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 cannot develop below a certain minimal size, or rather, since the tendency 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 produced, 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 intercalated at the anterior end during normal growth, nor does this take place during regeneration. At the posterior end of the earthworm we find a region of growth in which new segments are produced, 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 [Illustration: FIG. 27.--_Tubularia mesembryanthemum._ _A._ Minimal-sized piece that produced a hydranth. _B, C._ Pieces below minimal size. _D._ Ring produced by closing of small piece.]