remotest resemblance to the process in the crystal. It is so obvious from every point of view that the comparison is entirely a superficial one, that it seems useless to point out further differences between the two processes. Pflüger (’83) has given, in brief outline, an hypothesis to account for the process of regeneration. He states that since there is always replaced exactly what is lost, the new part cannot arise from a pre-existing whole germ. If, for instance, the leg of a salamander is cut off at any level, as much comes back as is removed. The assumption of a leg germ is insufficient to account for the fact that only as much comes back as is lost, and not always a whole leg. Pflüger, therefore, offers another hypothesis. He assumes that food material is taken up at the wounded surface and organized into the substance of the new part. The new material is laid down at the surface of the old material, and is then organized into the kind of tissue that lay just beyond that region in the whole limb. Upon this first layer a new layer is deposited that is organized into the next part of the limb, and so on, until the whole missing part is replaced. Pflüger does not give any idea of how the new material is deposited at the cut-surface, but from what we know of the histology of the process we must suppose, if we should adopt Pflüger’s interpretation, that new cells are produced by the old ones, and that these new cells form the successive layers out of which the new limb is produced. Pflüger speaks of an arranging molecular force, which we can only suppose, in the light of what has just been said, to act from cell to cell through the continuous protoplasm. Pflüger also pointed out that in certain cases the organization can take place only in a certain direction, that is, in some forms regeneration can take place from one side of a cut-surface, but not from the other. He interprets this as due to a polarization of the protoplasm, one surface having peculiarities that are absent in the other.