[Illustration: FIG. 12.--After Herbst. Diagram showing brain, eye, and “heteromorphic” antenna (in place of eye of one side) of palæmon. The animal had lived in a dark aquarium for five months.] “homomorphosis.”[17] Under this heading we may distinguish two cases, in one of which the entire lost part is at once, or later, replaced--holomorphosis; in the other the new part is less than the part removed--meromorphosis. When the new part is different from the part removed the process has been called by Loeb “heteromorphosis,” but there are at least two different kinds of processes that are covered by this definition. In one case the new part is not only different from the part removed, but is also an organ that belongs to a different part of the body (or it may be unlike any organ of the body). This we may call “neomorphosis.” As an illustration of this process may be cited the development of an antenna, when the eye of a crab or of a prawn is cut off near the base (Fig. 12); and as an example of an organ different in kind from any organ of the same animal, may be cited the case of _Atyoïda potimirum_, in which the new leg is unlike any other leg on the body. The name “heteromorphosis” can be retained for those cases in which the new part is the mirror figure of the part from which it arises, or more generally stated, where the new part has its axes reversed as compared with the old part. As an example of this may be cited the development of an aboral head on the posterior end of a piece of the stem of Tubularia (Fig. 15, _B_), or the development of a tail at the anterior end of a posterior piece of an earthworm (Fig. 2). The term “physiological regeneration” I shall use in the ordinary sense to include such changes as the moulting and replacement of the feathers of birds, the replacement of teeth, etc.,--changes that are a part of the life-cycle of the individual. In some cases it can be shown that these processes are closely related to ordinary regeneration, as when a feather pulled out is formed anew without waiting for the next moulting period, and formed presumably out of the same rudiment that would have made the new feather in the ordinary moulting process.