At present there are known two general ways in which regeneration may take place, although the two processes are not sharply separated, and may even appear combined in the same form. In order to distinguish broadly these two modes I propose to call those cases of regeneration in which a proliferation of material precedes the development of the new part, “epimorphosis.” The other mode, in which a part is transformed directly into a new organism, or part of an organism without proliferation at the cut-surfaces, “morphallaxis.”
In regard to the form of the new part, certain terms may be used that will enable us to characterize briefly different classes. When the new part is like that removed, or like a part of that removed, as when a leg or a tail is regenerated in a newt, the process is one of
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.
It is sometimes convenient to contrast the process of physiological regeneration with all other kinds. The use of the term “pathological regeneration” for the latter seems to me, as has been said, unsatisfactory. The two terms proposed by Delage,[18] viz. “regular regeneration” and “accidental regeneration,” have certain advantages, although there is nothing accidental, or at least occasional, in regard to the process itself, as it is entirely regular, although it may only occur after an accident to the animal. The term “regular regeneration” is, I think, more satisfactory than “physiological regeneration,” but the latter has the advantage that it has come into current use. For what is known as pathological or accidental regeneration, I propose the term “restorative regeneration,” and I shall continue to use the term “physiological regeneration” as generally understood.
CHAPTER II
THE EXTERNAL FACTORS OF REGENERATION IN ANIMALS
There is a constant interchange of material and of energy that takes place between a plant or an animal and its surroundings, and this interchange may be influenced by such physical conditions as temperature, light, gravity, etc., or by such chemical conditions as the composition of the atmosphere or of the water surrounding the organism. We can study the process of regeneration either by keeping the regenerating organism under the same conditions that it is subject to in its natural environment, or else we can change the surrounding physical or chemical conditions. In this way we can determine how far the regeneration is affected by external changes, and how far it is independent of them. If a change in the external conditions produces a definite change in the regeneration, then the new condition is called an external factor of regeneration.