n8 PROBLEMS OF RELATIVE GROWTH 



of multiplication among the cells of a Protozoon colony is 

 noteworthy. 



§ 3. The Two Phases of Growth 



Before going further we must consider the difficulty men- 

 tioned on a previous page — that it is hard to understand how 

 a regular growth-gradient could produce anything but a series 

 of appendages regularly graded in size, instead of the irregular 

 alternation of large, small and medium-sized appendages which 

 is what we actually find. The answer appears to be that it 

 would inevitably produce such a regular series if it were the 

 only factor concerned in appendage-growth, but that it is not 

 the only factor. 



I base this assertion on a study of the figures given by 

 Herrick (191 1) for the development of the American lobster, 

 by Giesbrecht (191 1) for that of various Stomatopoda, and by 

 Schmalhausen and Stepanova (1926) for that of the appendages 

 of the embryo chick. 



In all three cases, there appear to be two successive and 

 quite distinct phases of growth. In the first, the general 

 form of the part is being laid down, and this process is accom- 

 panied by very rapid alterations of form, and by marked 

 histological changes ; in the second, histological changes are 

 absent or of an entirely secondary nature, and the form- 

 changes are confined to quantitative alterations in the pro- 

 portions of the definitive structural plan. And it would appear 

 that the regular growth-gradients we have been considering 

 are manifested only (or mainly) in the second of these two 

 phases, some quite other mechanism being at work in the 

 first. Thus not only definitive form-plans, but also marked 

 differences in size, are established in the short first phase, 

 and effects of growth during the second phase are confined to 

 a quantitative modification of the already diversified organiza- 

 tion given at the close of the first phase. 1 



The second phase is thus in general of less morphological 

 importance than the first, although occasionally it exhibits 

 differences of growth-potential great enough to effect very 



1 The matter is still further complicated by the so-called law of 

 antero-posterior differentiation of appendages (and by the exceptions 

 to it !) ; this, however, merely means that the onset of the second 

 phase is not synchronous throughout the body. It is also quite pos- 

 sible that in the first phase, second-phase growth-effects are present, 

 but are masked by the much more radical first-phase effects (see §§ 6, 7) . 



