326 Action of the Genetic Material 



responsible. Brachet stated that the results are due to traces of nucleo- 

 proteins, and succeeded in obtaining induction with yeast and virus 

 nucleoproteins. He further isolated granules, microsomes, from differ- 

 ent cell types (amphibian or even mammalian) which contained pro- 

 tein, phosphatases, and RNA, and they acted as inductors, an action 

 which was stopped by treatment of the material with ribonuclease. It 

 was found that metliylene blue and cystine also acted as inductors. 

 When Barth found that the toxic digitamine was an inductor, the 

 suspicion arose that toxicity is the common factor of all actions, which 

 would mean that none of these experiments had anything to do with 

 natural induction. 



At this time, Holtfreter (whose dramatic presentation of the 

 subject in 1951 we are now following) showed that the insertion of a 

 piece of glass, moving to and fro, suffices for induction; Okada got in- 

 duction with silica and similar substances. This suggested to Holt- 

 freter that the real action is based upon the destruction of cells with 

 the liberation of products which enter the normal cells and act as 

 inductors. (As far as I am aware, nobody drew attention in this con- 

 nection to Haberlandt's wound hormones. He showed, 1935, that cells 

 at the surface of a cut potato do not divide when the surface is care- 

 fully washed. If it is not cleaned, and especially if it is covered with a 

 brei of broken cells, mitotic divisions start at once. He spoke of 

 wound hormones as instigators of mitosis. Clearly we are dealing with 

 related phenomena.) All the experiments mentioned can thus be 

 explained. 



Further progress was made when Barth found that pieces of 

 gastrula (without an inductor region) of an axolotl, cultured in 

 vitro, showed neural differentiation, while similar pieces of other 

 Amphibia would never do so. This led to a series of brilliant experi- 

 ments by Holtfreter. He assumed that the difiFerence between axolotl 

 eggs and those of other Amphibia is due to differences in permeability, 

 those of the axolotl being damaged by the saline solution used as 

 medium. Actually, axolotl eggs could be made to behave like Triton 

 or frog eggs and vice versa by changing the pH of the medium: at 

 4-5 no induction occurred in the axolotl egg; at 9.2 all eggs possessed 

 induction. Also the reciprocal experiment succeeded. Triton gastrulae 

 (the explanted cells of which would produce nothing but epidermis) 

 were treated with a low pH medium, which resulted in disintegration 

 of the cells. By neutralization the cells (in explantate) were made to 

 reunite, and now neural tubes, and so on, were formed; that is, the 

 Triton cells now behaved like the axolotl. Obviously, some cytolysis 



