170 



PROBLEMS OF RELATIVE GROWTH 



specialized biochemical knowledge, I will merely refer the 

 reader to his article (see Fig. 79). 



It is noteworthy that in other respects he has found an 

 association of sulphydryl with growth-potential : e.g. the 

 decline with age both of sulphydryl content and growth-rate 

 (found in Carcinus, Pandalus, Sacculina embryos and Peri- 

 planeta). The coincidence of a gradient in this important 

 ' key ' metabolic agent with observed growth-gradients is 



Fig. 79. — Gradients in content of various sulphur compounds and in oxygen 

 uptake in the Crustacea, Cancer and Homarus (above), and the earthworm 



(below) . 



obviously a fact of considerable interest. Interestingly enough, 

 the gradient in sulphydryl content does not coincide with the 

 gradient in oxygen metabolism (in earthworms), so that Per- 

 kins concludes that the total oxygen uptake, being concerned 

 more with katabolic than anabolic processes, is not a good 

 measure of any gradients primarily concerned with growth, 

 a fact which clearly has important bearings upon Child's work 

 on the metabolic basis of his axial gradients. 



§ 3. Other Gradient Theories 



This brings us face to face with the relation between the 

 growth-gradients here described and other types of gradient- 

 effect. The existence of such effects has been casually recorded 



