80 THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION 



approaches, an opening suddenly appears in the area of rupture. The mus- 

 culature within the theca interna around the foUicle then contracts, and the 

 egg rolls out through the opening in the rupture area like a big ameba (fig. 

 45B). As the egg passes through the aperture, it may assume an hourglass 

 shape (Smith, B. G., '16). After the egg is discharged, the follicle contracts 

 to a much smaller size (fig. 45B). It has been suggested that the rupture of 

 the external surface of the follicle might be produced by a digestive enzyme 

 (Rugh, '35, a and b). 



3) Hormonal Control of Ovulation in Lower Vertebrates. The hormonal 

 mechanism regulating ovarian rupture and egg discharge in the lower verte- 

 brate groups has not been as thoroughly explored in all of the vertebrate 

 groups as it has in the mammals. However, sufficient work has been done to 

 demonstrate that pituitary hormones are responsible in all of the major verte- 

 brate groups, including the fishes. Amphibian pituitary implants under the 

 skin or macerated anterior-lobe pituitary tissue injected into the peritoneal 

 cavity of various amphibia have been effective in producing ovulatory phe- 

 nomena (Rugh, '35a). More recently, purified mammalian follicle-stimulating 

 hormone, FSH, and luteinizing hormone, LH, have been used to stimulate 

 egg discharge in frog ovarian fragments, as well as in normal and hypophysec- 

 tomized females. However, the follicle-stimulating hormone alone will not 

 elicit ovulation (Wright, '45; Wright and Hisaw, '46). Accordingly, both 

 factors are necessary in the frog, as in mammals. In the hen, these two pi- 

 tuitary hormones have been shown to bring about ovulation when injected 

 intravenously (Fraps, Olsen, and Neher, '42; Romanoff and Romanoff, '49, 

 pp. 208-215). Also, Neher and Fraps ('50) present evidence which suggests 

 that progesterone plays a part in the physiological chain which elicits ovulation 

 in the hen. A close relationship between the physiological procedures effecting 

 ovulation in the hen and the mammal thus appears to exist. 



c. Comparison of the Immediate Factors Effecting Egg Discharge in the 



Vertebrate Group 



In the vertebrates thus far studied contraction of muscle tissue of the fol- 

 licle following the rupture of surface tissues presumably is the main factor 

 which brings about egg expulsion. In higher mammals, associated with muscle 

 contracture, there also may be an increase in follicular turgidity due to endos- 

 motic phenomena associated with the contained follicular fluid (Walton and 

 Hammond, '28). In the frog, hen, and mammal the changes involved in the 

 surface tissues leading to their rupture are associated with the following se- 

 quence of events: 



( 1 ) avascularity of the surface tissues, 



(2) a thinning of the surface tissues, and finally 



(3) a rupture of these tissues. 



