INDUCTION OF THE NERVOUS SYSTEM 59 



answered in view of the recent work of Niu (1956, 1958a, b). He 

 demonstrated that explants of the chordomesoblast (organizer) 

 produce ribonucleoproteins in the surrounding medium ; the latter 

 — which Niu (1956) calls a "conditioned medium" — induces 

 neuralization in explanted ectoblastic fragments. Thisneuralization, 

 according to Niu (1956), cannot be explained on the basis of a 

 release of an inducing or toxic substance by cytolyzing cells. Further- 

 more, ribonuclease inactivates the neuralizing factor produced by the 

 explanted organizer in Axolotl and Triturus torosus. The enzyme 

 has no inhibitory action, however, in the case of Triturus rivularis. 



Niu's (1958a, b) most recent papers show how controversial the 

 question of the role of RNA in induction remains. Working with 

 small explanted ectoblastic fragments, he studied the inducing 

 activity of ribonucleoproteins and purified RNA extracted from 

 various sources, especially thymus. He found that these prepara- 

 tions are active and that a treatment with trypsin inactivates them; 

 but the effect of trypsin is apparently not on the nucleoprotein but 

 on the explanted cells themselves, since it can be suppressed by the 

 addition of soya bean trypsin inhibitor. Treatment of the extracts 

 with ribonuclease produces only partial removal (40-70 %) of the 

 RNA and reduces the inducing activity. Niu's (1958a, b) conclusion 

 is exactly the opposite of that of Yamada (1958a, b) who believes 

 that there is a correlation between the amount of RNA and the fre- 

 quency of embryonic differentiation. Obviously, much more work 

 is required before a definite and general conclusion can be reached. 



In the foregoing discussion, only facts related to neural induction 

 have been presented. The evidence presented suggests that the 

 inductor is a ribonucleoprotein, in which the protein part may be 

 more important than the nucleic acid part. Such a conclusion would 

 not be valid for the induction of mesodermic tissues, which is so 

 conspicuous when caudal (and not cephalic) regions are induced. 

 All the available evidence suggests that the caudal organizer is of a 

 purely protein nature (Yamada, 1958a, b). 



If we wish to summarize present knowledge concerning the 

 inducing substances, all we can say is that its chemical nature re- 

 mains obscure and that it will be an exceedingly difficult task to try 



References p. 90/93 



