MATERIALS AND METHODS 97 



tial centrifugation of homogenates. A system consisting of micro- 

 somes and mitochondria has been very useful to Siekevitz (1952) in 

 his studies on the in vitro incorporation of amino acids into pro- 

 teins. Vishniac and Ochoa (1952) have studied, with advantage, the 

 biochemical events which occur when chloroplasts are mixed with 

 mitochondria isolated from animal tissues. Potter et al. (1951) and 

 Johnson and Ackermann (1953), in an effort toward an understand- 

 ing of the biochemical role of the nucleus, have followed the effects 

 of the addition of nuclei on oxidative phosphorylations in mito- 

 chondria. They found that, although the nuclei themselves lack 

 respiratory enzymes, they stimulate phosphorylation reactions. Ex- 

 periments of that type have definite biochemical interest and value 

 but are useless when the nature of the interaction between nuclei, 

 mitochondria and ergastoplasm in the intact living cell is our ob- 

 jective. For instance, stimulation of mitochondrial oxidative phos- 

 phorylations by the addition of nuclei may simply be caused by an 

 autolytic release of enzymes or cofactors from the nuclei ; such a 

 phenomenon might never occur in the nucleus of a living cell. In 

 fact, the indications given by this type of homogenate experiment 

 may be totally misleading if the importance of structural integrity 

 inside the cell is neglected. 



In the case of the nuclei at least, there is another reason for doubt- 

 ing the value of the experiments performed on mixed fractions re- 

 covered by differential centrifugation of homogenates. If we take 

 as a test of survival for isolated nuclei the capacity of dividing when 

 they are reintroduced into cytoplasm, there is no doubt that isolated 

 nuclei are very quickly inactivated by contact with the outside me- 

 dium. In experiments by Comandon and De Fonbrune (1939) on 

 the transplantation of a nucleus to an enucleate Amoeba fragment, 

 very close contact between the cell surfaces of the donor and the 

 receptor amoebae is indispensable; the slightest contact of the nu- 

 cleus with the outside medium results in "death" and elimination 

 of the transplanted nucleus. The same is true for the embryonic 

 nuclei transplanted by Briggs and King (1953) into enucleate un- 

 fertilized frog eggs; the nucleus must be protected against the 

 medium by the crushed cytoplasm of the cell out of which it is 



References p. 1331135 



