THE FEMALE 45 



The histology of the ovary shows that within its outer peritoneal 

 covering, the theca externa, are suspended thousands of individual 

 sacs, each made up of another membrane, the theca interna or cyst 

 wall, which contains smooth muscle fibers. This theca interna is 

 derived from the retro-peritoneal tissue. The smooth muscle fibers 

 can be seen histologically and can be demonstrated physiologically. 

 The theca interna surrounds each egg except for the limited area 

 bulging toward the body cavity, where it is covered by only the theca 

 externa. This is the region which will be ruptured during ovulation 

 to allow the egg to escape its follicle into the body cavity. The theca 

 interna, plus the limited covering of the theca externa, and the follicle 

 cells together comprise the ovarian follicle. These two membranes 

 make up the rather limited ovarian stroma of the frog ovary, and 

 they contain both blood vessels and nerves. Within each follicle are 

 found follicle cells, with their oval and granular nuclei, derived origi- 

 nally from oogonia. These follicle cells surround the developing 

 oocyte and are found in close association with it throughout those 

 processes of maturation which occur within the follicle. Enclosed 

 within the follicle cells, and closely applied to each mature egg, is 

 the non-cellular and transparent vitelline membrane, probably de- 

 rived from both the ovum and the follicle cells. This membrane is 

 developed and applied to the egg during the maturation process so 

 that it is not seen around the earlier or younger oogonia. Since the 

 bulk of the egg is yolk (vitellus), this membrane is appropriately called 

 the vitelline membrane. It is sometimes designated as the primary 

 (of several) egg membranes. After the egg is fertilized this mem- 

 brane becomes separated from the egg and the space between is then 

 known as the perivitelline space, filled with a fluid. The fluid may 

 be derived from the egg which would show compensatory shrinkage. 

 As the oocyte matures and enlarges, the follicle cells and mem- 

 branes are so stretched and flattened that they are not easily distin- 

 guished. It is therefore best to study these structures in the immature 

 ovary. 



The egg will mature in any of a variety of positions within its 

 follicle, the exact position probably depending upon the maximum 

 blood supply. As one examines an ovary the eggs will be seen in all 

 possible positions, some with the animal hemisphere and others with 

 the vegetal hemisphere toward the theca externa and body cavity. It 

 is believed that the most vascular side of the follicle wall will tend 



