THE TWO PHASES OF GROWTH 119 



striking transformations, as in the chelae of Uca, or the abdo- 

 men of Pinnotheres. None the less, it is of considerable 

 theoretical importance as revealing the existence of deep-seated 

 and regular growth-gradients which appear to be in the first 

 instance correlated with fundamental properties of the animal 

 body, such as polarity. 



Thus the original effects of such gradients upon growth 

 would be, relative to the growth-process itself, secondary ; 

 but since all changes of proportions that occupy a considerable 

 time must depend on its agency, in certain cases its growth- 

 affecting properties become modified so as to become of direct 

 importance to the process of growth of parts. 



My meaning will be clearer with the aid of an example. 

 There is (Fig. 61) a progressive decrease in relative growth-ratio 

 as we pass forward along the head-appendages of Eupagurus, 

 which obviously results in a slight and continuous change in 

 their proportionate size, relative to the body and to each other, 

 as life goes on ; I cannot conceive, however, that these par- 

 ticular changes in relative size are of any biological significance, 

 but am forced to regard them as accidental consequences of 

 the existence of a growth-affecting gradient in the body. The 

 comparatively enormous increase in relative size of the male 

 right chela, however (as of the female swimmerets), does 

 appear to have biological significance ; and here therefore we 

 must consider that the form of the gradient has been modified 

 so that its growth-affecting properties become utilized to 

 alter bodily proportions in an adaptive way. 



Although clearly much more work must be done before 

 the laws, properties and physiological mechanisms of these 

 two growth-phases are properly understood, general considera- 

 tions as well as the limited special analysis I have been able 

 to undertake make me feel that their existence is real enough. 



If that is so, we may enshrine the distinction in specific 

 terminology, and say that, following the period of chemical 

 predetermination at which the specific fates of different regions 

 of the embryo (or regenerating part) are invisibly determined, 

 there occur two further phases, one of tissue differentiation 

 and the assumption of the definitive general form-plan, and 

 one of subsequent quantitative growth-changes in proportions 

 (and presumably also changes due to functional activity). 

 To the first of these the term chemo-differentiation has already 

 been applied (Huxley, 1924c) and adopted by others (Gold- 

 schmidt, 1927 ; Needham, 1931) ; for the next I propose 



