SEGMENTATION 75 



reduced in size, and this deformity may perhaps have ensued as 

 a consequence of the same wound which excited the growth of the 

 extra pair. Its reduced size may be due to the same injury, 

 which may quite well have checked its growth to full proportions. 



Admitting doubt in these ambiguous cases it seems to be a 

 general rule that for the production of the extra pair the normal 

 limb should persist in connexion with the body. Moreover it is 

 practically certain that in no case can a single, viz. an unpaired, 

 duplicate of the normal appendage grow from it. Many examples 

 have been described as of this nature, but all of them may be with 

 confidence regarded as instances of a supernumerary pair in 

 which only the two morphologically anterior or the two mor- 

 phologically posterior surfaces are developed. We have thus 

 the paradox that a limb of one side of the body, say the right, 

 has in it the power to form a pair of limbs, right and left, as an 

 outgrowth of itself, but cannot form a second left limb alone. 



A very interesting question arises whether it is strictly 

 correct to describe the extra pair as a right and a left, or whether 

 they are not rather two lefts or two rights of which one is reversed. 

 This question did not occur to me when in former years I studied 

 these subjects. It was suggested to me by Dr. Przibram. 

 The answer might have an important bearing on biological 

 mechanics, but I know no evidence from which the point can 

 be determined with certainty. In order to decide this question 

 it would be necessary to have cases in which the paired repetition 

 affected a limb markedly differentiated on the two sides of the 

 body, and of course the development of the extra parts in order 

 to be decisive must be fairly complete. One example only is 

 known to me which at all satisfies these requirements, that of the 

 lobster's chela figured (after Van Beneden) in Materials for the 

 Study of Variation, p. 531, Fig. 184, III. 



Here the drawing distinctly suggests that one of the extra 

 dactylopodites, namely that lettered R, is differentiated as a 

 left and not merely a reversed right. For the teeth on this 

 dactylopodite are those of a cutting claw, not of a crushing claw, 

 whereas the dactylopodites R' and L' bear crushing teeth. The 

 figure makes it fairly certain also that the limb affected was a 



