120 PROBLEMS OF RELATIVE GROWTH 



to use the term histo-differentiation, since histological change 

 would here appear to be the most decisive factor ; and for 

 the last, the term auxano-diff eventration, since quantitative 

 growth-changes are now the most significant. 1 



Histo-differentiation would obviously be at work not only 

 in the first formation of organ-rudiments, but also during any 

 radical metamorphosis. This appears to be the case even 

 when limbs are converted from one structural plan to another 

 during development : e.g. when in a lobster a limb is con- 

 verted from a biramous swimming appendage to a jaw, or 

 even when the pereiopods lose their exopodites. In this latter 

 case, it is possible that negative heterogony of the auxano- 

 differentiative type is at work, but acts so quickly that between 

 one moult and the next the whole exopodite disappears, but 

 the presumption appears the other way. It would be interest- 

 ing to study the histology of the different parts of the limb 

 during the instar prior to the exopodite's disappearance. 



§ 4. Growth-changes Correlated with High Local 



Growth-intensity 



Another indication of the real existence of these fundamental 

 growth-gradients is afforded by the fact that it appears im- 

 possible to effect a marked change in the growth-ratio of one 

 appendage without at the same time effecting slight changes 

 in the growth-ratios of the neighbouring appendages — in other 

 words, that a localized growth-change is not in reality fully 

 localized, but must operate within the framework of the main 

 growth-gradient of the body, and affect its working. The 

 proof of this is afforded by the changes in relative size of 

 neighbouring appendages which are to be found correlated 

 with marked heterogony of a particular appendage. The 

 ' control ', by the difference from which the magnitude of the 

 correlated change is deduced, may be provided by contrasting 

 corresponding organs, either those of the opposite side of the 

 body in cases of asymmetrical heterogony of a part, or those 

 of the opposite sex when there is heterogony of an organ in 

 one sex only. As examples of the former we can take the 

 male fiddler-crab (Uca) ; and of the latter, those numerous 

 Crustacea in which one of the pereiopods is enlarged, in the 

 male only, to form a powerful chela. 



1 Most significant, that is, for our present purpose. If we were 

 more interested in the changes directly brought about by function, 

 another term would be needed. 



