8 J. Bracket 



radioactivity of the various constituents of its liver cells 

 (nuclei, mitochondria, microsomes and supernatant as obtained 

 by differential centrifugation). This technique is open to some 

 criticism. Results of the Mir sky group show that the methods 

 of preparation of isolated nuclei used entail serious losses of 

 some nuclear proteins. In this manner histones have been 

 shown to incorporate amino acids only slowly, while the rest 

 of the nuclear proteins do not differ much in activity from the 

 whole of cytoplasmic proteins. But it must always be kept 

 in mind that the current preparation processes may well 

 extract from the nuclei some proteins of considerable meta- 

 bolic importance. 



In the light of these objections, we have taken up a dif- 

 ferent aspect of the same problem, one which appears more 

 worthwhile from a biologist's standpoint. Together with a 

 group of co-workers, we have investigated protein meta- 

 bolism in nucleated and non-nucleated halves of unicellular 

 organisms. We have deliberately selected two widely separ- 

 ated species : the amoeba (Amoeba proteus), the non-nucleated 

 halves of which cannot regenerate, and the giant unicellular 

 alga, Acetabularia mediterranean in which the non-nucleated 

 stems remain capable of extensive regeneration, as shown by 

 the classical work of Hammerhng (1934, 1953). We shall next 

 consider the results obtained in both cases. 



If one cuts an amoeba into half, the non-nucleated frag- 

 ment soon rounds up and stops feeding. The nucleated half 

 keeps behaving normally and, if fed living micro-organisms, it 

 can resume growth and divide. Since the biochemical changes 

 in both halves should be studied under comparable conditions, 

 both fragments must be kept fasting in the course of the 

 experiment. Under these conditions the non-nucleated halves 

 remain alive for 10-15 days and the nucleated fragments for 

 about 3 weeks. 



Our experiments have led us to the following conclusions. 

 As already pointed out, the oxygen consumption of non- 

 nucleated halves remains unaffected, but their ATP content 

 rises aerobically. Under anaerobic conditions, however, 



