INTERNAL FACTORS OF REGENERATION IN ANIMALS 63 



tors, and something different from the part removed may be formed. 

 One example of this sort has already been discussed, namely, that in 

 which after the removal of much of the anterior end of the earthworm 

 or of a planarian, only the distal end comes back. Another case is 

 that in which something different from the part removed is regener- 

 ated. If the tip of the eye of the hermit-crab or of other crustaceans 

 is cut off a new eye is regenerated, but if the eye-stalk is cut off near 

 its base an antenna-like organ develops. Herbst has suggested that 

 the presence of the ganglion at the end of the stalk accounts for the 

 regeneration of a new eye, when only the tip of the stalk is cut off. 

 In the absence of the ganglion at the cut-edge the stalk does not pro- 

 duce an eye, but an antenna, as is shown when the eye-stalk is cut 

 off near the base. The factors that determine the development of an 

 antenna instead of an eye have not been discovered. Przibram has 

 shown that when the third maxilliped of portunas, carcinas, or of other 

 crustaceans is cut off near the base, the new appendage that develops 

 is different from the one removed, and resembles a leg in many ways, 

 but if the animal is kept until it has moulted several times the 

 appendage becomes more and more like the part removed. Another 

 remarkable case has also been described by Przibram for Alphens 

 platyrrliyncJius. In this decapod, the claws of the first pair of legs are 

 different from each other, one being much larger than the other and 

 having a different structure. 1 If the larger claw is thrown off at its 

 breaking-joint, and the smaller one left intact, the latter at the next 

 moult (or sometimes after two moults) changes into the character- 

 istic larger claw and the newly regenerated claw is like the smaller 

 one. If the experiment is repeated on this same animal, i.e. if the 

 newly acquired large claw is removed, then at the next moult the 

 smaller claw becomes the larger one and the new claw becomes 

 the smaller one the conditions now being the same once more as at 

 the beginning. If both claws of an animal are thrown off at the 

 same time, two new claws regenerate that are both of the same size, 

 and each is a small copy of the claw that was removed. As yet no 

 experiments have been made that show what factors regulate the 

 development of each kind of claw. 



Returning again to the question of the regeneration of parts simi- 

 lar to the ones removed, there are some interesting results that Peebles 

 has obtained in the colonial hydroids, podocoryne and hydractinia. 

 These colonies consist of three principal sorts of individuals : the 

 nutritive, the reproductive, and the protective zooids. Peebles has 

 found that if the stalks of these zooids are cut into pieces, each pro- 

 duces the same kind of zooid as was originally carried by that stalk. 

 Pieces of the stem of the nutritive zooid produce new nutritive zooids 



1 In normal animals some have the right claw the larger and some the left. 



