466 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



Fig. 98. Gonocytes in the hind-gut of a tortoise, Pseudemys. In this species the gonocytes 

 remain in the endoblast long enough to be incorporated in the epithehum. They are here 

 made evident by a periodic acid-Schiff reaction, preceded by digestion with saHva to 

 eliminate the glycogen. Some of the gonocytes are shown abandoning the epithelium, 

 and one is already in the somatopleura at the left of the mesentery. Photogr. courtesy of 



J. Milaire. 



This material is "involved in the induction of kidney tubules' but impenetrable to 

 the passage of the mobile cartilage-inducing factor produced in the ventral regions". 



{g) Gonadic inductions 



At least, an allusion must be made to inductive mechanisms at work in the gonad. 

 They have been discovered by Witschi {cf. 1950) in amphibians, the group where 

 they are now best known. This knowledge, however, is limited to the bare fact 

 that induction occurs, but its content illustrates more clearly than anywhere 

 else the relation of sexual differentiation with genes on the one hand, with hor- 

 mones on the other hand. 



We have already met induction in the formation of the urinary tract where 

 the prcjnephric duct activates the mesonephric blastema. This is a typical case of 

 intradermic induction, which is convenient to express in terms of morphogenetic 

 potential, the anterior and physiologically older material having the initiative. 

 The events occurring in the gonads of amphibians appear as a further step in the 

 same embryonic area: part of the activated cell cords of the metanephric blastema 

 glide into the young genital ridge. 



^ Grobstein rightly remarks that this function of the dorsal cord region has no obvious 

 in vivo function. He mentions, however, that at the stage of the open neural plate this 

 material was superposed on the nephrotome. This amounts to a suggestion that a normal 

 induction of the nephrotomes by the lateral part of the neural plate takes place. This 

 idea is, I think, incompatible with several experimental results with amphibians. The 

 nephrotomes are not induced, but express a given level of morphogenetic potential inside 

 the middle layer (p. 351). 



