58 GROWTH AND MORPHOGENESIS 



disappointing, since it soon became clear that many chemically un- 

 related substances (sterols, glycogen, nucleotides, fatty acids, etc.) 

 can induce neural differentiation in ventral ectoderm (see Brachet, 

 1944, for a detailed review of this work). 



It has been suggested by Brachet (1944) that ribonucleoproteins 

 might play a leading role in neural induction for the following 

 reasons. Ribonucleoproteins extracted from different tissues are 

 better neural inductors than proteins which have a lower RNA 

 content. Tobacco mosaic virus, which is a pure ribonucleoprotein, 

 is a very good inductor (Fig. 19, p. 61). Furthermore, removal of 

 RNA from the active ribonucleoproteins by a ribonuclease diges- 

 tion leads to a decrease in the inducing activity (Brachet, 1944). 



The strong inducing power of ribonucleoproteins (e.g. liver 

 microsomes, tobacco mosaic virus) has been confirmed by many 

 workers (Brachet era/., 1952;Kuusi, 1953; Yamada, 1958a, b, etc.). 

 But, on the other hand, it has proved impossible to confirm the 

 inhibitory effect of ribonuclease on abnormal inductors in later 

 experiments (Brachet etal., 1952 ; Kuusi, 1953 ; Yamada and Takata, 

 1955a; Englander and Johnen, 1957; etc.). The reason for the dis- 

 crepancy between our first results (1944) and those of more recent 

 workers is now clear; as shown by Hayashi (1958), a short treat- 

 ment of the ribonucleoprotein with proteolytic enzymes, such as 

 pepsin or trypsin, is enough to destroy the inducing power. At the 

 time of our first experiments (1944), no crystalline ribonuclease 

 was available and there is no doubt that the "purified" preparations 

 used in these experiments were contaminated with proteolytic 

 enzymes. That the active substance in ribonucleoprotein is protein 

 rather than RNA is further shown by the fact that RNA isolated by 

 mild methods (Yamada and Takata, 1955b; Tiedemann and Tiede- 

 mann, 1956) from various tissues, including embryos, is a mild in- 

 ductor only. These negative experiments carry, however, no great 

 weight in view of the difficulty often experienced in isolating non- 

 denaturated RNA. 



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

 view that the active portion in ribonucleoproteins is protein rather 

 than RNA, the question should not yet be considered as completely 



