1,18 INDUCTION AND ORGANISATION 



chemical nature of certain of the extracted products, and even 

 to obtain inductions with synthetic preparations of such sub- 

 stances. But a wide variety of organic compounds, such as fatty 

 acids, nucleotides and sterols, had the same effect. The con- 

 clusion was inevitable that no specific chemical actions are 

 involved, but that the ectoderm will react to the most diverse 

 influences with the same reaction, namely, the formation of 

 a neural plate. Recent experiments by Holtfreter (1945-47) 

 have made it likely that an injury to the host tissues is the 

 primary effect in many cases of so-called induction by means 

 of chemical substances, or of dead, or foreign tissue. The injury 

 is supposed to increase the water permeability of the ecto- 

 dermal cell walls, and this, in turn, to lead to enzymatic 

 decomposition of certain components of the cytoplasm. 

 Presumably, substances are thereby liberated, which direct the 

 differentiation of the cells into a course leading to a neural 

 plate. This makes it unlikely that experiments with such 

 abnormal agents can be used to deepen our insight into the 

 normal process. 



In later years, therefore, another course has been followed, 

 which took V/oerdeman's results (p. 106) as its starting point. 

 Woerdeman found a very rapid decomposition of glycogen in 

 cells of the marginal zone that were being rolled in. Heatley 

 and Lindahl (1937) confirmed this, using quantitative micro- 

 analytical methods. Although it was found that glycogen itself 

 could not be regarded as the evocator, yet attention was now 

 called to the metabolic processes in the marginal zone material. 

 Several investigations, in particular those of J. Brachet, showed 

 that the cell metabolism in the organisation centre differs from 

 that in the rest of the germ. Both the COo production, and the 

 respiratory quotient have higher values. Further research in- 

 dicated that the main role is played here by protein metabolism, 

 and not by carbohydrate metabolism. Attention was drawn to 

 the sulphydryl compounds which are highly important in the 

 oxidation and reduction processes in the cell, and later also 

 to the ribonucleic acids, with which the sulphydryl compounds 

 are closely linked. J. Brachet (1938, 1942) proved that in amphi- 

 bian embryos these compounds are specially accumulated at 



