BIOLOGY OF EGGS AND IMPLANTATION 



805 



cells in the germinal epithelium (Allen, 

 1911; Firket, 1914; Kingery, 1917). On the 

 otlier hand, Essenberg (1923), Butcher 

 (1927), Brambell (1927, 1928), and Swezy 

 and Evans (1930) postulated a dual origin 

 for the germ cells, i.e., they may arise both 

 from the primordial germ cells, and directly 

 from somatic cells. 



The ingrowth of new cells from the ger- 

 minal epithelium, resulting in the produc- 

 tion of new oocytes, was thought to have 

 been demonstrated for both the eutherian 

 mammals (Pincus, 1936; Duke, 1941; Slater 

 and Dornfeld, 1945), and birds (Bullough 

 and Oibbs, 1941). However, various opin- 

 ions flourished as to whether these oocytes 

 were produced continuously throughout the 

 reproductive life of the female (Robinson, 

 1918; Papanicolaou, 1924; Hargitt, 1930j, 

 or whether they arose from a cyclically 

 stimulated germinal epithelium. On the 

 basis of Allen's (1923) investigations on 

 the mouse, and Evans' and Swezy 's (1931) 

 work on a variety of mammalian species, 

 it was widely accepted that a large number 

 of oocytes make their appearance from the 

 germinal epithelium about the time of es- 

 trus. According to these investigations the 

 oocytic population reaches its peak during 

 the period of heat and ovulation. On the 

 other hand. Green and Zuckerman (1951a, 

 b, 1954) analyzed the difference in the 

 number of oocytes during the menstrual 

 cycle in 12 pairs of ovaries of Maccica 

 mulatta by both quantitative and statistical 

 methods. Their results did not support the 

 accepted view that the total number of 

 oocytes in the ovaries of the monkey varies 

 during the cycle and reaches a maximum 

 near the time of ovulation. They concluded 

 that there is no significant difference be- 

 tween the average total number of oocytes 

 present at the beginning, middle, and end 

 of the cycle. From the results of the ex- 

 periments of Papanicolaou (1924), Moore 

 and Wang (1947), Mandl and Zuckerman 

 (1951), Mandl and Shelton (1959), Enders 

 (1960), and others, one would assume that 

 the germinal epithelium is not essential for 

 oogenesis in the adult mammal. If oogenesis 

 is to continue after puberty in the absence 

 of a germinal epithelium, are there al- 

 ternative sources for the new oocytes? It has 

 been proposed that either the concentration 



of primordial germinal cells in the region of 

 the hilum of the ovary, redescribed by Vin- 

 cent and Dornfeld (1948), may be a source, 

 or that specialized cells, histologically in- 

 distinguishable from other stromal cells, 

 may be transformed into germ cells. In 

 support of the latter, Dawson (1951) sug- 

 gested that in polyovular follicles in which 

 there is a great disproportion in the size of 

 the ova, the accessory egg may have arisen 

 by delayed oocytic differentiation of a cell 

 temporarily incorporated in the follicular 

 epithelium. 



Of the numerous experimental approaches 

 to the problem of the origin of the germ 

 cells in the sexually mature animal, the 

 action of various hormones on the ger- 

 minal epithelium has received particular 

 attention. Bullough (1946) claimed that at 

 the time of ovulation the estrogen-rich fol- 

 licular fluid which bathes the ovary induces 

 mitotic activity of the germinal epithelium. 

 Stein and Allen (1942) demonstrated a 

 stimulating effect of estrogen on the pro- 

 liferation of the germinal epithelium of the 

 mouse when this hormone was injected di- 

 rectly into the periovarial sac. On the other 

 hand, thyroxine similarly applied retarded 

 mitoses of the germinal epithelium (Stein, 

 Quimby and Moeller, 1947). More recently 

 Simpson and van Wagenen (1953) reported 

 an enhancement of all the processes con- 

 cerned with the development of oocytes and 

 follicles in prepubertal monkeys (Macaca 

 mulatta) that had been injected subcu- 

 taneously with either highly purified follicle- 

 stimulating hormone (FSH) extracted from 

 the sheep pituitary or extracts from ho- 

 mologous pituitaries ( also see van Wagenen 

 and Simpson, 1957, and Simpson and Van 

 Wagenen, 1958). The germinal epithelium 

 was stimulated to such an extent that there 

 was an active ingrowth of germinal cords 

 which closely simulated the development of 

 Pfliiger's tubes. Small oocytes appeared to 

 be developing within the germinal cords and 

 there were evidences which one could in- 

 terpret as reactivated oogenesis. An attempt 

 was made to carefully quantify the response 

 of the ovaries by counting the number of 

 oogonia and growing follicles. In general 

 the follicular counts remained unchanged, 

 but primary follicles with a single granulosa 

 cell laver were fewer in the stimulated 



