210 THE FEMALE REPRODUCTIVE SYSTEM 



ovary degenerates, or, as in certain teleosts, the two ovaries fuse. 

 In the embryonic gonad, string-like continuations of the iniclerlying 

 mesenchyme divide the ei)itheHal cells into cords and carry along 

 developing blood vessels. Thus, each gland is composed of spherical 

 cells supported by connective tissue, and the whole co\ered by an 

 epithelial membrane continuous with the ])erit()neal lining of the 

 coelome. The first indication of ovarian differentiation appears in 

 the development of a layer of the cuboidal or polyhedral cells to 

 form follicle or nurse cells about larger primordial sex cells. Gen- 

 erally a single layer of cuboidal or columnar cells siu'rounds each 

 maturing ovum and assists in the elaboration of the nutrition for 

 its development. By the activity of these cells, yolk is added in 

 the case of yolk-bearing eggs so that adequate nutrition is stored 

 for future developmental demands to be met after fertilization. 



Not all the ova and their follicle cells complete their development. 

 A number api)ear to reach various stages of differentiation and then 

 disintegrate and are resorbed with an accompanying in\'asi()n of the 

 surrounding connective tissue. 



The condition of the ovary shows considerable variation in the 

 different grou])s of vertebrates and the de\'elopment of the OAum 

 likewise varies. Among the oviparous forms the fertilized egg 

 develops outside the body and must depend u])on its stored nutri- 

 tion. In some viviparous forms it may likewise have to (lc])end 

 entirely or in part u])on stored nutrition. In ])lacental mannnals, 

 no yolk storage occurs and nutrition is furnished the embryo directly 

 from the mother throughout its de\elopment. 



Fishes. — In myxinoid cyclostomes, an unpaired, elongate gonad 

 is held l)y a median dorsal mesentery. Xo genital ducts are ])resent, 

 and the ova and sperm are liberated into the !)0(1\ caAity from which 

 they pass to the urogenital sinus by two pores. The anterior 

 portion of this gonad ])ro(luces ova and the posterior portion ])ro- 

 duces s])erm, but both are not functional at the same time. In 

 this case an individual functions first as a male and then as a female. 

 In some cyclostomes only one i)orti()n may l)e functional, though 

 the other is ])resent. 



In elasmobranchs, such as the skate, the female has a single 

 ovary; the right ovary degenerates. In the functional left ()\ary, 

 ova periodical!}- matiu'e in round follicles on the ])crii)her\ and are 

 liberated to be i)icke(l u]) by a pair of oN'iducts ()])ening behind the 

 ])ericar(lium. The eggs of this grouj) are the larg(>st among the 

 fishes and usualh' onl\' a small number mature each season. 



