PLASMAGENES 403 



one of whose major components consists of genes which we beheve to 

 retain their identity throughout. The difficulty is only made the greater if 

 we have to suppose that the major factors in the cytoplasm also retain 

 their identity. 



As a third argument, one may point to the fact that the localisation of 

 different organs within the developing body may often be altered by 

 factors which operate after the segregation of plasmagenes in the egg cyto- 

 plasm must have been completed. For instance, one might be tempted to 

 attribute the localisation of the organs in a developing Drosophila to the 

 segregation of organ-forming substances or plasmagenes in the eggs, 

 which are known to belong to the mosaic type; yet we have seen (p. 141) 

 that environmental treatments apphed many hours after fertihsation can 

 divert the differentiation of particular regions into abnormal paths. 



The mere fact that a gene, like aristopaedia, can cause a mass of tissue 

 which should normally develop into an antenna to develop into a leo- 

 instead, shows that even if we try to attribute the major process of differ- 

 entiation to plasmagene-like bodies, these cannot be autonomous in their 

 properties but must be highly susceptible to modifications caused by inter- 

 action with genes. Finally, it would be still more difficult for a hypothesis 

 which attributed differentiation to be activities of autonomous plasma- 

 genes to account for metaplasia, which, though rare, does seem to occur 

 (p. 308). 



It appears, therefore, that the postulation of true plasmagenes as organ- 

 forming substances in the cytoplasm of the egg does not materially simpli- 

 fy the theoretical task of accounting for the phenomena of differentiation. 

 That does not necessarily mean, of course, that such bodies do not or can- 

 not exist; we should have to take account of them if there was unequivocal 

 evidence for the existence in the eggs of multi-cellular animals of cyto- 

 plasmic factors which had genetic continuity independently of the nucleus. 

 As yet there seems to be no compelling evidence to tliis effect, hideed, 

 attempts to assess the autonomy of cytoplasmic factors in the egg over 

 against the nucleus have been few and far between. The studies with 

 hybrid merogons in Amphibia (p. 358) are perhaps the most promising. 

 The facts there can probably all be accounted for in terms of a mere per- 

 sistence of cytoplasmic character, without the need to postulate that the 

 cytoplasmic factors can reproduce while retaining their specific nature. 

 The case which argues most strongly in the opposite direction is perhaps 

 that of Hadorn (1936), who found that epidermis derived from Tritums 

 palmatHS cytoplasm fertilised by T. cristatus sperm developed the typical 

 characteristics o£palmatus as late as after metamorphosis. This may indeed 

 be evidence of the existence of a true plasmagene, but it might equally be 



