THE OVARIES 211 



The teleost fishes ])r()(hice tlie smallest e<i<i;s (,^eiierally and also 

 the most numerous; sometimes millions may he formed eaeh season. 

 The ovary here arises as an eva,i,dnation of the emhryonie wall of 

 the body cavity on eaeh side of the mid-line, and becomes surrounded 

 by a fold of the ])eritoneum which encloses it like a sac and continues 

 caudally to form a duct connecting the ()\ary with the cloaca. In 

 this case the eggs do not ])ass into the general body cavity when 

 mature, but continue down this duct to tlie cloaca. 



In general, the oxaries vary from an inacti\-e stage, in which few 

 oogenic cells occur in the connective tissue of an elongate mend)ran- 

 ous fold, to the active stage, in which the ovary l)ec()mes filled with 

 developing ova. In the active stage the connective-tissue covering 

 of the ovary is extremely thin, and the mass of maturing ova are 

 visible through it. The oogenic epithelial cells of the ovary become 

 active before each breeding season, and ooc\'tes and follicle cells 

 are proliferated. Each o\um becomes surroimded by a layer of 

 flattened nurse or follicle cells and a delicate sheath, or theca, of 

 connective tissue su])])orting a ca])illary network. As the ova 

 mature, a storage of yolk occurs in the cytoplasm and they distend 

 the connective tissue until, at the time of ovulation, they l)reak 

 through the follicular sheath and oxary wall into the l)ody cavity. 

 Cells derived from the follicle fill in the cavity left by the ovum and 

 form a cor])us luteiun. 



Amphibians. — Nearly all of this group are oviparous and lay their 

 eggs in water. During the breeding season, when one opens a 

 female amphibian, such as a frog, a mass of eggs is found occu])ying 

 the greater portion of the abdominal cavity. These are not free 

 in the body cavity, but are held in a pair of swollen sacculated 

 ovaries covered by a reflected fold of peritoneum. Dorsally this 

 peritoneal fold extends from each ovary to attach the o\ary to the 

 dorsal wall of the body cavity and is continuous with the peritoneum. 

 This fold, the mesovarium, carries blood vessels, nerves, and lym- 

 phatics to each ovary. Beginning in the region of the mesovarium, 

 the oogenic cells of each sacculated portion of the OAary begin 

 proliferating for the reproductive season. Each maturing o\um 

 becomes surrounded by a layer of cuboidal cells that gradually 

 elaborate more and more yolk which is added to the cytoplasm of 

 the ovum. Surrounding this layer of nurse cells is a delicate sheath, 

 or theca, of connective tissue. Continued proliferation and conse- 

 quent maturation of the eggs distends the o\'ary and its peritoneal 

 covering to the point observed above. "When mature, the eggs 



