LARVAL DEVELOPMENT IN THE SEA URCHIN 105 



and these in turn are dependent upon the differences in rate of 

 reaction or in protoplasmic condition, permeability, aggregation, 

 enzyme activity or whatever designations we prefer, associated 

 with the differences in rate of reaction. The relation between 

 susceptibility, and metabolic rate is a very general one, and is 

 apparently independent, at least to a large extent, of the par- 

 ticular component reaction or condition of the protoplasmic 

 reaction system which is directly affected in a particular case. 

 It is dependent, rather, upon the fact that living protoplasm is a 

 system and that no very great changes in any essential compo- 

 nent of this system are possible without affecting the system as 

 a whole. 



It is evident that the alterations in form resulting from differ- 

 ential inhibition and from differential acclimation and recovery 

 are very closely associated with changes in the relative rate and 

 amount of growth at different levels of an axis. The so-called 

 normal form of the sea urchin, or of any other organisms, repre- 

 sents merely the usual relations of metabolism and growth be- 

 tween different parts. 



Experimental data of many kinds and from many fields show 

 that where nutritive supply is limited, a region of high meta- 

 bolic rate will grow or maintain itself more or less completely at 

 the expense of a region of lower rate; i.e., some of the products 

 of breakdown in the region of low rate go to the building up of 

 new molecules in the region of high rate because the physico- 

 chemical conditions determine a passage of these substances 

 toward the region where they are being most rapidly trans- 

 formed. In short, the region of higher rate of reaction robs the 

 region of lower rate. Where the nutritive supply equals the 

 demand in all parts, the region of higher rate of reaction shows, 

 in general, a higher rate of growth than a region of lower rate, 

 simply because it synthesises more molecules in a given time 

 (Child, '15 b, Chap. II). General form of body and proportions 

 of parts are, fundamentally, the expression of relations of this 

 character. They represent, so to speak, the metabolic balance 

 between regions of different rate of reaction under a particular 

 complex of conditions. The 'normal' form is merely one par- 



