LECTURE XXI
REGENERATION (continued)
Phyletic origin of the regenerative capacity—The liberating stimuli of regeneration—Production of extra heads and tails in Planarians (Voigt)—Regeneration in the Starfish—Atavistic regeneration in Insects and Crustaceans—Progressive regeneration—Regeneration has its roots in the differentiation of organisms—The nuclear substance of unicellular organisms is the first organ for regeneration—The ultimate roots of regeneration.
In the previous lecture we have considered many different forms of regeneration, and have recognized them as adaptive phenomena; we have now to inquire how such regeneration-adaptations have arisen, and this is a very difficult question even in general, while in particular cases it is often quite unanswerable at present. In regard to the case last discussed, the regeneration of the lens in the eye of Triton, our hypotheses would require to reach back to the time of the primitive vertebrates with an unpaired eye, for the lens of the paired vertebrate eye, from Mammals down to the lowest Fishes, does not arise in embryonic development from the retinal cells, but always from the corneal epithelium, as the elaborate researches of Rabl have recently shown. It is true that the unpaired parietal eye of some reptiles forms its lens from the cells of the retinal layer, but it would be difficult to demonstrate the possibility of a genetic connexion between it and paired eyes, and in the meantime we must refrain from elaborating a hypothesis as to the origin of the marvellous faculty the retinal cells possess of transforming themselves into lens-fibres.
But it is easier to form some sort of picture of the origin and adaptation of the faculty of regeneration in general.
We saw that the power of regenerating a part can be localized, and that it does not belong to all the cells of the body, but only to some of them, and we have to ask how and by what steps it has been imparted to these. The faculty depends on the possession of a regeneration-primordium (Anlage), and this again, in our mode of expression, consists of a definite complex of determinants, and as determinants are the products of an evolution, and thus are vital units which have arisen historically, they can nowhere suddenly originate anew in a species, but must be derived directly or indirectly from the sole basis which, in each species, forms the starting-point of the individual—that is to say, in the Metazoa, from the germ-plasm of the ovum. From it the determinant-complex of every regeneration-rudiment mast in the ultimate instance be derived.
We may think of the matter thus: all the determinants of the germ-plasm vary, grow slowly or quickly, and in certain circumstances may be doubled. In this way there arise what we may call 'supernumerary' determinants, which are not required in the primary building up of the body from the ovum, and which may remain in an inactive state in the nuclei of certain cells, ready to become active under certain circumstances and to produce anew the part which they control. Such regeneration-idioplasm will at first come to lie in the younger cells of the determinate organ, but it is conceivable that under the influence of selection it may be gradually shifted to other cells of a later developmental origin, or, conversely, to others in a less external position, so that, for instance, the regeneration-rudiment for the finger of a newt may be contained not merely in the cells of the hand, but in those of the fore-arm or even of the upper arm.
But all such segregation of determinant-groups cannot have taken place, as we might perhaps be inclined to think, at the periphery in the organ itself during its development; it must take place in the germ-plasm of the ovum, for otherwise it could not be transmissible, and could not be directed and modified by the processes of selection, as is actually the case, as I shall show in more detail later on.
I have already pointed out the importance of the rôle played by liberating stimuli in regeneration, and not only of extra-organismal stimuli, such as gravity, but above all of intra-organismal stimuli that is, the influences exerted in a mysterious manner by other parts of the animal on the parts which are in process of regeneration. It is a great merit of the modern tendency in evolution theory that it has demonstrated the importance of such internal influences. Although we are still far from being able to define the manner in which these influences operate, we may say so much, that it depends essentially on the nature and extent of the loss which parts are reproduced by the regenerating cells, and, also, on the position and direction of the injured surface from which the regeneration starts. The influences, still quite beyond our comprehension, which are exerted on the regenerating part by the uninjured parts constitute the liberating stimuli, which evoke the activity of one or other of the determinants contained in the regeneration-idioplasm.