The regeneration of small pieces of animals and of plants may often fail to take place, because, as Vöchting 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 possibilities 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 organization 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 hydranth_. 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 discovery, 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, _C-G_. Sometimes the inner tube, or cœnosarc, which is composed of the two layers of the body, ectoderm and