CONCLUSION 103 



the less the difference between the growth-coefficient of the 

 organ or region and that of the rest of the body, the less 

 marked and flatter is the gradient, so that in isogonic organs 

 there is scarcely any gradient, but all the parts grow at approxi- 

 mately the same rate as the body as a whole. There are, 

 however, some unusual gradients, as in the first antennae of 

 certain copepoda, in which the growth-coefficients of part of 

 the organ are above, the rest below unity. 



When one of two corresponding organs is positively, the 

 other negatively, heterogonic, the growth-gradients of the two 

 appear to be similar but reversed in sign, the same joint 

 being in one a centre of maximum, in the other a centre of 

 minimum growth. 



In most markedly heterogonic organs so far investigated 

 (chelae of Crustacea, abdomens of female crabs, limbs of 

 ruminants) the growth-centre is terminal or sub-terminal. 



When an organ is markedly heterogonic in both sexes, then, 

 even though the growth-coefficient of the whole organ may 

 differ considerably in male and female, the growth-gradient 

 within the organ appears to be essentially similar — i.e. there 

 is a certain qualitative type of growth-gradient needed to 

 produce an organ of a certain morphological type, though 

 quantitative details may be different. This is confirmed by 

 the change from a growth-gradient with sub-basal centre (as 

 in the pereiopods) to one with a sub-terminal centre (as in other 

 large chelae), when the chela of the male Eupagurus passes 

 from a slight heterogony, no greater than in the walking legs, 

 to the marked heterogony of maturity. 



The gradients for growth in the three planes of space may 

 be different ; applications of this are seen in the abdomen 

 of male and female crabs, and still more in the crusher and 

 nipper claws of heterochelous Crustacea. 



The existence of growth-centres and growth-gradients is an 

 empirical fact, whose physiological explanation is quite un- 

 known but may prove to be of importance for the study of 

 growth in general. 



