Potencies of the gonad-forming area in the chick as tested in chorio-allantoic grafts.

Abstract

1. This report deals with an analysis by chorio-allantoic grafting of the developmental potentialities of the gonad-forming areas of stages prior to the genital ridge in the chick embryo. The stages tested include a) those prior to the formation of the germinal epithelium (donors having from 29 to 34 somites) and b) those in which a germinal epithelium is visibly differentiated (donors having from 35 to 41 somites). In all of these stages germ-cells are present in the gonadforming areas. 2. a) In 34% of the 77 grafts histologically examined a gonad of specific sex has differentiated. Prior to the origin of the germinal epithelium such a gonad arises in 23.9% of the grafts whereas after its formation in 48.4% of the grafts; according to a former report (WILLIER '27) when the genital ridge stage is reached it forms in approximately 100% of the grafts. b) A gonad-like body of undetermined sex forms commonly (60%) in the grafts of the gonad-forming area whereas it rarely does so in grafts of the genital ridge. Prior to the 31-somite stage this type of gonad only has thus far formed. Several grades of organization of these bodies are recognized. c) The gonad-forming area yields somewhat frequently multiple gonads whereas a well-defined genital ridge gives as a rule a single gonad. d) The size of the gonad of specific sex which arises from the gonad-forming area is smaller than one from the genital ridge. It is thus evident that a progressive change in the developmental potentialities of the gonad-forming area occurs as it transforms into a genital ridge. This is interpreted as indicating that the gonad-forming area during its initial development exhibits an ascending organization which finally attains at the genital ridge stage a fully "determined" condition. 3. In the majority of the grafts of the gonad-forming area masses of germinal-cells are found external to the gonad in the mesenchyme or spaces within it. In such positions the germ-cells merely undergo repeated multiplication, exhibiting no tendency to form a gonad. If by chance germ-cells come to be situated in the wall of either the Wolffian or Müllerian duct, the overlying coelomic epithelium thickens into a layer simulating a germinal epithelium, yet the essential components of an avian gonad, namely, the sexual cords, fail completely to invaginate. The origin and differentiation of the germinal epithelium, on the contrary, is apparently independent of primordial germ-cells since a sterile gonad forms in grafts of whole blastoderms of early somite stages following the removal of the "germ-cell crescent" ofSWIFT. 4. Germ-cells in the mesenchyme remain cytologically undifferentiated, like the primordial germ-cells, whereas those which come to be situated in the sexual cords undergo specific differentiation. The conclusion is reached, therefore, that the germ-cell is dependent upon the specific tissue environment of the sexual cord for its sexual differentiation.