264 REGENERATION 



tion of the same substance, it becomes larger by the deposition of 

 new material over all its surfaces. The addition of new material may 

 be more rapid over the cut-surface than elsewhere, but it must not be 

 supposed that the more rapid " growth " takes place in order to com- 

 plete the form of the crystal, for the growth over the cut-surface fol- 

 lows precisely the same laws that regulate the " growth " over all the 

 other surfaces, that is taking place at the same time. In this respect 

 we find an essential difference between the regeneration of a crystal 

 and that of an animal, since in the latter the growth takes place only 

 over the cut-surface ; and, in forms that regenerate by proliferation, at 

 the expense of the old material, so that the old material is correspond- 

 ingly diminished as the new part grows larger. Regeneration may 

 even take place in an animal deprived of all food, and also in one that 

 is starving to death and diminishing in size. In those forms that re- 

 generate by a change in shape of the entire piece into that character- 

 istic of the typical form, the process bears not even the remotest 

 resemblance to the process in the crystal. It is so obvious from 

 every point of view that the comparison is entirely a superficial one, 

 that it seems useless to point out further differences between the two 

 processes. 



Pfliiger ('83) has given, in brief outline, an hypothesis to account 

 for the process of regeneration. He states that since there is always 

 replaced exactly what is lost, the new part cannot arise from a pre- 

 existing whole germ. If, for instance, the leg of a salamander is cut 

 off at any level, as much comes back as is removed. The assumption 

 of a leg germ is insufficient to account for the fact that only as much 

 comes back as is lost, and not always a whole leg. Pfliiger, there- 

 fore, offers another hypothesis. He assumes that food material is 

 taken up at the wounded surface and organized into the substance of 

 the new part. The new material is laid down at the surface of the 

 old material, and is then organized into the kind of tissue that lay 

 just beyond that region in the whole limb. Upon this first layer a 

 new layer is deposited that is organized into the next part of the limb, 

 and so on, until the whole missing part is replaced. Pfliiger does 

 not give any idea of how the new material is deposited at the cut-sur- 

 face, but from what we know of the histology of the process we must 

 suppose, if we should adopt Pfluger's interpretation, that new cells 

 are produced by the old ones, and that these new cells form the suc- 

 cessive layers out of which the new limb is produced. Pfliiger speaks 

 of an arranging molecular force, which we can only suppose, in the 

 light of what has just been said, to act from cell to cell through the 

 continuous protoplasm. Pfliiger also pointed out that in certain cases 

 the organization can take place only in a certain direction, that is, in 

 some forms regeneration can take place from one side of a cut-sur- 



