136 W. M. Wallace, W. B. Weil and A. Taylor 



TuDVAD, F., Birch-Andersen, A., and Marmer, I. L. (1957). Acta 



paediat., {Uppsala), 46, 329. 

 WiDDOWSON, E. M., and McCance, R. A. (1957). Brit. J. Nutr., 11, 198. 

 WiDDOwsoN, E. M., McCance, R. A., and Spray, C. M. (1951). Clin. 



Sci., 10, 113. 

 WiDDOWSON, E. M., and Spray, C. M. (1951). Arch. Dis. Childh.,26, 205. 

 Williams, J. N., Jr., and Elvehjem, C. A. (1949). J. biol. Chem., 181, 



559. 



DISCUSSION 



Widdowson: May I suggest, Prof. Wallace, that you started your 

 experiments far too late. If you had started at 21 "Adolph days" 

 instead of 21 "Wallace days", you might possibly have got different 

 results. We have the feeling that a great deal happens during these first 

 three weeks of suckling and the whole subsequent growth and develop- 

 ment of the rat depends upon the amount of milk it receives during that 

 time. Rats suckled in litters of three weigh two to three times as much 

 at weaning as others suckled in litters of 16-20. This difference in weight 

 persists even though all the animals receive unlimited food from weaning 

 onwards. The chemical maturation of the tissues of the body, particu- 

 larly the skeletal muscle, is more rapid in the fast-growing rats. 



Wallace: How are they different? Are they dilute? 



Widdowson: The proportion of extracellular fluid in the bodies and 

 muscles of all the rats decreases with development, and the proportion 

 of intracellular constituents, nitrogen and potassium, rises, but the 

 changes take place more quickly in the fast-growing animals, so that they 

 reach chemical maturity at an earlier age. 



Kennedy : We can say, too, that the general developmental history is 

 altogether different. For example, puberty in the female rat, as meas- 

 ured by vaginal opening, is at 30-35 days in the big rat and it may be 60 

 days in the small rat. All subsequent growth is also quicker. 



Wallace: What happens if the smaller young rats are specially fed? 



Kennedy: This experiment was first done by Parkes (1926 and 1929. 

 Ann. appl. Biol., 13, 374, and 14, 171). He did fantastic things like 

 suckling mice with rat foster-mothers and getting them up within 21 

 days to something like 75 per cent of an adult mouse's weight. I went 

 over this again, breaking the changes down week by week (1957. J. 

 Endocrin., 16, 9). I found the acceleration in growth rate due to an un- 

 limited milk supply was achieved almost entirely in the first week of life. 

 The difference between birth weight and the weight at the end of one 

 week might be fourfold; after that there was roughly a 50-60 per cent 

 increase per week and this went on after weaning, when food was un- 

 limited. Something happened within the early part of the suckling period 

 which determined the shape of the subsequent exponential growth curve, 

 and I think that one of the things was probably the development of 

 appetite regulation. The amount the animal ate became fixed in relation 

 to body weight, so naturally the bigger rat ate more and continued to 

 grow faster. 



