440 ON INCREASE IN SIZE [pt. iii 



the initial weight being taken as unity {k = Cvt). His results for various 

 organisms show interesting differences, thus : 



Sturgeon 



Pike 



Hen 



Mouse 



Rat 



Guinea-pig ... 



Pig 



Man 



Here we observe the effect of early hatching in the two aquatic 

 forms, which have the greater part of their growth still before them 

 at the time of leaving the tgg. The other figures demonstrate quanti- 

 tatively what is apparent to common sense, namely, that the em- 

 bryonic period is the time of greatest growth in terrestrial animals. 



2-10. The Growth of Parts 



We must now turn to the relative growth-rates of parts of the 

 embryonic organism. This is a field which has mainly been tilled 

 by anatomists, but it is of the greatest importance to the chemical 

 embryologist. For the increasing and decreasing intensities of physico- 

 chemical processes cannot be intelligently studied in the absence of 

 a knowledge of the distribution of the whole mass among the 

 different organs and tissues. The investigation of the relative 

 growths of endocrine glands, again, cannot but throw much light 

 on the development of the adult metabolism in the embryo. 

 D'Arcy Thompson sees an appreciation of this in the eighteenth- 

 century preformationists. "It was the apparently unlimited extent", 

 he says, "to which, in the development of the chick embryo, in- 

 equalities of growth could and did produce changes of form and 

 changes of anatomical structure that led Haller to surmise that the 

 process was actually without limits and that all development was 

 but an unfolding, an 'evolutio' in which no part came into being 

 which had not essentially existed before. In short the celebrated 

 doctrine of preformation implied on the one hand a clear recognition 

 of what, throughout the later stages of development, growth can do, 

 by hastening the increase in size in one part, hindering that of 

 another, changing their relative magnitudes and positions, and 

 altering their forms ; while on the other hand, it betrayed a failure — 



