Problems of Life and Mind. Second series by George Henry Lewes (best contemporary novels TXT) 📕

106. And this becomes clear when we go back to the earliest indications of development. Look at Fig. 15, representing a transverse section of the germinal membranes in a chick after eighteen hours’ incubation. Here the three layers, A, B, and C, have the aspect of simple cells very slightly differing among each other. Yet since each layer has ultimately a progeny which is characteristically distinguishable, we may speak of each not as what it now is, but what it will become. Although the most expert embryologist is often unable to distinguish the embryo of a reptile from that of a bird or of a mammal, at certain stages of evolution, so closely does the one resemble the other, yet inasmuch as the embryo of a reptile does not, cannot become a bird, nor that of a bird a mammal, he is justified in looking forward to what each will become, and in calling each embryo by its future name. On the same ground, although we cannot point to any such distinction between the layers of the blastoderm as I have indicated in the separation of Instrumental and Alimental Systems, nor specify any characters by which the cells can be recognized as epithelial, neural, and muscular, yet a forward glance prefigures these divisions. We know that the first result of the segmentation of the yolk is the formation of cells all alike, which in turn grow and subdivide into other cells. We know that these cells become variously modified both in composition and structure, and that by such differentiations the simple organism becomes a complex of organs.

107. But here it is needful to recall a consideration sometimes disregarded, especially by those who speak of Differentiation as if it were some magical Formative Principle, quite independent of the state of the organized substance which is formed. There is a luminous conception—first announced by Goethe, and subsequently developed by Milne Edwards—which regards the organism as increasing in power and complexity by a physiological “division of labor,” very similar to that division of employments which characterizes the developed social organism. But the metaphor has sometimes been misleading; it has been interpreted as indicating that Function creates Organ (see Problem I. § 88), and as if Differentiation itself were something more than the expression of the changes resulting from the introduction of different elements. In the Social Organism a “division of labor” presupposes that laborers with their labor-materials are already existing; the change is one of rearrangement: instead of each laborer employing his skill in doing many kinds of work, he restricts it to one kind, which he is then able to do with less loss of time and power. Thus is social power multiplied without increase of population, and the social organism becomes more complex by the differentiation of its organs. It is not precisely thus with the Animal Organism during its evolution. Indeed to suppose that the differentiation of the germinal membrane into special tissues and organs takes place by any such division of employments, is to fall into the ancient error of assuming the organism to exist preformed in the ovum. The unequivocal teaching of Epigenesis is that each part is produced out of the elements furnished by previous parts; and for every differentiation there must be a difference in composition, structure, or texture—the first condition being more important than the second, the second more important than the third. The word protoplasm has almost as wide a generality as the word animal, and is often used in forgetfulness of its specific values: the protoplasm of a nerve-cell is not the same as that of a blood-cell, a muscle-cell, or a connective-tissue cell, any more than a bee is a butterfly, or a prawn a lobster. No sooner has the specific character been acquired, no sooner is one organite formed by differentiation, than there is an absolute barrier against any transformation of it into any other kind of organite. The nerve-cell, muscle-cell, and epithelial cell have a common starting-point, and a community of substance; but the one can no more be transformed into the other than a mollusc can be transformed into a crustacean. In the homogeneous cellular mass which subsequently becomes the “vertebral plates,” a group of cells is very early differentiated: this is the rudimentary spinal ganglion, which becomes enveloped in a membrane, and then pursues a widely different course from that of the other cells surrounding it, so that “the same cell which was formerly an element of the vertebral plate now becomes a nerve-cell, while its neighbors become cartilage-cells.”133 Indeed all the hypotheses of transformation of tissues by means of Differentiation are as unscientific as the hypotheses of the transformation of animals. In the organism, as in the Cosmos, typical forms once attained are retained. There probably was a time in the history of the animal series when masses of protoplasm by appropriating different materials from the surrounding medium were differentiated into organisms more complex and more powerful than any which existed before. But it is obvious that from a common starting-point there could have been no variations in development without the introduction of new elements of composition: there might have been many modifications of structure, but unless these facilitated modifications of composition, there could never have resulted the striking differences observed in animal organisms.134

108. To return from this digression, we may liken the three primary layers of the germinal membranes to the scattered and slightly different masses of protoplasm out of which the animal kingdom was developed. In this early stage there are no individualized organites—no nerve-cells or muscle-cells. They are cells ready to receive modifications both of composition and structure, appropriating slightly different elements from the yolk, and according to such appropriation acquiring different properties. And this is necessarily so, since the different cells have not exactly the same relation to the yolk, nor are they in exactly the same relation to the incident forces which determine the molecular changes. The uppermost layer (epiblast) under such variations develops into epithelium and central nerve-tissue; the epithelial cell cannot develop into a nerve-cell, the two organites are markedly unlike, yet both spring from a common root. Another modification results in the development of muscle-cells from the inner layer.

109. Hence we can understand how the surface is sensitive even in organisms that are without nerve-tissue; and also how even in the highest organisms there is an intimate blending of epithelial with neural tissues. The same indication explains the existence of neuro-muscular cells in the Hydra, recorded by Kleinenberg, and of neuro-muscular fibres in the Beroë, by Eimer.135 In the simpler organisms the surface is at once protective, sensitive, and absorbent. It shuts off the animal from the external medium, and thus individualizes it; at the same time it connects this individual with the medium; for it is the channel through which the medium acts, both as food and stimulus. The first morphological change is one whereby a part of the surface is bent inwards, and forms the lining of the body’s cavity. Soon there follows such a modification of structure between the outer and inner surfaces (ectoderm and endoderm) that the one is mainly sensitive and protective, the other mainly protective and absorbent. The outer surface continues indeed to absorb, but its part in this function is insignificant compared with that of the inner surface, which not only absorbs but secretes fluids essential to assimilation. The inner surface, although sensitive, is subjected to less various stimulation, and its sensibility is more uniform.

110. The uppermost of the primary layers we have seen to be epithelial; and we know that the first lines of the central nervous system are laid there. A depression called the medullary groove is the first indication of the future cerebro-spinal axis. Some writers—Kölliker, for instance—regard this medullary groove as continuous with but different from the epithelial layer; others maintain that it lies underneath the epithelium, just as we see it in later stages, when the differentiation between epithelial and nerve cell has taken place. Since no one disputes the fact that when the groove becomes a closed canal its lining is epithelial, one of two conclusions is inevitable: either the cells of the primary layer develop in the two diverse directions, epithelial and neural; or else epithelial cells can be developed on the surface of neural cells and out of them. The latter conclusion is one which, involving the conception of transformation, would seem to be put out of court. I think, then, we must admit that the under side of the primary layer of cells becomes differentiated into nerve-cells; and this is in accordance with the observations of Messrs. Foster and Balfour.136

111. While there is this intimate morphological and physiological blending of epithelial and neural organites, there is an analogous relation between neural and muscular organites. As the neural layer lies under the epithelial, the muscular lies under the neural. The surface stimulation passes to the centre, and is reflected on the muscles. Embryology thus teaches why a stimulus from the external medium must be propagated to a nerve-centre before it reaches the muscles; and why a stimulus on one part of the surface may set all the organism in movement, by passing through a centre which co-ordinates all movements. This, of course, only applies to the higher organisms. In the simpler structures the sensitive surface is directly continuous with the motor organs.

It is unnecessary here to pursue this interesting branch of our subject; nor need we follow the analogous evolution of the second germinal membrane representing the Alimental System. Our attention must be given to what is known and inferred respecting the elementary structure of the nerves and centres, on which mainly the interest of the psychologist settles, since to him the whole of Physiology is merged in nerve actions.