98. Although analysis distinguishes the two elements of the neuro-muscular system, assigning separate properties to the separate tissues, an interpretation8 of the phenomena9 demands a synthesis, so that a movement is to be238 conceived as always involving Sensibility, and a sensation as always involving Motility.126 In like manner, although analysis distinguishes the various organs of the body, assigning separate functions to each, our interpretation demands their synthesis into an organism; and we have thus to explain how the whole has different parts, and how these different parts are brought into unity10. Embryology helps us to complete the fragmentary indications of Anatomy12 and Physiology13.
99. Take a newly laid egg, weigh it carefully, then hatch it, and when the chick emerges, weigh both chick and shell: you will find that there has been no increase of weight. The semifluid contents have become transformed into bones, muscles, nerves, tendons, feathers, beak14, and claws, all without increase of substance. There has been differentiation15 of structure, nothing else. Oxygen has passed into it from without; carbonic acid has passed out of it. The molecular16 agitation17 of heat has been required for the rearrangements of the substance. Without oxygen there would have been no development. Without heat there would have been none. Had the shell been varnished18, so as to prevent the due exchange of oxygen and carbonic acid, no chick would have been evolved. Had only one part of the shell been varnished, the embryo11 would have been deformed19.
99a. The patient labors20 of many observers (how patient only those can conceive who have made such observations!) have detected something of this wondrous21 history, and enabled the mind to picture some of the incessant22 separations and reunions, chemical and morphological. Each stage of evolution presents itself as the consequence of a preceding stage, at once an emergence23 and a continuance; so that no transposition of stages239 is possible; each has its appointed place in the series (Problem I. § 107). For in truth each stage is a process—the sum of a variety of co-operant conditions. We, looking forward, can foresee in each what it will become, as we foresee the man in the lineaments of the infant; but in this prevision we always presuppose that the regular course of development will proceed unchecked through the regular succession of special conditions: the infant becomes a man only when this succession is uninterrupted. Obvious as this seems, it is often disregarded; and the old metaphysical conception of potential powers obscures the real significance of Epigenesis. The potentiality of the cells of the germinal membrane25 is simply their capability26 of reaching successive stages of development under a definite series of co-operant conditions. We foresee the result, and personify our prevision. But that result will not take place unless all the precise changes that are needful serially27 precede it. A slight pressure in one direction, insufficient28 to alter the chemical composition of the tissue, may so alter its structure as to disturb the regular succession of forms necessary to the perfect evolution.
100. The egg is at first a microscopic29 cell, the nucleus30 of which divides and subdivides31 as it grows. The egg becomes a hollow sphere, the boundary wall of which is a single layer of cells, all so similar that to any means of appreciation33 we now possess they are indistinguishable. They are all the progeny34 of the original nucleus and yolk35, or cell contents. Very soon, however, they begin to show distinguishable differences, not perhaps in kind, but in degree. The wall of this hollow sphere is rapidly converted into the germinal membrane, out of which the embryo is formed. Kowalewsky (confirmed by Balfour) has pointed24 out how in the Amphioxus the hollow sphere first assumes an oval shape, and then, by an indentation240 of the under side, with corresponding curvature of the upper side, presents somewhat the shape of a bowl. The curvature increases, and the curved ends approaching each other, the original cavity is reduced to a thin line separating the upper from the under surface. The cavity of the body is formed by the curving downwards36 of this double layer of the germinal membrane.
101. This is not precisely37 the course observable in other vertebrates; but in all, the germinal membrane, which lies like a watch-glass on the surface of the yolk, is recognizable as two distinct layers of very similar cells. What do these represent? They are the starting-points of the two great systems: Instrumental and Alimental. The one yields the dermal38 surface; the other the mucous39 membrane. Each follows an independent though analogous40 career. The yolk furnishes nutrient41 material to the germinal membrane, and so passes more or less directly into the tissues; but unlike the germinal membrane, it is not itself to any great extent the seat of generation by segmentation. There are two yolks: the yellow and the white (which must not be confounded with what is called the white of egg); and their disposition42 may be seen in the diagram (Fig43. 14) copied from Foster and Balfour’s work. The importance of the white yolk is that it passes insensibly into a distinct layer of the germinal membrane, between the two primary layers.127 Each of the three layers of the germinal membrane has its specific character assigned to it by embryologists, who, however, are not all in agreement. Some authorities regard the topmost layer as the origin of the nervous system, the epidermis44, with hair, feathers, nails, horns, the cornea and lens of the eye, etc. To the middle layer are assigned the241 muscular and osseous systems, the sexual organs, etc. To the innermost layer, the alimentary46 canal, with liver, pancreas, gastric47 and enteric glands. Other authorities are in favor of two primary layers: one for the nervous, muscular, osseous, and dermal systems; the other for the viscera and unstriped muscles. Between these two layers, a third gradually forms, which is specially48 characterized as the vascular49.
