BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 485 



believe that, after ribonuclease treatment, an increase in the percentage of 

 cephalic inductions can be obtained. It should finally be pointed out that 

 there is no conclusive indication that yeast PXA or mononucleotides can 

 ever induce the neuralization of epidermal explants (Brachet,^^ Kuusi^O • 



These negative and rather discouraging results are of course only what 

 one should expect, now that we know that any agent inducing a condition 

 of sublethal cytolysis will promote the neuralization of ectodermal explants 

 (Holtfreter'*^) ; they do not mean that pentosenucleoproteins do not play 

 an essential role in normal induction, since sublethal cytolysis certainly 

 induces PNA synthesis. As has been pointed out earlier in this chapter, it 

 is almost certain that they represent, on the contrary, very important mor- 

 phogenetic factors. The task which chemical embryology will face in the 

 future is to follow the chemical changes undergone by ectoderm subjected 

 either to a normal organizer or to agents producing the sublethal cytolysis 

 condition. In such a study, PNA metabolism, as well as the composition of 

 the microsomes, should remain in the foreground. 



The fact that heat shocks, as reported earlier, inhibit inducing processes 

 while they break down the microsomes, suggests that these particles are of 

 special importance in normal induction; in keeping with this suggestion is 

 the fact, discovered recently by Mookerjee,^^ that sublethal cytolysis pro- 

 duced by heat shock does not promote the neuralization of epidermal 

 explants. It should be pointed out, however, that all attempts made to 

 induce the formation of a secondary nervous system by injecting micro- 

 somes in the ventral part of a cleaving egg have so far yielded only negative 

 results (Brachet and Shaver,'^ Brachet, Gothic, and Kuusi^*). 



On the other hand, embryologists have often insisted on the similarities 

 existing between normal inductive processes and virus infections (Dalcq,^^ 

 Needham,^^ Brachet,'^ etc.) : for instance, once the neural plate has been 

 induced, it becomes in turn an inductor (homeogenetic induction). It is 

 tempting to imagine that the microsomes, which have so much in common 

 with viruses as regards their size and chemical composition, behave like 

 viruses, i.e., as self-duplicating units, capable of spreading from cell to cell 

 (Holtfreter^^). This hypothesis has already found a certain amount of ex- 

 perimental support (Brachet^^) : for instance, if a cellophane membrane is 

 placed between an organizer and a piece of presumptive epidermis, induc- 

 tion is completely suppressed wherever the ectoderm is protected from the 

 organizer by the membrane. Similar results for the induction of the lens by 



" S. Mookerjee, Experientia 9, 340 (1953). 



" J. Brachet and J. R. Shaver, Experientia 5, 204 (1949). 



^* A. Dalcq, "L'oeuf et son dynamisme organisateur." A. Michel, Paris, 1941. 



65 J. Holtfreter, Symposia Soc. Exptl. Biol. 2, 17 (1948). 



" J. Brachet, Experientia 6, 56 (1950). 



