The cell physiology of early development 



The situation is rather different when we turn to the fourth category, that of gene- 

 initiated plasmagenes. If these were to play an important part in development we 

 should have to imagine that the various ooplasms of the egg differentially excite the 

 nuclei which enter them; that the particular genes which are activated in a given 

 region then cause the appearance of cytoplasmic factors, and that these factors, when 

 they have appeared, show a certain degree of autonomy, being able to reproduce 

 for a short time with repetition of their character even if the nucleus is later removed. 

 If one supposes that, once they have been formed, the autonomy of the plasmagenes 

 is complete, this suggestion would come up against the same difficulties as confronted 

 the hypothesis of organ-forming substances in accounting for the sequential character 

 of differentiation, and phenomena such as the metaplasia of retinal cells into lens in 

 Wolffian regeneration. We have seen, however, that in the best-studied examples of 

 gene-initiated plasmagenes the autonomy is by no means complete. If one waters it 

 down sufficiently, the difficulties which have just been mentioned could be overcome. 

 The hypothesis would then amount to the suggestion that during differentiation the 

 genes cause the appearance in the cytoplasm of bodies with a certain limited amount 

 of autonomy. There seems nothing impossible, or even very difficult, in such a sug- 

 gestion. Brachet (1944, 1952) has urged it with considerable persuasiveness. He 

 points to a type of cytoplasmic particle, the ultra-centrifugable ribose-nucleic acid- 

 containing microsomes, which he supposes to be the plasmagene-like factors in ques- 

 tion. The problem that still remains at issue is how far these particles, once their 

 character has been determined, become independent of the nucleus. Only the trans- 

 plantation either of the nuclei or of the particles from one type of differentiating cell 

 to another can settle the matter conclusively. There would, in my opinion, be nothing 

 surprising if experiment eventually showed that, in cells which are more or less com- 

 pletely determined and are in process of producing their final cytoplasmic consti- 

 tuents, the cytoplasm is able to carry on in synthesizing these more or less indepen- 

 dently of the nucleus. However, such a fact does not yet seem to have been demon- 

 strated. Whether, if it were, we should be justified in speaking of the effectiveness of 

 plasmagenes in differentiation would be largely a matter of definition; it would 

 depend on whether we are satisfied that such gene-initiated cytoplasmic factors of 

 limited autonomy are comparable to plasmagenes of the more classical kind. 



Turning now from plasmagenes, let us consider some of the more general charac- 

 teristics of the physiological processes in a developing cell. The basic fact which we 

 have to try to understand is that different cells in the body, although presumably all 

 containing the same genes, yet differentiate into quite different tissues. The funda- 

 mental mechanism must be one by which the different cytoplasms, or ooplasms, 

 which characterize the various regions of the egg, act differentially on the nuclei 

 so as to encourage the activity of certain genes in one region, of other genes in other 

 places. Such specific activation of particular genes at certain times and places can 

 actually be observed visually in favourable cases, for instance, in the important work 

 of Pavan (1954), Mechelke (1953) and Beerman (1952) on the polytene chromo- 

 somes in various tissues of chironomids. The fact of differential activation of genes is, 

 then, scarcely in doubt. But there has been as yet little discussion of how we may 

 envisage such a process in chemical terms. There are innumerable different types of 

 kinetic system which might be supposed to be in operation, but it is worth some further 



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