CHAPTER V.THE SPECIFIC FORM. ITS ACQUISITION. ITS REPARATION.
§ 1. Specific form not special to living beings—Connected with the whole of the material conditions of the body and the medium—Is it a property of chemical substance?—§ 2. Acquisition and re-establishment of the specific form—Normal regeneration—Accidental regeneration in the protozoa and the plastids—In the metazoa.
The Specific Form is not Peculiar to Living Beings.—The position of aspecific form—the acquisition of this typical form progressively realized—the re-establishment when some accident has altered it—these are the features which we consider distinctive of living beings, from the protophytes and the lowest protozoa to the highest animals. Nothing gives a better idea of the unity and the individuality of the living being than the existence of this typical form. We do not mean, however, that this characteristic belongs to the living being alone, and is by itself capable of defining it. We repeat that this is not a case with any characteristic. In particular thetypical formbelongs to crystal as well as to living beings.
The Specific Form depends on the sum of Material Conditions of the Body and the Medium.—The consideration of mineral bodies shows us form dependenton the physico-chemical conditions of the body and the medium. The form depends mainly on physical conditions in the cases of a drop of water falling from a tap, of the liquid meniscus in a narrow tube, of a small navel-shaped mass of mercury on a marble slab, of a drop of oil “emulsioned” in a solution, and of the metal which is hardened by hammering or annealed. In the case of crystals the form depends more on chemical conditions. It is crystallization which has introduced into physics the idea that has now become a kind of postulate—namely, that the specific form is connected with the chemical composition. However, it is sufficient to instance the dimorphism of a simple body, such as sulphur, sometimes prismatic, sometimes octahedric, to realize that substance is only one of the factors of form, and that the physical conditions of the body and of the medium are other factors quite as influential.
Is the Specific Form a Property of the Chemical Substance?—How much truer this restriction would be if we consider, instead of a given chemical compound, an astonishingly complex mixture, such as protoplasm or living matter, or the more complex organism still—the cell, the plastid.
Are there not great differences between the substance of the cellular protoplasm, or cytoplasmic substance, and that of the nucleus? Should we not distinguish in the former the hyaloplasmic substance; the microsomic in the microsomes; the linin between its granulations; the centrosomic in the centrosome; the archoplasmic in the attraction sphere; not to mention the different leucins, the vacuolar juice, and the various inclusions? And in the nucleus must we not consider the nuclear juice, the substance of thechromosomes, and that of the nucleoles? And is not each of these probably a very complex mixture?
However, it is to this mixture that we attribute the possession of a form, in virtue of and by extension of the principles of crystallization, which definitely teach us that these mixtures cannot have form; that form is the attribute of pure bodies, and is only obtained by the separation of the blended parts—i.e., by a return to homogeneity. There are therefore very good reasons for hesitating before we transfer the absolute principle of the dependence between chemical form and composition, as some philosophical biologists have done, from the physical sciences—where it is already subject to serious restrictions—to the biological sciences.
Le Dantec, however, has made this principle the basis of his biological system. He therefore finds in the crystal the model of the living being. He thus gives a physical basis to life.
Is it a question in this system of explaining this incomprehensible, this unfathomable mystery, which shows the egg cell attracting to itself materials from without and progressively building up that amazing structure which is the body of the animal, the body of a man, of any given man, of Primus, for example? It is said that the substance of Primus is specific. His living substance is his own, special to him; and that, too, from the beginning of the egg to the end of its metamorphosis. It only remains to apply to this substance the postulate, borrowed from crystallography, of the absolute dependence of the nature of substance on the form it assumes. The form of the body of the animal, of the man, of Primus, is the crystalline form of their living substance. It is theonly form of equilibrium that this substance can assume under the given conditions, just as the cube is the crystallized form of sea salt, the only state of equilibrium of chloride of sodium in slowly evaporated sea water. Thus the problem of the living form is reduced to the problem of the living substance, which seems easier; and at the same time the biological mystery is reduced to a physical mystery. It is clear that this way of looking at things simplifies prodigiously—and, we must add, simplifies far too much—the obscure problem of the relation of form to substance, simultaneously in the two orders of science. This may be summed up in a single sentence: There is an established relation between the specific form and the chemical composition: the chemical compositiondirectsand implies the specific form.
We need not now examine the basis of this opinion. If it is nothing but a verbal simplification, a unification of the language applied to the two orders of phenomena, it implies an assimilation of the mechanisms which realize them. To the organogenic forces which direct the building up of the living organisms it brings into correspondence the crystallogenic forces which group, adjust, equilibrate, and harmonize the materials of the crystal.
When it is a question of the application of a principle such as this, in order to test its legitimacy we must always return to the experimental foundations. Let us imagine, for example, a simple body, such as sulphur, heated and brought to a state of fusion—that is to say, homogeneous, isotropic, in an undisturbed medium the only change in which will be a very gradual cooling down. These are the typical crystallogenic conditions. The body wouldtake a given crystalline form. It is from experiments such as this that we derive the idea ofa specific form connected with a chemical constitution.
But in drawing this conclusion our logic is at fault. The real interpretation suitable to this case, as in all others, is that the specific form is suitable to the substance, and also to the physical, chemical, and mechanical conditions in which it is placed. And the proof is that this same substance, sulphur, which takes the prismatic form immediately after fusion, will not retain that form, but will pass on to the quite different octahedral form.
It is so with the specific form of the living being—that is to say, with the assemblage of its constituent materials co-ordinated in a given system—in a word, with its organization. This is suitable to its substance, and to all the material, physical, chemical, and mechanical conditions in which it is placed. This form is the condition of material equilibrium corresponding to a very complex situation, to a sum of given conditions. The chemical condition is only one of these. And further, it is hardly proper to speak of a “chemical substance” when we refer to an astonishingly complex mixture which is in addition variable from one point to the other of the living body. When we thus reduce phenomena to their original signification, false analogies disappear. To say with Le Dantec that the form of the greyhound is the condition of equilibrium of the “greyhound chemical substance” is saying much; and too much, if it means that the body of the greyhound has a substance which behaves in the same way as homogeneous, isotropic masses like melted sulphur and dissolved salt. It were better to say much less, if itmeans, as it will in the minds of the physiologists, that the body of the greyhound is the condition of equilibrium of a heterogeneous, anisotropic, material system, subjected to an infinite number of physical and chemical conditions.
The idea of connecting form, and by that we mean organization, with chemical composition did not arise in the minds of chemists or physiologists. Both have expressed themselves very clearly on this point.
“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.”
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 blendedwith the work of nutrition. It isdirected 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 anormal 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 thecrustaceæ; 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 anaccidental regenerationwhich 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., inpartial 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 hasbudded, 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,plasticandmorphoplastic. 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.