We meet here, I think, with precisely the same problem that we meet with in the regeneration of parts of adult animals. If a planarian is cut in two lengthwise, along the middle line, each half produces new tissue at the cut-side, out of which the missing half is formed. In this case the old median plane remains, more or less, as the median plane of the new worm, _i.e._ the structure of the new part is built up on that of the old. Very much the same result follows when the worm is cut longitudinally into two unequal parts. The larger piece retains its old plane of symmetry and adds to the cut-edge a new part that completes the symmetry. The smaller piece also builds up new material along the cut-edge, and a new plane of symmetry is formed between the old and the new parts. Here, also, a median plane is established at the edge of the old material, but in this case the material lay to one side of the old middle line, and this involves the changing over to a large extent of the old material, so that it fits in with the new structures of the new median plane. In those forms in which the readjustment takes place entirely in the old part, the change of conditions is more difficult to interpret. In some respects hydra gives us an intermediate condition, but since it is a radially symmetrical instead of bilaterally symmetrical form, the transformation is not so obvious. If a cylindrical piece is cut from the body, and is then cut lengthwise into two half-cylinders, each closes in and makes a cylinder of smaller diameter. A little new tissue may appear along the fused edges, but the missing half is not replaced, and a new hydra with a body of half size is formed from the piece. It is to all appearances a radially symmetrical form, and we must think, in this case, of the new axis of symmetry as having shifted to the middle of the piece. As yet no similar experiments have been made on a bilateral animal that regenerates by morphallaxis, so that we have nothing to appeal to for comparison with the bilateral egg, but the results, just described for the planarian and for hydra, indicate how a change might take place in pieces of adult animals that would lead to the formation in them of a new symmetrical structure. If we imagine a case of this sort, and suppose that after separating a piece from the side the cut-edge closed in and the piece assumed a symmetrical form, it is conceivable that a new plane of bilateral symmetry might soon appear in the middle of the piece owing to the symmetrical form of the piece; or the new plane of symmetry might slowly shift from the cut-edge toward the middle of the piece, after reaching which the balance or equilibrium would be attained. This statement, it must be confessed, is little more than a supposition, and rests on the unproven assumption that the internal symmetry may develop in response to a symmetrical change in shape of the piece as a whole, which is partly the outcome of purely physical factors.