GROWTH 289 



a remarkable fact that they so often do so. One simple physiological 

 explanation of such a system would be the hypothesis that the organs are 

 each competing, with constant efficiencies per unit mass, for a generally 

 available supply of nutrients. An attempt to test this has been made by 

 Twitty and Wagtcndonk (1940), who transplanted eyes of various ages 

 and sizes between different individuals of axolotl, which were fed at differ- 

 ent levels. They found that the simple scheme of constant efficiencies of 

 competition was certainly not the whole story, since, for instance, a 

 young eye transplanted to an older host which was starved might continue 

 to grow even when the host was declining in weight. They concluded 

 that the assimilative capacity of an organ must change (usually if not 

 always decreasing) during the course of development (cf. p. 298). A 

 similar conclusion emerges when one studies the growth of fragments of 

 an organ isolated in vitro : pieces of chick heart, explanted from embryos 

 of increasing age, show an increasing lag period before they start growing 

 and a decreasing rate of growth in the first few days, though these differ- 

 ences fairly soon disappear (Medawar 1940). 



Several authors, beginning with Teissier in 193 1 and Needham in 1932, 

 have applied the allometry formula to the increase in various chemical 

 entities during development (reviewed in Needham 1942). If wet weight 

 is plotted on double-log paper against dry weight, or glycogen, fat, pro- 

 tein, ash, calcium, phosphorus, etc., plotted against each other or against 

 wet or dry weight, a series of straight lines are obtained. This indicates 

 that the entities are related in the manner of the allometry equation 



log X = a log y + constant. 



The very interesting fact emerged that if we measure a number of differ- 

 ent substances, x, y, z, etc., in two different animals A and B, we find 

 relations of the kind 



log J =alog^=^log^, 

 ^1 ^2 -^3 



-Dl -D2 -D3 



in which A^, Bi, etc. are different constants, but a, j3, remaui the same, 

 whatever the animals in which x and y are measured. 



There is thus the same general relationship between a particular x and y 

 (say fat and glycogen) throughout the animal kingdom or a great part of 

 it. Needham spoke of tliis as a 'chemical ground-plan of animal growth'. 

 Wadduigton (1933c) suggested that one might envisage the situation in 

 terms of a general speeding up or slowing down of a basic chemical 



