. Analysis of development. Embryology; Embryology. 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 tha
. Analysis of development. Embryology; Embryology. 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 (, 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 (, 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 n
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Keywords: ., bookcentury1900, bookpublisherphiladelphi, booksubjectembryology
