1130 



SUBMAMMALIAX VERTEBRATES 



trophins are not as great as the ratio be- 

 tween gonadotrophic potency of pullets of 

 about 126 days and laying hens, but, on the 

 other hand, the extents of stimulation ob- 

 tained with the exogenous gonadotrophins 

 in the chickens of the two ages were not 

 directly comparable (the laying hens were 

 overstimulated, the immature pullets not 

 completely stimulated I . 



3. On exogenous gonadotrophin adminis- 

 tration the ovary is more sensitive when 

 estrogen is administered before gonado- 

 trophin injection or when administered si- 

 multaneously with gonadotrophins (Phil- 

 lips, 1959) . A more complete understanding 

 of the regulation of follicular growth during 

 the reproductive cycle will require more 

 quantitative data on the gonadotrophic po- 

 tency of pituitaries, especially during the 

 period between the end of laying and the 

 emergence of the new crop of follicles, more 

 data on the sensitivity of the follicles to ex- 

 ogenous gonadotrophins, and pure gonado- 

 trophic hormones (see chapters by Greep 

 and by Young on the ovary). 



2. Estrogen 



Breneman (1955, 1956) investigated the 

 effects of injection of 0.5, 1.0, 5.0 and 25.0 

 /Ag. estradiol per day for 10 days on the 

 ovary of 30-day-old pullets and found no 

 significant difference from control ovarian 

 weight. Histochemically, there was evidence 

 that 1 /Ag. estradiol caused increased choles- 

 terol deposition in the follicle. Phillips 

 (1959) injected 12.5 mg. DES per week into 

 6-week-old pullets and obtained a 32 per 

 cent increase in ovarian weight (p < 0.01). 

 Similarly, 10 mg. DES per day, given to 

 adult, nonbreeding black ducks {Anas pla- 

 tyrkynchos) , resulted in a 95 per cent in- 

 crease in ovarian weight (p < 0.01 ) . In 

 neither of the experiments was there any 

 yellow yolk deposition. Chu and You (1946) 

 also failed to induce maturation of follicles 

 by estrogen injections in hypophysectomized 

 pigeons. It is not clear from their paper 

 whether any stimulation of ovarian weight 

 occurred. Schonberg and Ghoneim (1946) 

 reported that feeding of stilbene to pullets 

 100 days of age resulted in egg production 

 at 114 days of age, whereas egg production 

 in DES-fed pullets did not start until 162 



days and egg production in control pullets 

 did not start until 146 days of age. These 

 results suggest that there may be differences 

 in effect between estrogens and it also sug- 

 gests that stilbene feeding just before pul- 

 lets reach sexual maturity may cause earlier 

 egg production. The difference between the 

 two estrogens may be caused, for instance, 

 by differences in inhibition of the pituitary 

 gonadotrophin secretion. Experimental evi- 

 dence that estrogens may synergize with 

 exogenous gonadotrophins suggests that an 

 estrogen which does not inhibit gonado- 

 trophin secretion but mobilizes yolk pre- 

 cursors could cause a somewhat earlier egg 

 production. Clavert (1958) has, largely on 

 theoretical grounds, defended the proposi- 

 tion that estrogens should augment the ac- 

 tion of exogenous gonadotrophins with re- 

 spect to stimulation of follicular growth. 

 Clavert's hypothesis was that estrogen 

 would mobilize yolk precursors immediately 

 and thus facilitate yolk deposition in the 

 follicles under the influence of gonado- 

 trophin. If gonadotrophins were given 

 alone, estrogen would have to be secreted 

 under the influence of gonadotrophin and 

 yolk mobilization could occur subsequently. 

 Tliis hypothesis was tested with nonbreeding 

 black ducks by Phillips (1959). In one ex- 

 periment 4 out of 6 birds treated with the 

 combination of CAP and DES had large 

 follicles with yellow yolk, whereas none of 

 the birds treated with either hormone alone 

 had yellow yolk. In the second experiment 

 3 out of 5 birds on the combined treatment 

 had yellow yolk in the follicles, whereas 

 none of the CAP-treated birds contained 

 yellow yolk (there were no DES-treated 

 birds in this experiment) . It should be noted 

 that similar experiments wdth 6-week-old 

 pullets failed to show any large follicle for- 

 mation with either CAP, DES, or the com- 

 bination. This may be the result of the un- 

 responsiveness of the immature ovary, a 

 factor which was discussed previously. 



The administration of estrogen causes de- 

 lay of the next ovulation by suppressing LH 

 release (Fraps, 1954). Progesterone admin- 

 istration under identical conditions results 

 in LH release and premature ovulation. The 

 significance of these findings will be dis- 

 cussed under regulation of the laying and 



