484 J. BRACKET 



is already present in the presumptive epidermis, in the form of an inac- 

 tive complex: evocating agents act in breaking down this inactive complex 

 and in unmasking the active substance. This viewpoint has been further 

 strengthened by Holtfreter's''^ important experiments, showing that neu- 

 ral ization occurs in isolated pieces of presumptive epidermis whenever 

 they undergo a certain type of sublethal cytolysis: a shift in the pH of the 

 culture medium, either towards the acid or the alkaline range, is enough to 

 produce spontaneous neuralization in almost all of the ectodermal ex- 

 plants. In more chemical terms, the whole process can be visualized by 

 assuming that the inducing substance is the prosthetic group of some inac- 

 tive conjugated protein; any agent producing a reversible denaturation of 

 the protein and liberating the prosthetic group will act as an unspecific 

 evocator. 



It is obvious, in these circumstances, that the search for a single specific 

 evocating substance becomes a rather hopeless proposition. We shall, how- 

 ever, briefly summarize what is known about PNA as an evocating sub- 

 stance, since this is still a controversial subject. 



It has been shown (Brachet^^ '*^) that pentosenucleoproteins, especially 

 after alcohol treatment, are very good evocators, whatever their origin or 

 their method of preparation; it was claimed that this evocating activity 

 was markedly decreased if, prior to their implantation into gastrulae, the 

 nucleoproteins were treated with ribonuclease. But later work in the 

 writer's laboratory (Kuusi,^^"^^ Brachet, Gothic, and Kuusi^^), while con- 

 firming that pentosenucleoproteins (tobacco mosaic virus or alcohol-fixed 

 microsomes, for instance) are excellent evocators, failed to substantiate the 

 claim that ribonuclease abolishes the activity of these substances. It is 

 probable that Brachet 's^*-^* earlier results were due to proteolytic con- 

 taminants in the ribonuclease preparations used: such an explanation is 

 especially likely since it has been shown meanwhile that proteolytic en- 

 zymes inactivate various evocators (Toivonen''^), while blocking the — NH? 

 groups with formalin (Lallier^") or ketene (Kuusi''^) markedly decreases the 

 evocating activity of various pentosenucleoproteins. 



Recently, Englander et a/.^' have confirmed that ribonuclease has little 

 inactivating effect on the evocating power of various tissues; they even 



« J. Holtfreter, /. Exptl. Zool. 106, 197 (1947). 



^^ J. Brachet, Bull. acad. roy. Belg. 29, 707 (1945). 



*^ T. Kuusi, Experientia 7, 299 (1951). 



^* T. Kuusi, Ann. Zool. Soc. Zool. Botan. Fennicae Vananio 14, 1 (1951). 



" T. Kuusi, Arch. biol. (Liege) 64, 189 (1953). 



^8 J. Brachet, S. Gothic, and T. Kuusi, Arch. biol. (Liege) 63, 429 (1952). 



*^ S. Toivonen, Ann. Zool'. Soc. Zool. Bot. Fennicae Vanamo 4, 28 (1949). 



s" R. Lallier, Experientia 6, 92 (1950). 



" H. Englander, A. G. Johnen, and W. Vahs, Experientia 9, 100 (1953). 



