GERM-CELL ORIGIN 



121 



the following question: Will the gonad develop into a functional structure 

 without the presence of the primordial germ cells? Experiments performed 

 by Humphrey ('27) on Ambystoma, and the above-mentioned workers — 

 Benoit ('30), Goldsmith ('35), and Willier ('37) — on the chick, suggest that 

 only sterile gonads develop without the presence of the primordial germ cells. 



Finally, another facet of the germ-cell problem is this: Are germ cells 

 completely self differentiating? That is, do they have the capacity to develop 

 by themselves; or, are the germ cells dependent upon surrounding gonadal 

 tissues for the influences which bring about their differentiation? All of the 

 data on sex reversal in animals, normal and experimental (Witschi, in Allen, 

 Danforth, and Doisy, '39), and of other experiments on the development 

 of the early embryonic sex glands (Nieuwkoop, '49) suggest that the germ 

 cells are not self differentiating but are dependent upon the surrounding 

 tissues for the specific influences which cause their development. Furthermore, 

 the data on sex reversal shows plainly that the specific chromosome complex 

 (i.e., male or female) within the germ cell does not determine the differen- 

 tiation into the male gamete or the female gamete, but rather, that the influ- 

 ences of the cortex (in the female) and the medulla (in the male) determine 

 the specific type of gametogenesis. 



The table given on pp. 121-124 summarizes the conclusions which some 

 authors have reached concerning germ-cell origin in many vertebrates. It is 

 not complete; for more extensive reviews of the subject see Everett ('45), 

 Heys ('31), and Nieuwkoop ('47, '49). 



Species 



Place uf Origin, etc. 



Author 



Entosphenus wilderi 

 (brook lamprey) 



Germ cells segregate early in the em- Okkelberg. 1921. 



bryo; definitive germ cells derived J. Morphol. 35 



from "no other source" 



Petroinyzon muriniis 

 unicolor (lake 

 lamprey) 



Definitive germ cells derive from: 

 a) early segregated cells, primor- 

 dial germ cells, and b) later from 

 coelomic epithelium. Suggests that 

 primordial germ cells may induce 

 germ-cell formation in peritoneal 

 epithelium 



Butcher. 1929. 

 Biol. Bull. 56 



Sqiialus acanthias 



Germ cells segregate from primitive 

 entoderm; migrate via the meso- 

 derm into site of the developing 

 gonad 



Woods. 1902. 

 Am. J. Anat. 1 



Amia and Germ cells segregate early from en- Allen. 1911. 



Lepidosteiis toderm; continue distinct and mi- J. Morphol. 22 



grate into the developing gonad 

 via the mesoderm (see fig. 60) 



