EXTERNAL CONDITIONS 201 



temperature-difference between tail and body, and would 

 therefore accelerate the tail's relative growth. 1 



Information perhaps more important, and certainly more 

 directly correlated with Perkins's results, is afforded by our 

 knowledge of the differential effect upon bodily proportions 

 produced by malnutrition, by hormones, and by teratological 

 embryonic agencies. 



Jackson (1925) has published interesting data concerning 

 the effect of malnutrition upon differential growth in rodents. 

 An important fact is that in young animals held at constant 

 body-weight, certain organs, notably the skeleton, continue 

 growth, and continue on the whole in the same differential 

 way as in the normally-fed animal — a pretty demonstration of 

 the existence of inherent growth-potentials and their variation 

 in different parts of the same organ-system. 



The actual details of the process vary according to the age 

 at which the animals are subjected to the treatment. (In 

 acute inanition leading to actual weight-loss, the changes, as 

 expected, are different in degree.) 



In albino rats kept at maintenance from three weeks of 

 age, the tail actually grows in absolute length, and consider- 

 ably in relative length. The absolute head- weight is slightly 

 increased. When the experiment started at birth, the increase 

 in head-weight was much more marked. The brain and the 

 testes are other organs which show increase of weight when 

 growing animals are kept at constant weight, especially in 

 the new-born (Fig. 89). Full details are given by Jackson. 



Hammond (1928, 1930, 1931) has also some interesting 

 remarks on the effect of undernutrition at various periods 

 upon relative growth in mammals (sheep, rabbit, etc.). In 

 an adult, the loss of weight occurs first mainly from fat, then 

 mainly from muscle, last of all the bones and the more ' vital ' 

 organs, such as heart, lungs, etc. 



In a young growing animal the vital organs appear to have 

 first call on nutritive material : during moderate under- 

 nutrition they thus continue their growth more or less un- 

 affected, while that of muscles (and fat) is markedly inhibited : 

 since muscles show positive heterogony in juvenile life, the 

 animal will retain juvenile proportions (and will, of course, 

 not be so valuable as a carcass). 



In general, when organs have their period of maximum 

 absolute growth at different times (from whatever physio- 



1 See also Abe (1931), Endokrinol., 9, and Przibram, ibid., no, for 

 effect of internal temperature upon relative cell-size in rats. 



