INTERNAL FACTORS OF REGENERATION IN ANIMALS 57 



but there is no evidence that this relation can in any way influence 

 the result. Whether the difference in surface tension could prevent 

 the small piece from assuming the typical form and hold it, as it 

 were, in a spherical form is not known, but there is little probability 

 that this is the explanation of the phenomena. 



The regeneration of small pieces of animals and of plants may 

 often fail to take place, because, as Vochting has pointed out, the 

 injury caused by the cutting may extend so far into the small piece 

 that its repair may be impossible. In other cases there may be an 

 insufficient reserve supply of food stuff, although, if a proportionate 

 form of any size could be produced, it is difficult to see how this could 

 be the case. There can be no doubt, however, that pieces taken from 

 parts of the body that are dependent on other parts for their food, 

 oxygen, etc., will die for lack of these things, and even if they can 

 live for some time their further development may not take place in the 

 absence of sufficient food to carry on the process. After these pos- 

 sibilities have been given due weight, there remain several cases in 

 which there can be little doubt that the failure of a small piece to 

 regenerate is owing to the lack of sufficient material to produce even 

 the smallest possible form for that sort of material, i.e. for the organ- 

 ization to be formed on so small a scale. 



There are some facts in connection with the regeneration of small 

 pieces of tubularia that have an important bearing on this question of 

 organization size. If long pieces of the stem are cut off, the new 

 hydranth, that develops out of the old tissue at the end of the piece, 

 occupies, within certain limits, a region of definite length. If pieces 

 of the stem are cut off that are only twice the length of the hydranth- 

 forming region, the length of the latter will be reduced to half the 

 length that it has in longer pieces, and if still smaller pieces are cut 

 off, the hydranth-forming region may be reduced, as Driesch has 

 shown, to seventy per cent of the normal length. The hydranths 

 that develop from the smaller pieces have also a reduced number of 

 tentacles, as I have found. It was first shown by Bickford, and later 

 by Driesch, and by myself, that in many cases very short pieces of 

 the stem of tubularia produce only the distal parts of a hydrantli. 

 This happens most often when the length of the piece is less than 

 the average normal length of the hydranth-forming area, but it may 

 also take place in pieces that are much longer than the minimal size 

 of the least hydranth-forming region. Driesch made the further dis- 

 covery, which I have confirmed, that pieces from the distal end of the 

 stem are more likely to produce these partial structures than are 

 pieces from the more proximal part. Some of these partial structures 

 are represented in Fig. 28, CG. Sometimes the inner tube, or coeno- 

 sarc, which is composed of the two layers of the body, ectoderm and 



