INTERNAL FACTORS OF REGENERATION IN ANIMALS 55 



cannot develop. The interpretation is fully in accordance with what 

 we know to be the case for small pieces of hydra and of other forms 

 that, below a certain minimal size, do not regenerate. The question 

 as to how many segments are formed out of the new part is determined, 

 not only by the amount of new material, but also by the number of 

 segments to be replaced, at least up to five segments. Beyond this 

 limit we may think of the maximum possible number of segments 

 appearing in the new material. That a relation of some sort obtains 

 between the old and the new parts, that may have an influence 

 on the number of the new segments which are formed, is shown by the 

 fact that, when one, two, three, four, or five are cut off, just this 

 number comes back. A sort of completing principle exists as a 

 factor in the result, but when so much has been cut off that the old 

 part cannot complete itself in the new material that is formed, then 

 other factors must determine how many segments will be produced. 



In planarians we find a similar phenomenon. If much of the 

 anterior end is cut off, only a head is formed at the anterior cut- 

 surface of the posterior piece, and the intermediate region is 

 absent. I interpret this in the same way as the similar case in the 

 earthworm. As soon as enough new material has been formed 

 for the anterior end to appear, it begins to develop, and since it can- 

 not develop below a certain minimal size, or rather, since the ten- 

 dency to produce a head approaching the maximum size is stronger 

 than the tendency to produce as much as possible of the missing 

 anterior end, all the new material goes into the new head. In the 

 planarian the possibility of subsequently replacing the missing region 

 behind the head exists, and the intermediate part is later pro- 

 duced, the head being carried farther forward. The same is true of 

 the new posterior end of the earthworm, in which a growing region 

 is established at a very early stage in front of the tip of the tail, 

 but no such growing region is present at the anterior end in the 

 earthworm. These differences appear to be connected with the 

 general phenomena of growth in these forms. In the planarian 

 interstitial growth can take place in any part of the body, hence the 

 possibility of producing a missing region is present in all parts of 

 the worm ; but in the earthworm we never find new segments inter- 

 calated at the anterior end during normal growth, nor does this 

 take place during regeneration. At the posterior end of the earth- 

 worm we find a region of growth in which new segments are pro- 

 duced, and we find the same thing is true in the regeneration of the 

 posterior end. In other words, the growing region in front of the 

 last segment is also regenerated. 



It has been found in several forms that pieces below a certain size 

 do not regenerate. In those cases in which a small piece dies soon 



