i68 



PROBLEMS OF RELATIVE GROWTH 



youth, and has grown to a considerable size while still the 

 possessor of two symmetrical male-type claws. 1 



When either of these is amputated, it regenerates male-type. 

 Thus we have the remarkable fact that while in early youth 

 the amputation of both male-type claws is followed by a loss 

 of all the male-type potentialities, and the amputation of one 

 by a loss of male-type potentialities on that side of the body, 

 this does not hold when the symmetrical double-male-clawed 

 stage has lasted to a considerably later period of life. 



One possibility that suggests itself is as follows. We know 

 that during the phase of chemo-differentiation, prospective 



potencies are sharply localized. 

 It is reasonable to suppose that 

 the potency for growing into a 

 large instead of into a small 

 chela is localized in the claw's 

 growth-centre, viz. the propus. 

 Whatever the chemical substances 

 responsible, they are then wholly 

 removed by amputation, just as 

 those responsible for limb-differ- 

 entiation in Amphibia are wholly 

 removed if a particular disc of 

 material be removed during em- 

 bryonic life. But we have seen 

 that in the male-type chelae of 

 Uca there is a growth-gradient. 

 If, as a result of this during growth, 

 the substance determining heter- 

 ogony and masculine type should 

 spread proximally to the breaking- 

 joint, then amputation should now permit the regeneration 

 of a male-type chela. 2 



1 In the only specimen of this type which I came across, the two 

 male claws together weighed more than the mean for a normal single 

 male-type claw for that body-weight, but considerably less than the 

 sum of two normal single claws. The range of individual variation 

 being considerable, however, one should have a number of specimens 

 before attempting to generalize. 



2 In this connexion, reference should also be made to the results 

 obtained by Haseman (1907A and b) on the direction of differentiation 

 in segmenting Crustacean appendages. He finds that some regenerate 

 basipetally, others centripetally ; further, there is sometimes (e.g. in 

 many antennae), but not always, a particular segment which produces 



Fig. 78. — Sketch to show asym- 

 metry in the thoracic portion 

 of the central nervous system 

 of the male fiddler-crab, Uca 

 piiguax. The shaded regions are 

 the ganglia supplying the chelae. 



