“We must distinguish,” said Berthelot, “between the formation of the chemical substances, the assemblage of which constitutes organized beings, and the formation of the organs themselves. This last problem does not come into the domain of chemistry. No chemist will ever claim to have formed in his laboratory a leaf, a fruit, a muscle, or an organ.... But chemistry has a right to claim that it forms direct principles—that is to say, the chemical materials which constitute the organs.” And Claude Bernard in the same way writes:—“In a word, the chemist in his laboratory, and the living organism in its apparatus, work in the same way, but each with its own tools. The chemist can make the products of the living being, but he will never make the tools, because they are the result of organic morphology.”
§ 2. The Acquisition and Re-establishment of the Specific Form.
Acquisition of the Typical Form.—The acquisition of the typical form in the living being is the result of ontogenic work which cannot be examined here. In the elementary being, the plastid, this work is blended with the work of nutrition. It is directed nutrition. It consists of a simple increase from the moment the element is born by the division of an anterior element, and of a necessarily restricted differentiation. It is a rudimentary embryogeny. In the complex being, metazoan or metaphyte, the organism is constituted, starting from the egg, by the growth, by the bipartition of the elements, and their differentiation, accomplished in a certain direction and in conformity with a given plan. This, again, is directed nutrition, but here the embryogeny is complex. The directing plan of operations is no doubt the consequence of the material conditions realized each moment in the organism.
Normal Regeneration.—Not only do living beings themselves construct their typical architecture, but they re-establish it and continually reconstitute it, according as accidents, or even ordinary circumstances, tend to destroy it; in a word, they become rejuvenescent. This regeneration consists in the reformation of the parts that are altered or carried away in the normal play of life, or by the accidents which disturb its course.
Thus there is a normal physiological regeneration, which is, so to speak, the prolongation of the ontogenesis—i.e., of the work of formation of the individual. We have examples in the reconstitution of the skin of mammals—in the throwing off of the epidermic products constantly used up in their superficial and distal parts and regenerated in their deeply-seated parts; in the loss and the renewal of teeth at the first dentition and in certain fish in the fact of successive dentitions; in the periodical renewal of the integument in the larvæ of insects, and in the crustaceæ; and finally in the destruction and the neo-formation of the globules of the blood of vertebrates, of the glandular cells, and of the epithelial cells of the intestine.
Accidental Regeneration in Protozoa and Plastids.—There is also an accidental regeneration which more or less perfectly renews the parts that are lost. This regeneration has its degrees, from the simple cicatrization of a wound to the complete reproduction of the part cut off. It is very unequally developed in zoological groups even when they are connected. In the elementary monocellular beings—i.e., in the anatomical elements and in the protozoa,—the experiments in merotomy, i.e., in partial section, enable us to appreciate the extent of this faculty of regeneration. These experiments, inaugurated by the researches of Augustus Waller in 1851, were repeated by Gruber in 1885, continued by Nussbaum in 1886, Balbiani in 1889, Verworn in 1891, and have been reproduced by a large number of observers. They have shown that the two fragments cicatrize, and are repaired, building up an organism externally similar to the primitive organism, but smaller. The two new organic units do not, however, behave in the same way. That which retains the nucleus possesses the faculty of regeneration, and of living as the primitive being lived. The protoplasmic fragment, which does not contain the nucleus, cannot rebuild this absent organ; and though it has functional activity in most respects, just as the nucleated fragment, yet it is distinguished from it in others of great importance. The anucleated fragment of an infusorian behaves as the nucleated, and as the whole animal so far as the movements of the body, the cilia, prehension of food, evacuation of fæces, and the rhythmical contraction of the pulsatile vesicules are concerned. But Balbiani’s researches in 1892 have shown us that secretion, complete regeneration, and the faculty of reproduction by fission can take place only in the nucleated fragment—i.e., in the nucleus.
Accidental Reproduction in the Metazoa.—Among multicellular beings the faculty of reproduction is met with in the highest degree in plants, where we find it in the process of propagation by slips. In animals it is the most marked in Cœlenterata. Trembley’s experiments are a striking instance. We know that when the hydra is cut into tiny pieces it reproduces exactly as many complete beings. Among the worms, Planaria afford a similar example. Every fragment, provided its volume is not less than a tenth of that of the whole, can reproduce a complete, entire being. The snail can produce a large part of its head, including the tentacles and the mouth. Among the Tritons and the Salamanders the faculty of regeneration reproduces the limbs, the tail, and the eye. In the Frog family, on the contrary, the work of regeneration does not go beyond cicatrization, and it is the same with Birds and Insects.
It is really startling to see in a vertebrate like the Triton the stump of an arm with its fragment of humerus reproducing the forearm and the hand in all their complexity, with their skeleton, blood vessels, nerves, and teguments. We say that the limb has budded, as if there were a germ of it which develops like the seed of a plant, or as if each transverse portion of the limb, each slice, so to speak, could re-form the slice that follows.
The mechanism of generation and that of regeneration alike raise problems of the highest importance. Does the part become regenerated just as it was formed at first? Does the regeneration repeat the ontogeny? Is it true that a lost organ is never regenerated (the kidney for instance)? Does the symmetrical organ enjoy a compensating and hypertrophic development, as Ribbert has asserted? And further, if the organ be removed and transplanted to another position, can it be grafted there, as Y. Delage maintains? These are very important questions; but if we dwell upon them, we shall be diverted from our immediate object. Our task is to look at these facts from the point of view of their significant and characteristic meaning in vitality. Flourens invoked on their behalf the intervention of vital forces, plastic and morphoplastic. But, as we shall see later, these phenomena of cicatrization, of reparation, of regeneration, these more or less complete efforts for the re-establishment of the specific form, although they are found in all living beings in different degrees, are not exclusively confined to them. We find them again in some representatives of the mineral world—in crystals, for instance.