INTERACTION OF GROWTH-RATIOS 



191 



ratio is still considerably higher than the old nipper, and at 

 the next moult it will almost inevitably overhaul its partner 

 (Fig. 84). This hypothesis fits the facts very prettily; it 

 only remains to confirm it quantitatively by appropriate ex- 

 periments. It should, however, be pointed out that in any 

 case it only explains why one of two alternatives is realized 

 on a given side of the body in each particular case : it gives 

 us little light on the ques- 

 tion of the nature and 

 determination of the two 

 alternative types them- 

 selves. The difference be- 

 tween crusher and nipper 

 is doubtless in part one of 

 absolute growth-coeffici- 

 ent ; but as we have earlier 

 seen, there are also differ- 

 ences between them in the 

 spatial distribution of 

 growth-potential, and 

 qualitative differences in 

 teeth, etc. The changing 

 growth-ratio between re- 

 generating and normal 

 claw acts primarily as a 

 ' realization factor ', to use 

 a German term, just as 

 does the female sex- 

 hormone of birds in effect- 

 ing the difference in colour 



and form between male and female feathers. The determina- 

 tion of the difference in growth-gradients and in teeth is not 

 in the least explained by this. 



Fig. 84. — Diagram to illustrate Przibram's 



hypothesis as to reversal of chelae in 



heterochelous Crustacea. 



C, crusher ; N, nipper claw. 



In each case the crusher has been removed, and its 

 regeneration rate is assumed to remain constant in spite 

 of age. Dotted lines indicate growth. The height of 

 the lines is not supposed to indicate absolute size, but 

 is adjusted so that greater height indicates capacity to 

 develop into a crusher and to inhibit the other claw, 

 causing it to become a nipper. 



(a) In young animals the nipper grows relatively fast 

 and can thus differentiate into a crusher at the next 

 moult, causing the regenerating crusher to become a 

 nipper ; [b) in old animals the nipper grows relatively 

 slowly ; the regenerating claw can thus differentiate 

 into a crusher again at the next moult, and the nipper 

 remains a nipper ; (c) at an intermediate stage, neither 

 claw gets sufficient advantage to develop into a crusher 

 by the first moult ; but at the succeeding moult the 

 old crusher will become a crusher again. 



§ 6. Specific Growth-intensities and their Interaction 



A series of highly important experiments bearing on our 

 problems has been carried out by Harrison (1924, 1929), and 

 later by Twitty and Schwind (1931), in grafting organs from one 

 species of urodele larva to another which has a different normal 

 growth-rate. The species used were Ambly stoma tigrinum, 

 which has a high growth-rate, and A . pimctatuni which, even 

 when fed as fully as possible, grows at a considerably lower 



