Urinogenital System 



manner of migration (for a discussion see 

 Willier, '39). 



FORMATION OF THE GENITAL RIDGE 



In a strict sense gonadogenesis begins with 

 the formation of the genital ridge. Two 

 pi-oblems are presented: (1) the role of ex- 

 trinsic or regional factors in the origin of the 

 ridge, and (2) the relative importance of 

 the primary gonad constituents: germ cells 

 vs. structural elements. The first problem is 

 inseparable from the larger question of re- 

 gional organization and has not been com- 

 pletely analyzed. A recent study indicates 

 that general as well as local factors are in- 

 volved in the production of the ridge (Nieuw- 

 koop, '47, '50). In the urodele, Triton, dif- 

 ferentiation depends first on early contact 

 between lateral plate material (future peri- 

 toneum) and the entoderm. Later, folds of 

 coelomic epithelium, closely resembling gen- 

 ital ridges, may be induced independently 

 by notochord. Wolffian duct, and probably 

 by mesonephric tissue. In later development 

 gonads may be present in the absence of 

 mesonephros (e.g., Humphrey, '28a,b; Griin- 

 wald, '37). 



It is well established that the germ cells 

 are not essential for the origin of a gonad. 

 In amphibian embryos sterile genital ridges 

 may develop after removal of the germ cells 

 (e.g., Humphrey, '27, '28a; Nieuwkoop, '47); 

 and elimination of the primordial germ cells 

 of the chick before they reach the gonad 

 region does not prevent the formation of 

 gonads (for a summary see Willier, '39). 

 Furthermore, germ cells in ectopic situations 

 are unable to initiate gonad formation 

 (Humphrey, '28b; Willier, '33; Witschi, '34). 

 These results are opposed to the view that 

 the primordial germ cells act as inductors 

 (Dantschakoff, '32) or are otherwise essen- 

 tial for the origin of a gonad. The evidence 

 indicates that (1) the local appearance of 

 a genital ridge is conditioned by regional 

 influences; (2) primordial germ cells alone 

 cannot indvice a genital ridge and are not 

 essential for its origin; and (3) the formation 

 of the ridge is an activity of the structural 

 elements. 



THE ROLE OF GERM CELLS VS. 



STRUCTURAL ELEMENTS IN 



SEX DIFFERENTIATION 



The conclusion last stated may be ex- 

 tended to cover the period of sex differentia- 

 tion. The characteristic structure of testis or 



473 



ovary may be developed in the absence of 

 germ cells. Irradiation of the vegetal pole 

 of the frog's egg results later in well dif- 

 ferentiated testes and ovaries which are 

 completely sterile (Bounoure, '37, '50), and 

 tadpoles that develop from overripe eggs 





Fig. 179. Sterile gonad, of testicular structure, in 

 a chorio-allantoic graft derived from a small piece of 

 blastoderm containing Hensen's node, taken from 

 an embryo of the head-process stage (from Willier, 

 '39). 



Fig. 180. Diagram of an experiment demonstrat- 

 ing the role of the structural elements (intermediate 

 mesoderm) in determining the sex-type of the gonad 

 (after Humphrey, '33). A, Frog embryo, showing 

 the area of intermediate mesoderm (broken lines) 

 containing the gonad-forming elements. When this 

 area is transplanted to a corresponding position in 

 an embryo of different sex, a situation is set up as 

 in B, in which the structural elements of one sex 

 (area A-B') are associated with the primordial germ 

 cells (GC) of the other. The sex-type of the gonad 

 which develops is always determined by the struc- 

 tural elements. 



have defective gonads which are typically 

 sterile (Witschi, '51a). When the gonad- 

 forming area of the chick blastoderm is iso- 

 lated, in the absence of germ cells, sterile 

 testicular structures are produced (Fig. 179; 

 Willier, '37, '50); and gonads of both sexes 

 differentiate after destruction of the germ 

 cells by irradiation (Salzgeber, '50; Dul- 

 becco, '46). 



It has been shown further that the struc- 



