244 CELLS, TISSUES, AND ORGANISMS 



ectoblastic fragments, he studied the inducing activity of ribonucleo- 

 proteins and purified RNA extracted from various sources, especially 

 thymus. He found that these preparations are active and that a treat- 

 ment with trypsin inactivates them, but the eflPect of trypsin apparently 

 is not on the nucleoprotein but on the explanted cells themselves, since 

 it can be suppressed by the addition of soya-bean trypsin inhibitor. In 

 contrast to Hayashi's claim (1959), treatment of the extracts with 

 ribonuclease produces only partial removal (40 to 70 per cent) of the 

 RNA and reduces the inducing activity.* Niu's conclusion is exactly the 

 opposite of that of Yamada: he believes that there is a correlation be- 

 tween the amount of RNA and the frequency of embryonic differentia- 

 tion. Obviously, much more work is required before a definite and gen- 

 eral conclusion can be reached, besides the well-established one that 

 ribonucleoproteins are very active inducing agents. 



In the foregoing discussion, only facts related to neural induction 

 have been presented. The general conclusion that the inducer is a 

 ribonucleoprotein is not valid any more for the induction of meso- 

 dermal tissue, 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, 1958, a, b ) . 



If we wish to summarize present knowledge concerning the in- 

 ducing substance, all we can say is that its chemical nature remains 

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

 date it along the lines just discussed. The menace of a non-specific 

 release of a neuralizing substance already present in the ectoblast in a 

 masked form will always loom up before the experimenter. The more 

 complex the experiments become, the more difficult is their interpreta- 

 tion. For instance, inhibition of induction by agents such as ribonu- 

 clease, proteolytic enzymes, etc., may be due to an effect on the ecto- 

 blast cells themselves rather than to the blocking of a specific chemical 

 group in the inducing substance. The reacting system— i.e., the ecto- 

 blast— may be directly affected by changes in the surrounding medium 

 in two opposite ways : ( 1 ) stimulation of neural differentiation ( spon- 

 taneous neuralization ) , or (2) loss of competence, which would make 

 the ectoblast incapable to react to inducing stimuli. 



In view of these uncertainties and the difficulty of solving them, 

 another approach must be used. This is why many investigators have 

 preferred to study RNA distribution and metabolism in intact eggs, 

 placed either in normal or in experimentally changed conditions. 



* According to a private communication of Dr. Niu, his joint experiments with 

 Hayashi have led to the conclusion that about 30 per cent of the RNA initially 

 present in ribonucleoproteins resists ribonuclease digestion; if so, the experiments 

 showing no loss in inducing activity after ribonuclease treatment would lose a good 

 deal of their meaning and interest. 



