CHEMODIFFERENTIATION 



61 



placed it by the hypothesis of "determining substances" which 

 would determine the cells to a certain development. In recent 

 years, however, the view has gained ground that this conception 

 is still too schematic, and that the actual relationships are 

 probably even more complicated. Presumably the differentiation 

 of a cell is not determined by only one determining substance. 

 It is more likely that the direction taken by the development 

 of any cell depends on the mutual quantitative relations of a 

 great number of substances, which together govern the cell 

 metabolism, e.g. enzymes, vitamins, etc. Slight differences in the 



Fig. 24. Abnormal embryos, formed by centrifuged eggs of 

 ascidians. Left: living larvae; the lines mark the places where 

 muscle contractions have been observed. Right: the same animals 

 after fixation and benzidine treatment for the demonstration of 

 peroxidases. Positive reaction (dotted areas) in the regions where 

 muscle cells occur. After Ries, 



