In other vertebrates the power of regeneration is more limited. A lizard can regenerate its tail, but not its limbs. A dog can regenerate neither its limbs nor its tail.
It has been stated that the new limb of the salamander is at first smaller than the one removed, but it may contain all the elements of the original limb. We find this same phenomenon in other forms, and since it is a point of some theoretical interest, a few other examples may be given. If the tail of a fish that has a bilobed form is cut off near the base, as indicated in [Fig. 40], G, there appears over the exposed edge a narrow band of new material. The new part
Fig. 2.—A. Allolobophora fœtida. Normal worm. B-F. Anterior ends of worms, which, after the removal of one, two, three, four, and five segments, have regenerated the same number. G. Anterior third cut off. Only five head-segments regenerated. H. Worm cut in two in middle. A head-end of five segments regenerated. I. Worm cut in two posterior to middle. A heteromorphic tail regenerated at anterior end.
now begins to grow faster at two places than at intermediate points, as shown in [Fig. 40], H. The new tail, although very short, assumes, as a result, the characteristic bilobed form. The point of special interest is that the new material that appears over the exposed edge does not first grow out at an equal rate at all points until it reaches the level of the original fork, and then continue to grow faster in two regions to form the lobes of the tail, but the two regions of most rapid growth are very soon established in the new tail. Subsequent growth in all parts of the new tail enlarges it to the full size.
Fig. 3.—A, B. Short head-ends of A. fœtida that did not regenerate at posterior surface. C, D, E. Longer anterior pieces, that made new segments at their posterior ends. F. After Hazen. A piece consisting of five (3 to 7) anterior segments grafted, in a reversed position, upon the anterior end of another worm. A heteromorphic head of about two segments regenerated at the free end, which is the posterior end of the piece.
In some cases of regeneration, in which the new part is at first smaller than the part removed, the new part represents at first only the distal portion of the body, and although the new part may grow to the full size, the whole of the part removed may never come back. This is illustrated in the regeneration of the anterior end of the earthworm; for example, in the red-banded earthworm, or brandling (Allolobophora fœtida).[7] If one segment of the anterior end is cut off, one segment is very quickly regenerated ([Fig. 2], B); if two segments are cut off, two come back ([Fig. 2], C); if three segments are cut off, as many are regenerated ([Fig. 2], D); if four are cut off, generally four come back ([Fig. 2], E); when five are cut off, four or five come back ([Fig. 2], F); but if six or more are cut off, only four or five are regenerated ([Fig. 2], G). It is found in this case that a limit is soon reached beyond which fewer segments are produced than have been removed. The new segments form the anterior end or head that enlarges to the characteristic size; but the missing segments behind the new head are never regenerated, and the worm remains shortened throughout the rest of its life. If the reproductive region has been removed with the anterior part, new reproductive organs are never formed and the worm remains incapable of reproducing itself.