148 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



Nothing is definitely known about the causes of repetition. It looks at first sight 

 as if the deformities in the appendages of the lobster and other arthropods could be 

 explained as phenomena of regeneration, though in this case there is addition rather 

 than replacement. 



We have seen that where the cheliped is thrown oft' by reflex muscular contrac- 

 tion the bud of anew leg forthwith appears, and under favorable conditions grows into 

 a perfect limb. The tissues of this limb are developed out of the formed histological 

 elements of the stump. The process of regeneration is begun in this case as a direct 

 result of a loss or injury. The plane of fracture lies between the second and third 

 joints, and all parts peripheral to the second joint are reproduced. Repetition of parts, 

 however, occurs in many insects and vertebrates where no such regeneration of lost 

 appendages is known. However, the power of regeneration, which is present in all 

 organisms, differs rather in degree than in kind, and Weismanu has shown that it has 

 probably been developed in many cases as a means of defense and protection to the 

 individual. (See p. 107.) 



In the .specimen of Palinurus (No. 808, Bateson, originally described by Leger 

 in 1886), where the left penultimate leg bears two supernumerary legs, both of which 

 spring from the basipodite, it certainly looks as if what would have taken place in 

 the case of loss of the original limb — namely, the growth of a new one from this joint 

 — had happened repeatedly, so that instead of the regeneration of one limb at a 

 time there is the superaddition of two; but where the limb is not cast off the first 

 superadded one is in secondary symmetry and belongs to the opposite side. If such 

 an interpretation will apply to this case, the various other repetitions and abnormal 

 growths which arise in more peripheral joints, as upon the sixth and seventh, fall into 

 the same general category. In the case of the growth of a new limb in consequence 

 of loss, however, all parts which are external to the plane of fracture are reproduced. 

 In the cases of repetition this is not usually the case, as Bateson has shown. The 

 bud which arises on the propodus (as in figs. 187, 190) may by fission give rise to a 

 second propodus, but not usually, if ever, to a dactyl. 



It seems as impossible to suppose that such a deformity as that seen in fig. 187 or 

 fig. 189 is congenital as that it is the result of injury. The monstrosities which occur 

 in the embryo, which are considered in another place, are, however, in some cases at 

 least, the result of injury or unfavorable conditions. 



Autotomy, or the casting of the claw at the second joint, is probably directly 

 accountable for the rarity of abnormal growths in the limbs of the higher Crustacea. 

 It is extremely improbable that any deformity at the extremity of a limb could sur- 

 vive autotomy, but the experiments to settle this interesting point have yet to be 

 made. While it would appear that the various deformities which have been described 

 can not be explained as the results of injuries and the attempted regeneration of 

 injured parts, since the limb is usually thrown off in such cases to be completely 

 renewed or it is retained to be completely restored, yet I can not escape the convic- 

 tion that the problem is in some way directly concerned with that of regeneration. 

 The mechanism by which so complete a structure as a limb is regenerated can not be 

 regarded as simple. As Weismanu says, the machinery of a cotton factory can not be 

 made out of a few simple levers. It is probably exceedingly complex, and it is no 

 wonder that the parts do not always work harmoniously, that the thread is sometimes 

 knotted or the product useless. 





