INDUCTION OF THE NERVOUS SYSTEM 57 



cell nucleus are difficult during early cleavage. All one can say is 

 that the Feulgen reaction is of apparently normal intensity in the 

 swollen interphase nuclei of the blocked eggs. The facts that, accord- 

 ing to Brachet and Ledoux (1955), ribonuclease produces an 85% 

 inhibition of the incorporation of amino acids into proteins in 

 amphibian eggs and that, according to autoradiographic observa- 

 tions, this incorporation is almost entirely limited to the nuclei 

 during early cleavage, shows that the synthesis of the nuclear pro- 

 teins is very sensitive to ribonuclease in cleaving eggs. It is therefore 

 likely that RNA is involved in the synthesis of these nuclear pro- 

 teins. But the reason why the ribonuclease-treated eggs never enter 

 into prophase remains obscure. One possibility is that ribonuclease 

 might interfere with the reduplication of the centrosome, which is 

 known to contain RNA. Such a hypothesis would agree with the 

 above-mentioned experiments of Shaver (1953) on the nature of 

 the "second factor" in parthenogenesis. But, for the time being, 

 this is no more than a hypothesis and further work is required in 

 order to test it experimentally. 



2. RNA AND THE INDUCTION OF THE NERVOUS SYSTEM 



As soon as it was shown by Bautzmann et ah (1932) that organizers 

 killed with alcohol, heating or freezing are still capable of inducing 

 a neural tube, it was realized that induction must be a chemical 

 process and attempts have been made to identify and isolate the 

 "active" inducing substance in pure form. Experiments by Weh- 

 meier (1934) and Holtfreter (1935) gave considerable hope that 

 such a goal might be reached. They found that the "inducing sub- 

 stance" (also called the "evocator") is a very widespread one. Al- 

 most all tissues of adult vertebrates and invertebrates, especially if 

 they have been killed beforehand, induce neuralization of the ecto- 

 blast if they are grafted into the blastocoele cavity of young 

 gastrulae. 



The next step, as was quickly realized by Needham, Waddington, 

 F. G. Fischer, Barth and others, was to try to isolate the inducing 

 substance from an adult tissue, e.g. liver. The results were, however. 



References p. 90/93 



