THE ROLE OF RIBONUCLEIC ACDD AND SULFHYDRIL CROUPS 243 



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Figure 1. Large neural tube induced after implantation of tobacco-mosaic 

 virus into the blastocele cavity of an axolotl gastrula (Brachet, 1950) . 



ribonucleoprotein is probably protein rather than RNA is further sug- 

 gested by the fact that RNA isolated by mild methods (Yamada and 

 Takata, 1955b; Tiedemann and Tiedemann, 1956) from various tissues, 

 including embryos, is a poor inducer. These negative experiments, 

 however, carry no great weight, in view of the difficulty often experi- 

 enced in isolating non-denatured, biologically active RNA. 



Although there is, as we have just seen, some evidence for the view 

 that the active portion in ribonucleoproteins is protein rather than 

 RNA, the question should not yet be considered completely answered 

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

 show that explants of the chordomesoblast (organizer) produce ribo- 

 nucleoproteins in the surrounding medium; the latter— which Niu calls 

 a "conditioned medium"— induces neuralization in explanted ectoblastic 

 fragments. This neuralization, according to Niu, cannot be explained 

 on the basis of a release of an inducing or toxic substance by cytolyzing 

 cells. Furthermore, ribonuclease inactivates the neuralizing factor pro- 

 duced by the explanted organizer in Axolotl and Triturus torosus; the 

 enzyme has no inhibitory action, however, in the case of Triturus 

 rivularis. 



Niu's most recent papers show how controversial the question of 

 the role of RNA in induction remains. Working with small, explanted 



