400 PRINCIPLES OF EMBRYOLOGY 



have originated by a comparatively slight alteration in the normal cell 

 particles, to which one would therefore have to attribute a capacity for 

 self-duplication. Finally, the tumour-viruses sometimes show extreme 

 specificity in the type of tissue they can infect; indeed Rose [19 sib) has 

 shown that a frog virus may be so specific that it travels through the body 

 and settles in only one particular region of the skeleton (cf, the reference 

 to regional specificity on p. 308). Now Rose has found that if the agent 

 can be caused to grow in an unusual tissue, it may acquire a new specificity 

 from it; its original specificity is sometimes retained, sometimes lost. 

 He suggests that the new character may be picked up by an interchange 

 process, akin to bacterial 'crossing-over' or 'transformation', with cyto- 

 plasmic constituents of the normal cells which are essentially similar to 

 the virus particles (but cf. Luria 1953). The suggestion is interesting, 

 and considerable developments may be expected in this field; but at 

 present the whole of this group of phenomena is so little understood that 

 it seems dangerous to use it as a basis for a general theory of differentiation. 

 For instance, according to Rose these tumour-inducing agents seem, after 

 passage through a number of different hosts, to lose nearly all their tissue 

 or even species specificity, which makes one wonder whether they may 

 not be damaged forms of the normal cell microsomes, the damage being 

 of such a kind as to render them insensitive to the normal control of the 

 nucleus; if this were so, they could not be taken as evidence that the cyto- 

 plasm of a healthy cell contains particles which are independent of the 

 nucleus. 



Another type of phenomenon, which may involve the activity of 

 plasmagenes, is that known as enzymatic adaptation (Reviews: Monod 

 1947, 1950, Monod and Cohn 1952, Spiegelman 1950, Gale and Davies 

 1953)- There is abundant evidence which strongly suggests (though 

 perhaps it does not completely prove) that in bacteria, yeasts and other 

 lowly organisms suitable for such studies, the formation of most enzymes 

 attacking exogenous substrates is specifically increased by the presence of 

 the substrate and in fact hardly occurs at all in its absence. The formation 

 of the adaptative enzymes involves the synthesis of protein, and the 

 physiology of the process has been studied by several authors (e.g. Spiegel- 

 man and Sussman 1952). The formation of an adaptive enzyme in the 

 presence of any particular substrate requires the activity of a corresponding 

 nuclear gene, and we therefore have here a good example of the influence 

 of substrates on the activities of genes. But it has been suggested that some- 

 thing more is involved. Lindegren and Spiegelman [Cold Spring Harbor 

 Syrrip. 1946) originally put forward the hypothesis that the production 

 of the adaptive enzyme was a property of a cytoplasmic plasmagene, 



