Gametogenesis, Fertilization and Parthenogenesis 



175 



parts of the gonad can develop normally in 

 the absence of the germ cells. 



Somewhat similar experiments have been 

 performed in frogs by Bounoure ('35a, b, 

 '37a, b, c). He irradiated the vegetal pole 

 of the egg and obtained a great reduction in 

 the number of germ cells of the larvae that 

 developed. While none of the larvae were 

 completely agametic, at least two cases 

 were obtained in which one gonad was en- 

 tirely devoid of germ cells. Here, too, the 



upon the occurrence of a migration of 

 primordial germ cells to the genital ridge 

 from some other region of the embryo or 

 from extra-embryonic areas, such as de- 

 scribed by Allen ('07) in frogs and by 

 Swift ('14) in birds. For a recent example 

 of such studies reference may be made to 

 the work of Witschi ('48) on human em- 

 bryos, in which migration of the germ cells 

 from the yolk sac to the genital folds is 

 convincingly described. 



\ 

 A \ 



Jl- 



Fig. 51. Maps showing distribution of germ cells in chick blastoderms. A, head-process stage; B, 3-somite 



stage. (From Willier, '37.) 



somatic constituents of the agametic gonad 

 appeared normal. 



Other Experiments Concerning the Origin 

 of the Germ Cells in Vertebrates. The ques- 

 tion of the origin of the germ cells in 

 vertebrates has been the subject of much 

 controversy centering primarily upon 

 whether or not they may be formed from 

 "differentiated" somatic cells in various 

 stages of embryonic development and in the 

 adult. The subject has been frequently and 

 extensively reviewed (Willier, '39; Bounoure, 

 '39; Dantschakoff, '41; Everett, '45; Nieuw- 

 koop, '46). The present discussion will re- 

 strict itself mainly to some of the more 

 recent experimental work. 



There is now fairly general agreement 



In many respects birds have provided 

 the more favorable experimental material in 

 attempts to deprive the embryo of germ cells, 

 and several workers have reported success 

 upon subjecting the area of Swift's germinal 

 crescent (see Fig. 51) to extirpation (Reagan, 

 '16; Willier, '33, '37), ultraviolet irradiation 

 (Benoit, '30), and cauterization (Dantscha- 

 koff et al., '31). In Willier's ('37) experi- 

 ments on chorio-allantoic grafting of por- 

 tions of chick blastoderms, it has been 

 demonstrated that a gonad free of germ cells 

 may develop and claims (e.g., Dantschakoff 

 et al., '31) of the necessity of the presence 

 of germ cells for differentiation of the gonad 

 are refuted. Willier cautions againsi inter- 

 preting the results of his experiments, and 



