The process of regeneration has been often compared to the process by which a broken crystal completes itself. Herbert Spencer, in particular, has elaborated this idea. In his book on the _Principles of Biology_, he says: “What must we say of the ability an organism has to recomplete itself when one of its parts is cut off? Is it of the same order as the ability of an injured crystal to recomplete itself? In either case new matter is so deposited as to restore the original outline. And if, in the case of a crystal, we say that the whole aggregate exerts over its parts a force which constrains the newly integrated molecules to take a certain definite form, we seem obliged, in the case of the organism, to assume an analogous force.” Spencer has called attention to a superficial resemblance between the renewal of a part of a crystal and the regeneration of an animal, and without further inquiry into the profound differences between the processes, assumes that “analogous forces” are at work. Now that we know something more of both processes, we find so much that is totally different, that there remains no basis for Spencer’s conclusion, namely, that analogous forces must be present. Furthermore, Spencer’s statement that the whole crystal aggregate exerts over its parts a force of some kind is diametrically opposed to our idea as to the method of “growth” of a crystal in a saturated solution. The new material is added always at the surface of the crystal, and the growth of each point is self-determining. There is no central force that controls the deposition of new material in the different regions. Rauber’s work on the so-called regeneration of the crystal has given us a clearer conception of how the process is brought about. He has shown that when a piece is broken from a crystal, and the crystal suspended in a saturated solution of the same substance, it becomes larger by the deposition of new material _over all its surfaces_. The addition of new material may be more rapid over the cut-surface than elsewhere, but it must not be supposed that the more rapid “growth” takes place in order to complete the form of the crystal, for the growth over the cut-surface follows precisely the same laws that regulate the “growth” over all the other surfaces, that is taking place at the same time. In this respect we find an essential difference between the regeneration of a crystal and that of an animal, since in the latter the growth takes place only over the cut-surface; and, in forms that regenerate by proliferation, at the expense of the old material, so that the old material is correspondingly diminished as the new part grows larger. Regeneration may even take place in an animal deprived of all food, and also in one that is starving to death and diminishing in size. In those forms that regenerate by a change in shape of the entire piece into that characteristic of the typical form, the process bears not even the