98 ROLE OF THE CELL NUCLEUS 



taken if the nuclear transplantation is to be successful. None of the 

 many media tried by Briggs and King (1953) has proved satisfactory 

 so far. Until a medium is found where nuclei can be kept for a 

 long time without losing their biological activities, experiments 

 made on nuclei isolated from homogenates will have limited bio- 

 logical interest only. Such a medium will probably be difficult to 

 find, since we know from the work of Cutter et al. (1955) that the 

 nuclei which swim freely in coconut liquid endosperm are only 

 capable of degenerative amitosis. 



3. EXPERIMENTAL RESULTS 



a. Intact cells 



As already mentioned, autoradiography, despite its shortcomings, 

 is the only method which can give information on what happens 

 in situ at the cytological level. 



All the autoradiographic experiments published so far clearly 

 establish the greater metabolic lability of nuclear RNA compared 

 to that of cytoplasmic RNA. The autoradiographic observations of 

 Ficq (1955a, b) on starfish and frog oocytes have introduced an 

 important point; the rapid incorporation of labelled adenine or 

 orotic acid into RNA is especially characteristic of the nucleolus. In 

 the germinal vesicle of amphibian oocytes, however, the incorpo- 

 ration of radioactive adenine also proceeds very quickly in the RNA- 

 containing loops of the lampbrush chromosomes (Brachet and 

 Ficq, 1956). In Acetabularia also, autoradiographic observations 

 indicate a very rapid incorporation of adenine into the nucleoli 

 (Fig. 32, p. 109). 



Similar observations have been made with radioactive phosphate 

 as a label, by Vincent (1954) for starfish oocytes, by Odeblad and 

 Magnusson (1954) for mouse oocytes, by Stich and HammerUng 

 (1953) for Acetabularia, and by Taylor (1953, 1954) and Taylor and 

 McMaster (1955) for insect glands. 



There is thus no doubt that the nucleus — and especially the nu- 

 cleolus — is the site of a particularly active RNA anabolism. All 

 these observations are obviously compatible with the view that 



