ACTIVITIES OF THE OVARY 79 



of the ovary under the influence of luteotrophin, LTH. The presence of 

 progesterone, in small quantities together with increasing amounts of estro- 

 gen, stimulates the anterior lobe to discharge increased amounts of the luteiniz- 

 ing hormone, LH (ICSH). (S,ee figs. 22, 53, 59.) The elevated level of 

 estrogen, according to this theory also causes a decreased output of FSH until 

 it reaches a minimal level at the period shortly before egg discharge (figs. 

 53, 59). As a result, the increased quantity of LH together with FSH has an 

 added effect upon the follicle which brings about the chain of events leading 

 to egg discharge. Evans and Simpson in Pincus and Thimann ('50) give the 

 proportion of 10 parts of FSH to 1 of LH (ICSH) as the proper hormonal 

 balance in effecting ovulation in the hypophysectomized rat. 



In those mammalian species where ovulation is dependent upon the act of 

 copulation, a nervous stimulus is involved which increases the output from 

 the pituitary gland of the gonadotrophic factors, particularly LH. 



b. Ovulation in Vertebrate Groups Other Than the Higher Mammals 



The physical mechanism involved in the ovulatory procedure in the lower 

 vertebrate classes is different from that found in higher mammals. Two forms, 

 the hen and the frog, have been studied in detail. These two animals represent 

 somewhat different types of ovulatory behavior. 



1) Hen. As the hen's egg develops in the ovary, it gradually pushes the 

 ovarian surface outward; it ultimately becomes suspended from the general 

 surface of the ovary by means of a narrowing stalk, the pedicle (figs. 31, 47). 

 When the ovulatory changes are initiated, the musculature of the ovarian 

 wall overlying the outer surface of the egg appears to contract, and an elon- 

 gated narrow area along this outer surface becomes avascular. This avascular 

 area represents the place where the ovarian surface eventually ruptures to 

 permit the egg to leave the ovary; it is called variously, the rupture area, 

 stigma, or cicatrix. Gradually, the cicatrix widens and finally a slit-like open- 

 ing is formed by a tearing apart of tissues in the central region of the cicatrix. 

 Contractions of the smooth muscle fibers appear to be responsible for this 

 tearing procedure (Phillips and Warren, '37). The egg eventually is expelled 

 through the opening and in many instances it rolls into the infundibular funnel 

 of the oviduct which at this time is actively engaged in an endeavor to engulf 

 or "swallow" the egg (fig. 31). 



2) Frog. The egg of the frog projects into the ovarian cavity within the 

 ovary and is attached to the ovarian wall by means of a broad area or stalk 

 (fig. 45B). As the egg enlarges, it tends to push the ovarian surface outward, 

 and the egg and its follicle thus forms a mound-like protuberance from the 

 ovarian surface (figs. 45A, B; 72F). The egg and the surrounding ovarian 

 tissue thus lies exposed on one aspect to the outer surface of the ovary. The 

 outer surface of exposure is the stigma or area of rupture, and in the older 

 follicles this area does not contain blood vessels (fig. 72F). As ovulation 



