376 PRINCIPLES OF EMBRYOLOGY 



with the importance of RNA in protein synthesis (c£ Serra 1949, Schultz 



1952). 



The 'inactivation' of the genes in these 'unstable' chromosome re- 

 arrangements is probably to be understood as a mutation to an inactive 

 allelomorph, since in certain cases at least it may occur in germinal tissue 

 and then breed true in the inactivated form. It has been suggested by 

 McClintock (195 1) that differentiation might depend on the occurrence in 

 the different tissues of gene-mutations controlled by some mechanism of 

 this sort, involving an interaction of heterochromatic and euchromatic 

 segments of the chromosomes. But the hypothesis appears rather far- 

 fetched. In the examples known at present the mutations occur in a dis- 

 orderly fashion, giving rise to flecks and spots which have little relation 

 to the main anatomical features of the organism. Moreover, to explain 

 differentiation we should need not only the orderly mutation of one gene, 

 but of the whole complex set of genes active in the tissues concerned. 



Another mechanism which may be related to gene mutation should 

 be mentioned. Some years ago Avery showed that the characters of certain 

 strains of bacteria {Pneumococcus) can be transferred to other strains by 

 cell-free extracts. The strain thus 'transformed' continues to multiply in 

 its new form. The 'transforming principles' have been shown in some 

 cases to be composed of DNA, apparently unmixed with compounds of 

 any other type. A considerable amount of work has been done on the 

 genetical analysis of this most interesting phenomenon (cf. Ephrussi- 

 Taylor 195 1) but the mechanism of the transforming action is still quite 

 obscure: the principles may operate by inducing specific gene mutations 

 or in some other way. There are obvious analogies between these bacterial 

 transforming principles and embryonic evocators, but there is httle means 

 of deciding as yet whether these are merely formal parallels or whether 

 there is in fact any important similarity between the two mechanisms. 

 The occurrence of embryonic induction by unnatural evocators would at 

 first make it seem unlikely that the two phenomena are closely related; 

 but this argument would have little force if the unnatural evocators 

 operate by setting free the normal evocator from an inactive complex. 



3. Arguing from the substance to the genes 



The alternative mode of approach to the problem of the production of 

 substances by genes is to identify the substances and to try to trace them 

 back to the genes. A very great deal of information has been obtained in 

 this manner, but there is a fundamental difhculty in deriving a complete 

 theory in this way, because we can never be certain, as we trace a substance 

 back through its precursors, that we have reached the last stage from 



