igo PROBLEMS OF RELATIVE GROWTH 



Further, in practically all species, even in those where 

 reversal will take place after visible heterochely has appeared, 

 it will be either slowed down or totally prevented after the 

 attainment of a certain quite small body-size. Above a 

 cephalo-thorax length of about I cm., if the crusher be ampu- 

 tated, both chelae appear of nipper type at the next moult 

 following amputation of the crusher ; and in most such cases the 

 regenerating claw usually turns into the crusher again after one 

 or more further moults. (The only exceptions are hermit- 

 crabs, in which asymmetry of the whole body, and consequently 

 of the chelae, sets in during the larval development, and 

 appears to be invariably determined by the start of post- 

 larval life.) 



Przibram ingeniously suggests that these facts are due to 

 the ratio between rate of regenerative and of normal growth 

 at the time of operation. Rate of regenerative growth is 

 always high. We have little information as to its decline 

 with total size, but the previously cited experiments of Zeleny 

 on Portunus sayi (1. c.) show that there need be no falling-off. 

 Probably rate of regenerative growth remains about constant 

 where regeneration is possible at all. Normal growth, on the 

 other hand, falls off very markedly with absolute size-increase. 

 Thus the growth-ratio of regenerating crusher to normally- 

 growing nipper will increase with increase of absolute size. 

 There is, further, some evidence which suggests that a smaller 

 organ (such as the nipper) can in the absence of competition 

 from a corresponding larger organ (such as the crusher) draw 

 on the blood for a greater amount of nutriment. 



Przibram accordingly suggests that when the growth-ratio 

 of regenerating crusher to normal nipper is relatively low, the 

 impetus given to the nipper's growth by removal of the crusher 

 can reach the threshold of growth-coefficient needed to pro- 

 duce a crusher before the regenerate can catch up, upon which 

 the regenerate is then partially inhibited and must remain as 

 a nipper ; but when this growth-ratio is high, the regenerate 

 can differentiate as a crusher at the next instar, because the 

 amount of excess growth achieved by the nipper is relatively 

 so low. An intermediate condition is seen when at the next 

 instar both chelae regenerate as nippers, though the original 

 nipper is still the larger ; the nipper has not yet got sufficient 

 impetus to differentiate into a crusher at the first instar ; 

 and although the growth-ratio of the regenerate has not been 

 sufficiently high to reach crusher-type at this instar, its growth- 



