CELL NUCLEUS AND MORPHOGENESIS 89 



1954) and Urodeles (Zeller, 1956), cytochemical methods demon- 

 strate the presence of an excess of RNA in the nuclei of the blocked 

 hybrids. It looks as if an abnormal RNA, which cannot be utiUzed 

 by the cytoplasm, is synthesized by the foreign nucleus. More will 

 be said about the relationships of nuclear and cytoplasmic RNA in 

 the next chapter. 



Before we leave the problem of morphogenesis, a final problem 

 should be briefly discussed (see Brachet, 1957, 1960, for more 

 detailed discussions). Do the nuclei undergo some sort of differen- 

 tiation during development? Such a differentiation has been 

 postulated by Morgan (1934), who advanced the view that during 

 cleavage equipotential nuclei (thus undifferentiated nuclei) would 

 be distributed in a chemically heterogeneous cytoplasm. Owing to 

 this heterogeneity of the surrounding cytoplasm, genes would be 

 activated in certain nuclei and inactivated in others. The nuclei 

 would then no longer be equipotential but differentiated. Differ- 

 ences in genetic activity among the nuclei would, in turn, produce 

 deeper modifications in the chemical composition of the surrounding 

 cytoplasm, and would ultimately lead to embryonic differentiation. 



This theory is well supported by the most remarkable experi- 

 ments of "nuclear transfer" performed by Briggs and King (1953, 

 1955, 1957). They were able to inject into an enucleate recipient of 

 unfertilized eggs, the nucleus from a cell taken at the blastula, 

 gastrula or neurula stage. Their main result is that nuclei are equi- 

 potential before gastrulation and that they are still undifferentiated 

 at this stage, since they can support normal and complete develop- 

 ment. But the nuclei become different from each other and become 

 irreversibly differentiated when a late gastrula stage is reached. 

 Autoradiographic observations on ^^COa incorporation by Tencer 

 (1958) show that nuclear differentiation is closely linked to changes 

 in metabolic activity. In the blastula, despite the existence of the 

 animal-vegetal gradient, all the nuclei become labeled to the same 

 extent. In the gastrula, on the other hand, the intensity of the label- 

 ing in the various nuclei becomes parallel to the intensity of the 

 cytoplasmic RNA gradients. 

 There is thus no doubt that, in the gastrula, strong interactions 



References p. 90/93 



