General Discussion 29 



siK-li aninials Iroiii devclojunental causes? I'm tiMii^ to sflllc this at the 

 iiiojucnt by keeping fish, hut I liad to eoiupromise luy whole investioa- 

 tion by keeping a small speeies oi" fish which is known to have a limited 

 life-span. And aeeordingly, the results I i>('t may not be generally valid. 

 However, in Lchistcs, at any rate in tlie laboratory, it seems very much 

 as if the form of the life-table is going to be very nearly the same as in a 

 mammal. As regards gro\\i:h-pattern, the male of Lebistes ceases to 

 grow at a relatively early age, whereas the female goes on growing like 

 the female plaice, we think probably throughout life; the measurements 

 are not yet complete. Yet the sex differential in longevity is not a very 

 great one. 



I agree very much w ith Dr. Lansing, that the work of Dr. McCay is 

 the hub of the whole thing and the point from which we should open our 

 investigation. Is it the delay of maturation in the retarded rat which 

 makes it live longer or is it the prolongation of gro\\i:h? In other words, 

 are we to consider that gTO\Hh per se has some medicinal virtue in pro- 

 longing the life of the animal and a\ arding off senescence, or is it that the 

 animal senesces when a certain sequence of operations has been fed into 

 it? I have interpreted Dr. McCay's work — Fd very much like to hear 

 what he has to say about this — as simply slowing down the perforated 

 tape you're feeding to the calculating machine, and senescence as taking 

 place when the tape is exhausted; if you play the tape slowly then the 

 whole cycle is stretched out in time. If it's reall}^ a matter of gro^^i:h 

 rather than of sequence of operations, it should be possible, in the light 

 of some work Moon* and his colleagues have recently published on 

 giving gro\\i:h hormone to mice, to maintain some measure of growth in 

 adults — adult rodents, at any rate — and see whether that does have any 

 detectable effect on the life-table. I would like very much to know, tirst 

 whether anybody has any data whatsoever regarding the senescence or 

 non-senescence of animals other than birds and mammals, and in the 

 second place I'd like to hear w^hether Dr. McCay has anj^thing to tell us 

 now about his more recent work which would throw light on how far 

 gro\\'th and how far differentiation and development are the essential 

 components in determining the age of senescence in mammals like the 

 rat. 



McCay: I can't answer the basic problem at all, though we have 

 worked with other species, with insects and with trout. We did about 

 fifteen years' work with trout of various species and it seems to hold 

 there. But we failed with dogs; experimental methods are so extremely 

 difficult with dogs. The dog seems to seal its epiphyses at about ten or 

 twelve months, unlike rodents, and the retarded dog is much more 

 subject to parasites than the normal dog. 



Comfort: I wonder if I might ask you further whether you have any 

 evidence on another point. You delayed your rats and kept them at a 

 low rate of growth; you then allowed them to grow up and go through 

 their life-cycle. Could that be done in steps? Could they be allowed to 

 grow part of the way and then, when they had become sexually mature, 



*Moon, H. D., Simpson, M. K., Li, C. H. and Evans, H. i\I., 19.52, Cancer lies., 

 12, 448. 



