242 CELLS, TISSUES, AND ORGANISMS 



both the RNA and the -SH groups in morphogenesis could be hnked 

 together and they might lead to a common conclusion: that funda- 

 mentally morphogenesis is a problem of specific protein synthesis. 



The role of ribonucleoproteins and RNA in induction 



Ever since Bautzmann et al. ( 1932 ) showed that organizers killed 

 with alcohol, heating, or freezing are still capable of inducing a neural 

 tube, it has been realized that induction must be a chemical process, 

 and attempts have been made to identify and isolate the "active" in- 

 ducing substance in pure' form. Experiments by Wehmeier (1934) and 

 Holtfreter (1935) showed that the "inducing substance" (also called 

 the "evocator") is a very widespread one. Almost all tissues of adult 

 vertebrates and invertebrates, especially if they have been killed be- 

 forehand, induce neuralization of the ectoblast if they are grafted into 

 the blastocoele cavity of young gastrulae. 



The next step was to try to isolate the inducing substance from an 

 adult tissue— for instance, liver. The results, however, were disappoint- 

 ing, since it soon became clear that many chemically unrelated sub- 

 stances (sterols, glycogen, nucleotides, fatty acids, etc.) can induce 

 neural differentiation in ventral ectoderm ( see Brachet, 1944, for a de- 

 tailed review of this work ) . 



It was suggested by the author (1944) that ribonucleoproteins 

 might play a leading role in neural induction on the following grounds: 

 ribonucleoproteins extracted from diflFerent tissues (liver microsomes, 

 for instance) are better neural inductors than proteins with a lower 

 RNA content; tobacco mosaic virus, a pure ribonucleoprotein, is a very 

 good inducer (see Figure 1); furthermore, removal of RNA from the 

 active ribonucleoproteins by a ribonuclease digestion leads to a de- 

 crease in the inducing activity. 



The strong inducing power of ribonucleoproteins (liver micro- 

 somes or tobacco-mosaic virus, for instance) has been confirmed by 

 many workers (Brachet et al., 1952; Kuusi, 1953; Yamada, 1958 a, b; 

 etc.). But, on the other hand, it has been impossible to confirm the 

 inhibitory effect of ribonuclease on abnormal inductors in later experi- 

 ments (Brachet et al., 1952; Kuusi, 1953; Yamada and Takata, 1955a; 

 Englander and Johnen, 1957; etc.). The reason for the discrepancy 

 between our first results in 1944 and those of the more recent workers 

 is now clear; as shown by Hayashi (1958), a short treatment of the 

 ribonucleoprotein with proteolytic enzymes, such as pepsin or trypsin, 

 is enough to destroy the inducing power. At the time of our first experi- 

 ments, no crystalline ribonuclease was available, and there is little 

 doubt that the "purified" preparations used in those experiments were 

 contaminated with proteolytic enzymes. That the active substance in 



