225J REGENERATION OF CRAYFISH APPENDAGES 37 



c. There are no external signs of differentiation, as constric- 

 tions or folds in the tissue, and the cuticle is not even hardened. 

 Apparently nothing is present but a knob of indifferent tissue. 

 Sections of this knob show nothing but a cuticle consisting of 

 numerous lamellae and a small amount of connective tissue. In 

 Fig. 29, PI. IV, the uninjured left eye and the stump of the 

 right eye of another individual are indicated. In this case, a 

 moult took place about three months after removal of the entire 

 eye, yet the regeneration here is apparently not further advanced 

 than in the preceding case. The regenerated portion shows no 

 resemblance to an eye or any other appendage. The optic nerve 

 still persists, for it can be seen passing into the mass of tissue on 

 the right {op). No more new tissue has been formed than 

 would equal in volume the stalk of the normal eye. It does not 

 seem probable in this and in the preceding instance that any 

 definite structure would have been formed, if the crayfish had 

 lived for a much longer period. 



(c.) Theoretical Considerations. In considering these 

 cases of heteromorphosis, it is noteworthy that in no instance is 

 there anything like a basal protopodite present. In every case 

 the main structure arises from a slightly rounded eminence 

 which is in no way similar to a protopodite. This is again dif- 

 ferent from the majority of cases cited by Herbst. In all of his 

 figures, except one, there is shown some sort of a base from 

 which the principal portion of the appendage arises. The base 

 is not altogether similar to a typical protopodite, yet there is 

 sufficient resemblance to suggest it. 



Undoubtedly, as Herbst suggests, the regenerated hetero- 

 morphic structure receives a plausible explanation on the ground 

 of reversion to an ancestral condition. The question as to 

 whether or not the eyes are modified appendages is difficult 

 of solution, and the study of embryology has not yielded a defi- 



