles is thus seen as playing a vital role in determining 

 the rate of follicular growth. As the largest follicle approaches its ovulatory 

 size, the amount of blood flowing through its vascular system is thought to 

 be proportionally less than the amount available to smaller follicles. If this 

 assumption is correct (preliminary observations support it) then the amount 

 of hormone available per unit of follicular cells is also lower in the largest 

 follicles than in the smaller ones. Because of this reduction in the concentra- 

 tion of gonadotropic hormones, the largest follicle is viewed as having 

 reached a stage of "physiological atresia", during which hormone concentra- 

 tion is inadequate to maintain active proliferation of the cellular components 



