100 ROLE OF THE CELL NUCLEUS 



vesicle, epididymis, nerve cells), but Pelc (1956) does not state 

 whether the radioactivity of the nuclei is higher than that of the 

 cytoplasm or not. There is one instance, however, in which the 

 nuclei always show a much higher radioactivity than the cytoplasm 

 after treatment with a labeled amino acid. This occurs, as already 

 mentioned, during the early development of amphibian and avian 

 embryos (Brachet and Ledoux, 1955; Waddington and Sirlin, 1954; 

 Sirlin, 1955). This is not surprising since, in these embryos, we are 

 dealing with very actively dividing cells; extensive synthesis of 

 nuclear proteins is only to be expected under such circumstances. 



It is interesting to note, however, that according to a recent 

 autoradiographic study by Sirlin and Waddington (1956), incorpo- 

 ration of amino acids into proteins of chick embryos is highest in 

 the nucleoli, the nucleolus-associated chromatin and the cytoplasm. 

 Such conclusions are in obvious agreement with Caspersson's (1941, 

 1950) ideas. 



On the other hand, in recent experiments, Carneiro and Leblond 

 (1959) were unable to find any incorporation of tritium-labeled 

 amino acids into nucleolar proteins of many cells. 



in conclusion, present autoradiographic experiments suggest that 

 there are considerable differences between various cells as far as 

 protein metabolism is concerned. Except for rapidly dividing cells, 

 protein anabolism is not necessarily stronger in the nucleus than in 

 the cytoplasm. On the other hand, a very active RN A metabolism in 

 the nucleus is the general rule. We shall now try to ascertain whether 

 similar conclusions can be drawn from the merotomy experiments 

 performed on Amoeba proteus and Acetabularia mediterranea. 



b. Amoeba proteus 



In Amoeba proteus y the removal of the nucleus is quickly followed 

 by loss of motility; even after a few minutes, pseudopod formation 

 becomes sporadic and the enucleate halves soon become spherical. 

 They are unable to feed on living prey, in contrast to normal 

 amoebae or nucleate halves, but they survive almost as long as the 

 latter; if both are kept fasting, enucleate halves may survive for as 

 long as 2 weeks, while nucleate ones die after 3 weeks. 



