448 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



afford macromolecules able to penetrate into the cells of the ectoblast offered to them, and to per- 

 turbate their inner metabolism. It is plain that a sheet of competent ectoblast can only 

 behave in a limited number of ways. It can stretch in an undifferentiated epiblast 

 layer; or proceed along the above described events of notogenesis; or adopt the ac- 

 tivities leading to acrogenesis; or — not to neglect more exceptional eventualities — 

 develop either into a deutomerit, either into a (chordo)mesoblast layer or a spinal 

 chord. Each of these potencies needs, for its realisation, quite an array of reactions 

 among metabolites and enzymes. Any kind of macromolecules which are apt to 

 penetrate into the ectoblast cells, not being destroyed in this new milieu, and inter- 

 fering with some of the reactions, will be likely to shift the trend of biochemical 

 processes along one of the two or three progressive ways open to them. If this 

 speculation has some value, it would explain the astonishing extension of the 

 inductive properties, their frequent variability from species to species, their 

 occurrence even in vegetal tissues. The patient search for xeno-inductors, which 

 has absorbed such a huge work, would amount to detecting the macromolecvdes 

 able to penetrate the cells of the reactor and perturbate their inner equilibria in 

 a non-lethal but profitable way, with the resulting haphasard choice of destinies. 

 From the available data, it appears that such macromolecules would be preferably 

 either (ribo)nucleoproteins or simple proteins. It is likely that the normal in- 

 ducing agents appertain to the same biochemical categories, but this does not 

 imply any real identity. 



These remarks mean that the biochemical problem of induction has certainly 

 benefitted a lot from the study of xeno-inductors, but imperiously needs a direct 

 attack at the embryological level. Personally, I am inclined to admit, as a common 

 feature of xeno- and auto-inductors, the one-way transformation of the notogenic 

 into the acrogenic agent (s). In my opinion this process is the characteristic event 

 which takes place in the normal prechordal primordium. I consider that agents 

 isolated by the Nagoya workers from the kidney and the liver of well-fed guinea- 

 pigs are the best available models of the two normal inductors. On one hand, they 

 preexist in the organs as ribonucleoproteins, and are probably synthetized as such, 

 on the other hand, it has recently appeared that their active protein moiety 

 shifts from the noto- to the acrogenic eflfect by becoming more stable, reducing its 

 molecular size and acquiring more ability for cell-to-cell diffusion. Even if the 

 intimate nature of the xeno- and auto-agents is different, these properties seem to 

 be immediately transposable to the normal inductor as present in the archenteron 

 roof. Before trying to make a further and last step in this theoretical discussion, we 

 must acquire additional knowledge of the secondary and minor inductions. 



E. Secondary and mi?ior inductions 



In considering the primary aspect of neurogenic induction, I have tried to con- 

 centTP le on recent data and especially on the results which help us to understand 

 the iniLiction of the normal neural plate. This procedure should a fortiori be 

 followed for inductions of a secondary character. From this selective and restricted 

 standpoint we shall briefly review the main advances in this active field of research. 



