THE FEMALE 67 



accumulations of spherical, black pigment granules. With each cleav- 

 age, subsequent to fertilization, this superficial coat is divided between 

 the blastomeres, being an integral part of the living cell. There is no 

 clear-cut demarcation between this surface coat and the inner cyto- 

 plasm and yolk. It is believed that these growth changes of the egg 

 are under the influence of the basophilic cells of the anterior pitui- 

 tary gland, which cells are greater in number at this time than at 

 any other. 



During the growth period the vitelline membrane appears on the 

 surface of the oocyte as a thin, transparent, non-living, and closely 

 adherent membrane. It is formed presumably by a secretion from the 

 egg itself, aided by the surrounding follicle cells. It appears to be 

 similar in all respects to the membrane of the same name found 

 around the eggs of all vertebrates. 



Ovulation and Maturation. Ovulation, or the liberation of the 

 egg from the ovary, is brought about by a sex-stimulating hormone 

 from the anterior pituitary gland. Just before and during the normal 

 spring breeding period there is a temporary increase in the relative 

 number of acidophilic cells in the anterior pituitary. It is believed that 

 this is not coincidental but a causal factor in sex behavior in the frog. 

 However, until such time as extracts of specific cell types can be 

 made, this will be difficult to prove conclusively. Attempts on the 

 part of the male to achieve amplexus are resisted by the female not 

 sexually stimulated. However, such a female can be made to accept 

 the male by injecting the female with whole anterior pituitary glands 

 from other frogs. It is very probable that environmental factors such 

 as light, temperature, and food may act through the endocrine sys- 

 tem to prepare the frogs for breeding when they reach the swampy 

 marshes in the early spring, after protracted hibernation. It must 

 be pointed out, however, that there are frogs in essentially the same 

 environments which breed in July (Rana clamitans) and August 

 {Rana catesbiana) , so that the causal factors appear to be either com- 

 plex or possibly different for different species. The pituitaries of the 

 hibernating frogs do contain the sex-stimulating factor, but apparently 

 to a lesser degree than the glands of frogs approaching the breeding 

 season. The injection of 6 glands from adult female frogs will cause 

 an adult female of Rana pipiens to ovulate as early as the last week 

 in August, some 8 months before the normal breeding period. One 

 or two such glands will accomplish the same results if used early in 



