404 PRINCIPLES OF EMBRYOLOGY 



the result of a plasmagene initiated in the egg cytoplasm by the maternal 

 genes during the maturation of the egg. One may conclude that there is 

 hardly any evidence that plasmagenes with complete autonomous genetic 

 continuity exist in metazoan eggs, and that it seems most improbable that 

 the major phenomena of differentiation can be attributed to them. 



The same conclusion appHes even more forcibly to the plasmagenes of 

 category (3), namely microscopically visible cytoplasmic particles with 

 genetic continuity. These certainly occur in certain special cases, as for in- 

 stance in ciliates, but in general the histological evidence makes it clear that 

 differentiation does not consist to any large extent of the mere sorting out 

 of the already existing visible particles in the egg cytoplasm. Such particles 

 probably play an important part in development, but not by the mere 

 retention of their original characteristics. 



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 particu- 

 lar 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 removed or 

 changed. 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 differ- 

 entiation 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 suggestion. As was pointed out earlier (p. 212), 

 Brachet (1944, 1952^) has argued with considerable persuasiveness for the 

 importance of the ultra-centrifugable ribose-nucleic-acid-containing 

 microsomes, and he does not hesitate to refer to these as plasmagene-like 

 in character. The problem that still remains at issue is how far these parti- 

 cles, once their character has been determined, become independent of the 

 nucleus. Only the transplantation either of the nuclei or of the particles 

 from one type of differentiating cell to another can settle the matter con- 



