SOME PROBLEMS OF REGENERATION. 203 



thus cause an eye to appear. It was also conceivable that in 

 the absence of light an antenna might develop, since an eye 

 would be of no value to an animal in the dark, while an antenna 

 would be a useful organ. But these results did not follow, since 

 eyes and antennae appeared in equal numbers in the animals in 

 the dark and in the light. In the hermit crab also I have found 

 that sometimes eyes and sometimes antennae appear after the 

 eye-stalk is cut off, and I have discovered that when only the tip 

 of the eye-stalk is cut off a new eye reappears, but when the 

 eye-stalk is cut off near its base an antenna-like organ develops. 

 In this case it seems that the factor that determines whether an 

 eye or an antenna develops comes from within and not from 

 without. We find here a process of heteromorphosis depend- 

 ing, so far as we can see, on internal factors. Does this kind of 

 heteromorphosis belong to the same class of phenomena shown 

 by the hydroids } In the latter the stimulus that determines 

 the kind of regeneration comes from outside the animal. 



VII. 



It is of some theoretical interest to know whether the old 

 cells form directly the new tissue, or whether reserve cells are 

 present that bring about the result. From the work already 

 done it seems probable that this may vary with different forms. 

 In some cases, as Randolph has shown for Limtbricnlus, reserve 

 cells are present that assist in the regeneration of the meso- 

 derm ; in other cases, as Hescheler has shown for the earth- 

 worm, the old cells seem to give rise to the new ones. In 

 Tubularia, as Bickford has shown, the old cells go directly 

 over into the new tissue. 



Not only do the old cells give rise to new ones, but in the case 

 of Planaria maciilata the old part may itself change its form as 

 a whole, and in this way play an important part in the develop- 

 ment of the new worm. There is an extensive remoulding of 

 the old tissue, so that the new parts form a miniature copy of 

 the adult worm. It is difficult, even impossible, to form any 

 idea of what internal changes are taking place during this 

 period of change, and our ideas of reorganization and of correla- 



