THE PLAN OF CELLULAR REPRODUCTION 177 



plasmic protein synthesis ) seems reasonable, appealing, and consistent 

 with many facts, though it certainly has no rigorous foundations. 



One peculiarity of nucleolar reproduction is that the nucleolus ( or 

 at least much of its mass and all of its normal staining chracteristic ) 

 disappears during division. Moreover, Drs. Das and x'Mfert have shown 

 that the silver-staining nucleolar component is lost from the chromo- 

 some arms as well as the compact nucleoli during division and reap- 

 pears on them around the end of anaphase. Thus nucleolar reproduc- 

 tion is not describable as a 1 -^ 2 reproduction but, so far as we can 

 see, as a 1 -^ — » 2 reproduction. This is not strange, for cytologists do 

 not think of the nucleolus as a whole as a self-reproducing body but 

 rather as a product of sites on the chromosomes which are self- 

 reproducing. 



If we are dealing with a 1 ^ -^ 2 reproduction of material limit- 

 ing the growth rate of cells, we may expect to find that cells do not 

 grow during the period of division. And this is indeed the finding in 

 many cases. Again, I shall refer to Dr. Zeuthen's talk at this symposium, 

 because he has done so much of the work demonstrating a "block for 

 protein synthesis around division." In the fission yeast, Mitchison 

 found an interruption of growth in volume but not of growth in mass 

 during division. The amoebae studied by Prescott ( 1955 ) did not grow 

 during division. Studies on this question are now being made by our 

 laboratory and others, especially on cells in which the chromosomal 

 cycle can be followed. Meanwhile, it remains an interesting hypothesis 

 that physiological reproduction, the doubling of the growth potential, 

 is based on the reproduction of the chromosome conception, and that 

 perhaps the completion of the nucleolar cycle, where it occurs, is a 

 visual expression of the completion of the reproductive process. 



Conclusions 



In speaking of a "plan" of cell reproduction, I may be using a mis- 

 leading word. All I mean is that the cell, a discrete body of matter 

 which involves many kinds of molecules and much structural precision, 

 does reproduce itself very exactly and must do so in the face of the 

 limitations of what molecules can do. The solution of the problem de- 

 pends on the activities of a relatively few molecules which can ac- 

 tually reproduce themselves and can at the same time direct the syn- 

 thesis and assembly of other molecules which cannot reproduce them- 

 selves. From these two properties of a limited number of special 

 molecules, we can hope to derive the generative reproduction of multi- 

 molecular "organs" such as centrioles and chromosomes and the genera- 

 tive reproduction of the cell as a whole. The latter includes biosynthetic 

 growth, the mobiHzation of energy, the preparations for division, and 



