46 Discussion 



I think that interactions with the environment are of negligible im- 

 portance, but that we should separate these things quite clearly from one 

 another. 



Best: I can see, Prof. Danielli, that you were driven to the experiment 

 of preventing your unicellular organism from dividing. But you had to 

 do something different to accomplish that, you had to make a change. 

 Can you do this in a variety of ways and get the same result every time ? 



Danielli : The experiment I have described to you was done simply by 

 restricting the amount of food that was provided. I cannot say whether 

 it can be done in other ways, though later I may be able to do that. 



Tunbridge: I understand you have taken cells of different ages and 

 transferred the nucleus, putting it into cytoplasm of a different age ; to 

 what extent has such work enabled you to determine the duration of life 

 of the cytoplasm of the cell ? 



Danielli: We have not done any work yet on nuclear transplantation 

 in this particular connection. The experiments I described to you were 

 made to define the field where such operations were possible. 



Olbrich : Does the fluid content diminish in these ageing cells of yours ? 



Danielli: No, there is no characteristic change in that, as far as our 

 measurements have gone, but we have not made accurate assessments 

 of dry weights of single cells. 



Best: I don't quite follow that one-half of a dividing cell is old and 

 one-half new. 



Danielli: Of the macromolecules which are present in the cell just 

 before it divides, at least half (since it is doubled in dry weight) will be 

 new macromolecules. 



Best: But these will be divided between the two cells? 



Danielli: Perhaps you could even, in some cases, take a single 

 macromolecule and say half of it is new and half of it is old. But since 

 the amount of living matter has doubled and it is doubled in a finite length 

 of time, say two days, the life of at least half of that new cell has been 

 only two days. 



Comfort: Have you any information about those ciliates, and other 

 such organisms, in which you can identify which half of the dividing cell 

 gets the old macromolecule and which half gets the new ? I understand 

 that in some cases you can see which is the daughter cell which gets the 

 new organelle and which is the mother cell which gets the old one. 



Danielli: French protozoologists, e.g. Lwoff, have done fascinating 

 work on that. As far as I recall, the usual thing that happens with 

 kinetosomes is that the daughter cells will both get some old kinet- 

 osomes, but then as the new organism grows after cleavage, each of these 

 will multiply in some way, so that after division has taken place 

 repeatedly you have no cells which contain 100 per cent old kinetosomes. 



Comfort: In an organism like Stentor, you could make a form of 

 life table. 



Danielli: Yes, that could be done by isotopic labelling and would be 

 an interesting experiment. 



Landowne: To what extent could the cell that did not divide be 

 considered to be old because of the variable percentage of replaceable 



