Discussion 209 



regeneration rate falls with the growth rate. Growth eventually 

 declines to zero, and the regeneration rate to its basal level, which 

 persists even in starving fish. 



Danielli: Are these measurements in terms of percentage re- 

 generation of what was removed ? 



Comfort: Yes. They are not absolute measurements. We had to 

 adjust the size of the amputate to the size of the fish. The general 

 finding seems to be that as somatic growth flattens out, the regenera- 

 tion rate comes down to its basic level; as growth is restarted, so 

 regeneration is restarted. I would predict that this occurs also with 

 nitrogen utilization, as in Dr. Gerking's experiments. 



As G. V. Samokhvalova has shown (1952. J. gen. Biol., Moscow, 

 12, 153), in guppies during the first part of life the number of young 

 per brood is a function of the size of the female. I have kept them 

 up to four and a half years of age. Brood size declines fairly rapidly 

 even though body growth continues, and there is quite a long post- 

 reproductive period during which females may produce one or two 

 broods if remated, but generally they do not. 



Gerking: It seems that my paper would have been much more com- 

 plete if your experiments had been published a little earlier. 



Holt: Some light could probably be thrown on the regeneration 

 question if the growth could be modified not just by changing the 

 food supply, but also by changing the temperature. You could 

 then see whether the regeneration curves behaved in the same way. 

 In our terminology your food supply is changing the Loo (the 

 asymptotic size), whereas the temperature would change K (the 

 rate of approach to the asymptote). 



Comfort: I am now engaged in temperature experiments. If two 

 batches of fish are allowed to regenerate at different temperatures 

 the final percentage restoration is identical, but it is reached at a 

 different rate. It is striking that, as far as I have got, quite large 

 shifts in temperature do not alter the final percentage of restoration; 

 they only alter the rate at which it is approached, as you suggested, 

 Mr. Holt. 



Rotblat: One of your graphs [not printed] appears to be the 

 differential of the other. 



Comfort: In general but not invariably. If very old retarded fish 

 are kept very long, as is often the case, some of them will not restart 

 growth. They grow only very sluggishly or very little. Nevertheless 

 the regeneration rate still rises. 



Rotblat: Even when there is a small change in weight, the initial 

 slope appears to increase. 



Comfort: Some of them do not increase at all or show barely 



