CORRELATED GROWTH-CHANGES 121 



In Uca, it has been known for some time that other parts 

 besides the chelae are asymmetrical in males, notably the 

 carapace and the walking legs, which also are enlarged on the 

 side of the large chela. The only quantitative data on this 

 subject appear to be those of Yerkes (1901), which have been 

 further analysed in this laboratory (Huxley and Callow, 

 unpublished). Unfortunately Yerkes' measurements apply 

 only to large specimens (10-15 mm. carapace-length). Over 

 this range, the large chela is, of course, enormously much 

 larger than the small (at least twenty times as heavy, from 

 my data on U. pugnax) : the merus, measured by Yerkes, is 

 about 60 per cent, greater in length. The only other relevant 

 measurements taken by Yerkes are (1) the merus-length of 

 the first walking leg, which is about eight per cent, larger 

 on the large chela side, and (2) the lateral margin of the cara- 

 pace, which is rather over 5 per cent, larger on this side. 



What most concerns us is that for the merus of the first 

 walking legs, the excess of the large-chela side increases dis- 

 tinctly, over the size-range measured, with absolute increase 

 in size of the animal, from below 7-5 per cent, to over 9 per cent. ; 

 and that for the carapace margins, in spite of some irregularity, 

 appears to increase slightly (from below 5 per cent, to 5-5 per 

 cent.), i.e. the asymmetry is progressive. 



Work is in progress by Miss Tazelaar in this laboratory on 

 this subject. Her preliminary data indicate clearly that all 

 the walking legs are implicated in the increase of size on the 

 side of the large chela, that the first after the chela (second 

 pereiopod) is most affected, the second next, while the effect 

 on the last two is slight and somewhat irregular ; e.g. six 

 males of mean carapace-length 14-3 mm. gave the following 

 result for the percentage excess of the limb of the large side 

 over that of the small side : 



Pereiopod 



. A . 



Second Third Fourth 



Excess on large side, per cent. . 2-6 0-9 0-2 



A graded effect is clearly visible. 



In Uca, measurements have unfortunately not yet been 

 taken of the appendages lying just anterior to the chelae. 

 This has, however, been done in several cases of sexual differ- 

 ence in heterogony, and in every case we meet with the sur- 

 prising fact that whereas, as in Uca, marked heterogony of 

 an organ is associated with a slight increase of relative size 



