SECT. 2] AND WEIGHT 431 



rates of carbon dioxide production at the 14th day with the change 

 in mode of respiration from aquatic to terrestrial which takes place 

 late in incubation. This is quite a convincing correlation, but his 

 suggestion that the first break (at four days), before which the 

 instantaneous growth-rate is about 100 per cent., and the Minot 

 growth-rate 1000 per cent., is associated with a general critical 

 period occurring at that time is not really so satisfactory. For 

 almost any process has its critical moments during develop- 

 ment — for example, the peak in protein metabolism at 8-5 days. 

 In cases where there is no a priori reason for assuming corre- 

 lations except the one fact that their peaks coincide or are 

 converse to each other, the utmost caution should be used in 

 so correlating them. Wholesale correlations of apparently unrelated 

 phenomena may be chemically misleading. Thus Brody cites 

 Tomita's peak in total lactic acid content at the 5th day (see Fig. 292) 

 as evidence of a critical period corresponding to the abrupt break 

 in his growth-rates of carbon dioxide production and to the peak 

 in Payne's mortality curve (see Fig. 443). 



Brody is not the only investigator who has occupied himself with 

 the growth-rates of different chemical processes and amounts in 

 the embryo, but, before passing on to discuss these points, which 

 will lead naturally to the question of the growth-rates of parts of 

 embryos, a further word must be said about Brody's work. 



At present it is not possible to tell much from the comparison of 

 embryos of different kinds, though it is obvious that an immense 

 field of research is opened up here for the comparative embryologist 

 of the future. Thus the equation for the development of the chick 

 embryo in weight according to Murray is W^=o-668^^^, corre- 

 sponding to instantaneous growth-constants of 0-47, 0-33, and 0-2 1 

 successively,* while the equation for Stotsenberg's rat embryo figures, 

 according to Brody, is W^ = 0-000065^°^^*, corresponding to a steady 

 rate of 53 per cent, per day instantaneous. On the steadiness of this 

 rate Brody says, "If there is no fallacy in this reasoning we have 

 reached a new and an extremely important conclusion. While all 

 investigators of the time relations of growth have reached the con- 

 clusion that the percentage growth-rate continuously and rapidly 

 declines with age, our conclusion is that the instantaneous per- 

 centage growth-rate remains constant for the relatively enormously 



* A later value, due to Vladimirov & Danilina, is W=o-'^2^fi'^. 



