806 



SPERM, OVA, AND PREGNANCY 



ovaries than in the controls, indicating 

 that more of them had been started on the 

 course of fm^ther development. From the evi- 

 dence presented in the monkey and from a 

 variety of other observations one must con- 

 clude that, once reproductive life has begun, 

 there is no neonatal growth of germinal epi- 

 thelium. 



One of the major difficulties is the prob- 

 lem of distinguishing germinal epithelial 

 cells from adjacent oogonia. A similar diffi- 

 culty is encountered when attempts are 

 made to remove only the germinal epithelial 

 cells by surgical or chemical means (]Moore 

 and Wang, 1947; Mandl and Zuckerman, 

 1951). This problem is further emphasized 

 by Everett (1945) when he states, "It seems 

 probable that the cells of the epithelium, 

 which form functional sex elements, are 

 not and never were a part of the mesothelial 

 covering, but are cells which were segre- 

 gated early and are merely stored in the epi- 

 thelium." 



From some of the earlier work, it was felt 

 that much would be gained if some tech- 

 nique were devised whereby individual cells 

 could be marked and their subsequent fate 

 determined. Latta and Pederson (1944) 

 initiated such experimentation when they 

 injected India ink into the periovarian space 

 and examined the ovaries at varying in- 

 tervals thereafter. Ova and follicular cells 

 with carbon particle inclusions were seen in 

 various stages of growth and maturation 

 and these observations were interpreted as 

 demonstrations of the origin of ova and 

 follicular cells from "vitally stained" ger- 

 minal epithelium. It is suggested, however, 

 in light of recent evidence that many cells 

 are capable of moving such particles across 

 the cells and transferring them to others 

 (Odor, 1956; Hampton, 1958), that the va- 

 lidity of using colloidal particles for labeling 

 epithelial cells should be re-evaluated. 



Theoretically, the study of tissue culture 

 preparations of fetal and adult ovaries by 

 phase contrast and time-lapse cinematog- 

 raphy might be a better approach to the 

 problem of the neoformation of oocytes in 

 mammals and a few experiments of this 

 type have been performed. Long (1940) re- 

 ported oocytes developing from newborn 

 and adult mice ovaries growing in vitro. 

 These findings were not confirmed by simi- 



lar studies of Ingram (1956) in which he 

 found no signs of oogenesis in tissue culture 

 preparations of either mouse or rat ovaries. 

 Gaillard (1950) suggested that the germinal 

 epithelium was essential for survival of ex- 

 plants of human embryonic ovaries in that 

 explants without germinal epithelium in- 

 variably died. On the other hand, Martino- 

 vitch (1939) cultured fetal mouse ovaries 

 for as long as 3V2 months. Although the 

 ovarian epithelium disappeared after one 

 week in vitro, the ovocytes continued to 

 grow. 



The covering epithelium of the ovary is 

 capable of proliferation, and mitotic figures 

 are frequently demonstrable. As the size of 

 the ovary changes during the normal cycle 

 or upon stimulation with exogenous hor- 

 mones, the covering epithelium must keep 

 pace with the changing surface contour. As 

 mentioned above, the primordial germ cells 

 in the embryo are strongly phosphatase- 

 positive. Careful evaluation of the cells 

 arising from the germinal epithelium have so 

 far shown negative enzymatic reactions. 



Furthermore it is a consistent finding that 

 when mice are x-rayed in late fetal life or at 

 birth with sufficient dosages to eliminate the 

 ovogonia, no new ovocytes form from the 

 cells of the germinal epithelium (Brambell, 

 Parkes and Fielding, 1927; Mintz, 1958) . 



It is an obvious conclusion that any 

 attempt to ascertain the origin of germ cells 

 cannot be considered adequate without thor- 

 oughly investigating the entire germ-cell 

 cycle from tlie very earliest stages to the 

 formation of the definitive sex elements in 

 the fetal and postnatal periods. This must 

 include also the origin of the functional 

 germinal cells in the sexually mature ani- 

 mal. There is an urgent need for a compre- 

 hensive comparative study of the cytology, 

 distribution, and migration of these cells. 

 Inasmuch as the germ cells often contain 

 nuclear and cytoplasmic features which are 

 highly characteristic, they offer unusual ad- 

 vantages for various experimental analyses 

 using some of the moi'e modern techniques 

 of experimental embryology, tissue culture, 

 and microscopy. 



Even though we have confined our re- 

 marks here to the chick and mammal, we 

 recognize the importance of the considerable 

 body of descriptive and experimental in- 



