Discussion 23 



staining, etc., Goldstein and Plant found that all of the label could be 

 removed both from the nucleolus and the cytoplasm by ribonuclease, it 

 seems that what has been marked really is RNA. But I do not think 

 that the experiment shows more than the fact that cytoplasmic RNA 

 is labelled under these conditions. We cannot be sure that the nuclear 

 RNA has gone directly into the cytoplasm, because it is quite possible 

 that intermediary biochemical stages break down this RNA. Goldstein 

 and Plant say that RNA does not go the other way round, i.e. from the 

 cytoplasm to the nucleus. But, to be really sure, a normal nucleus 

 should be introduced into very strongly labelled cytoplasm, and this 

 experiment has not been done. 



As regards Dr. Popjak's first question, I believe that quite a number 

 of changes occur in a non-nucleated Amoeba. Why they ultimately die 

 off, I do not know, but this is a slow, progressive process; the overall 

 lifespan is always somewhat lower without the nucleus than with it and 

 the same applies to Acetabularia. The very first changes which occur 

 are unfortunately not known to us: when you cut an Amoeba in half, 

 you can identify the enucleated half 15 minutes later, because it has 

 rounded up ; there is something going on very quickly in the membrane 

 which is not yet understood. 



Hoivard: Prof. Brachet, what would happen if you added the nucleus 

 of one species of Acetabularia to the non-nucleated half of a different 

 species ? Would it grow a hat of the species from which the nucleus came ? 



Brachet: I don't think the experiment has been done. The only thing 

 that has been done by Hammerling is the transfer of a nucleus between 

 two species and what then happens is this: if you cut Acetabularia 

 mediterranea just before the hat is formed, you will get a small, but 

 typical Acetabularia mediterranea hat. If you graft a nucleated half of 

 another species — for instance Acetabularia crenulata — in the stem of 

 Acetabularia mediterranea before the formation of the hat, then you get 

 hybrid hats. The purpose of the hat is the reproduction of the alga. 

 This hybrid hat is never fertile; it can be replaced by a second hat, 

 which will now be a typical crenulata hat. 



These experiments of Hammerling show that there are morphogenetic 

 substances produced under the influence of the nucleus ; whether they 

 originate from the nucleus or are due to nucleocytoplasmic interactions, 

 we do not know. 



Roller: You mentioned that in Acetabularia when you cut off the hat, 

 the enucleated stem will develop another hat of the same kind as the 

 original. This shows that protein synthesis in the enucleated part is 

 still under genetic control. It would be interesting to see how far 

 genetic control remains in operation. By removing the regenerated hat 

 from the enucleated Acetabularia and repeating the process it might be 

 possible to distinguish morphogenetic substances which are derived 

 from the nucleus from those which are produced in the enucleated part. 



Brachet: I cannot answer that question because we have not done the 

 experiment. I believe that, probably very soon after the hat has been 

 formed, you will no longer be able to form a hat. We have carried out 

 experiments where we have tried to find out how long the non-nucleated 



