520 MOLECULAR MECHANISMS OF DIFFERENTIATION 5 



more or less purified chemical compounds were hailed with considerable enthu- 

 siasm. However, as these investigations continued it became apparent that a great 

 V'ariety of chemically unrelated and rather simple substances would elicit a response 

 in the primitive ectoderm. But these observations did not provide further insight 

 into the nature of the actually occurring embryonic inductive processes. A role of 

 unspecific cytoplasmic agents in induction was suggested by Holtfreter (ig48b and 

 1955). It is difficult to say at the present time whether the effects of various 

 chemical fractions obtained during this valiant search are actually without 

 biological significance. However, even if the failure of such a search for an 

 "Organizer" substance was only an apparent one, it gave the problem of in- 

 duction the appearance of even greater inaccessibility than it had before the 

 start of this biochemical Odyssey. 



In view of the pivotal significance of induction, several lines of research have 

 been recently initiated to establish a sounder basis for a renewed attack on this 

 knotty problem. Most important seems the progress made in the biological analysis 

 of the induction phenomenon. Through the ingenious experimentation of Nieuw- 

 koop and his associates (Nieuwkoop et al., 1952; Eyal-Giladi, 1954; Nieuwkoop 

 andNigtevecht, 1954; Nieuwkoop et al., 1955; Nieuwkoop, 1955) it could be shown 

 that the transformation of the primitive ectoderm cell into a highly specialized 

 cell can be divided into two phases. First, a stimulus emanating from the mesoderm 

 underlying the primitive ectoderm elicits a process in the ectoderm itself which, 

 without further control, leads to the development of pro-encephalic structures 

 in all parts of the ectoderm that react to this initial stimulus (activation). 



The second principle supplied by the mesodermal primordium becomes 

 effective in controlling the response of the ectoderm resulting in the formation of 

 structurally and functionally different components of the central nervous system 

 (Nieuwkoop's "transforming principle"). The two principles contributed by the 

 mesoderm can be experimentally distinguished. During the development of the 

 mesoderm the appearance fo the "activating" principle precedes the appearance 

 of the "transforming" principle and also the spatial distribution of the two factors 

 is different in the mesodermal primordium. Also the activating factor is heat 

 stable, while the "transforming" principle is strictly specific and heat labile. 



Apart form this exemplary re-examination of the biological characteristics of 

 neural induction, several other biological approaches have been pursued which 

 may eventually provide a fruitful basis for chemical investigations. Notable are the 

 advances of Grobstein (1955a, b, 1956 and 1957) and Grobstein and Holtzer (1955) 

 in establishing conditions for very striking induction phenomena in vitro. Zwilling's 

 (1955; 1956a, b, c, and d) successful experiments in removing and transplanting 

 the apical ectoderm of the limb bud may provide possibilities for the study of the 

 mechanism of the effect of this tissue on the limb bud mesoderm. Interactions 

 between neural tube tissue and somite mesoderm have been analyzed in greater 

 detail by Holtzer and Detwiler (1953 and 1954) ; Watterson et al. (1954) ; Avery, 

 Chow and Holtzer (1956) and S. W. Holtzer (1956) and the dependence of meso- 

 dermal differentiation in amphibians upon the cellular environment demonstrated 

 experimentally by Yamada (1937 and 1940) was elaborated by Muchmore (1951). 

 The intriguing phenomenon of production of lens proteins by iris cells of adult sala- 



