EXPERIMENTAL RESULTS 127 



A recent autoradiographic study of Abd-el-Wahab and Pante- 

 louris (1957) leads, however, to somewhat different results from 

 those which have just been described for sea urchin and newt eggs. 

 These workers found that the isolated polar lobes of the mussel 

 Mytilus (i.e. enucleate cytoplasm) show a considerable and rapid 

 decrease in the incorporation of amino acids and adenine in the 

 insoluble materials. No certain conclusion can be drawn from these 

 experiments, because the authors did not study the uptake and 

 concentration of the soluble precursors. It might well be that the 

 permeability of the polar lobe (which is known to differ strongly 

 from the rest of the cytoplasm by its viscosity and its low respiration) 

 to the amino acids and purines is abnormally low. 



The present evidence shows that there are quantitative, rather 

 than qualitative, differences in the effects produced by nucleus 

 removal on RNA and protein metabolism. In Acetabularia, when 

 optimal conditions are chosen for regeneration, protein and per- 

 haps RNA synthesis can be stimulated in enucleate fragments. The 

 synthesis of specific proteins, including probably hemoglobin, 

 whose synthesis is genetically controlled, apparently occurs in the 

 enucleate reticulocytes ; the turnover of RNA and protein is not 

 affected by removal of the egg nucleus. In amoebae, the protein 

 and, especially, RNA synthesis and content decrease markedly in 

 enucleate fragments. In all cases, RNA is more affected by enuclea- 

 tion than protein, and the long-term effects of nucleus removal are 

 inhibitory for both protein and RNA anaboHsm. 



A few words should now be said about results obtained by the 

 third method of study, i.e. the isolation of nuclei by centrifugation 

 of an homogenate. 



e. General properties of isolated nuclei 



The results of the many experiments in which precursors of either 

 RNA or protein were given to a Ifving animal whose liver was then 

 homogenized and centrifuged can be summarized in a few sentences. 

 Since Marshak's (1948) and Jeener and Szafarz's (1950) pioneer 

 experiments, it has been repeatedly confirmed that, whatever the 

 precursor, incorporation proceeds much faster into nuclear RNA 



References p. 133/135 