Fig. 14.—Diagrammatic section of an unincubated hen’s egg. bl, blastoderm; w y, white yolk; y y, yellow yolk; v t, vitelline membrane; x and w, layers of albumen; ch l, chalaza; a ch, air-chamber; i s m, internal layer of shell membrane; s m, external layer; s, shell.
102. Messrs. Foster and Balfour, avoiding the controverted50 designations of serous, vascular, and mucous layers, or of sensorial, motor germinative51, and glandular52 layers, employ designations which are independent of theoretic interpretation, and simply describe the position of the layers, namely, epiblast for the upper, mesoblast for the middle, and hypoblast for the under layer. From the242 epiblast they derive53 the epidermis and central nervous system (or would even limit the latter to the central gray matter), together with some parts of the sense-organs. From the mesoblast, the muscles, nerves (and probably white matter of the centres), bones, connective tissue, and blood-vessels54. From the hypoblast, the epithelial lining55 of the alimentary canal, trachea, bronchial tubes, as well as the liver, pancreas, etc.128 K?lliker’s suggestion is much to the same effect, namely, that the three layers may be viewed as two epithelial layers, between which subsequently arises a third, the origin of nerves, muscles, bones, connective tissue, and vessels.129
103. The way in which the history may be epitomized is briefly56 this: There are two germinal membranes57, respectively representing the Instrumental and Alimental Systems. Each membrane differentiates58, by different appropriations59 of the yolk substance, into three primary layers, epithelial, neural, and muscular. In the epiblast, or upper membrane, these layers represent: 1°, the future epidermis with its derivatives—hair, feathers, nails, skin glands, and chromatophores; 2°, the future nervous tissue; 3°, the future muscular tissue.130 (Bone, dermis45, connective tissue, and blood-corpuscles are subsequent formations.)
The hypoblast, or under membrane, in an inverted61 order presents a similar arrangement: 1°, the unstriped243 muscular tissue of viscera and vessels; 2°, the nervous tissue of the sympathetic system; 3°, the epithelial lining of the alimentary canal with its glands.
Fundamentally alike as these two membranes are, they have specific differences; but in both we may represent to ourselves the embryological unit constituted by an epithelial cell, a nerve-cell, and a muscle-cell. All the other cells and tissues are adjuncts, necessary, indeed, to the working of the vital mechanism62, but subordinated to the higher organites.
104. This conception may be compared with that of His in the division of Archiblast and Parablast assigned by him to the germ and accessory germ.131 We can imagine, he says, the whole of the connective substances removed from the organism, and thus leave behind a scaffolding in which brain and spinal63 cord would be the axis64, surrounded by muscles, glands, and epithelium, and nerves as connecting threads. All these parts stand in more or less direct relation to the nervous system. All are continuous. By a similar abstraction we can imagine this organic system removed, and leave behind the connected scaffolding which is formed from the accessory germ; but this latter has only mechanical significance; the truly vital functions belong to the other system.
105. The researches of modern histologists have all converged65 towards the conclusion that the organs of Sense are modifications66 of the surface, with epithelial cells which on the one side are connected with terminal hairs, or other elements adapted to the reception of stimuli68, and are connected on the other side through nerve-fibres with the perceptive69 centres. It has been shown that nerve-fibres often terminate in (or among) epithelial cells—sensory70 fibres at the surface, and motor-fibres244 in the glands.132 Whether the fibres actually penetrate71 the substance of the cell, or not, is still disputed. Enough for our present purpose to understand that there is a physiological72 connection between the two, and above all that sensory nerves are normally stimulated through some epithelial structure or other.
Fig. 15.—Transverse section of a Blastoderm incubated for eighteen hours. The section passes through the medullary groove73, m e. A, epiblast. B, mesoblast. C, hypoblast. m f, medullary fold, c h, notochord.
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 expert245 embryologist is often unable to distinguish the embryo of a reptile74 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 justified75 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 specify76 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 subdivide32 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 luminous77 conception—first announced by Goethe, and subsequently developed by Milne Edwards—which regards the organism as increasing in power and complexity78 by a physiological “division of labor,” very similar to that division of employments which characterizes the developed social organism. But the metaphor79 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”246 presupposes that laborers80 with their labor-materials are already existing; the change is one of rearrangement: instead of each laborer81 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 prawn82 a lobster83. No sooner has the specific character been acquired, no sooner is one organite formed by differentiation, than there is an absolute barrier against any transformation84 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 crustacean85. In the homogeneous cellular86 mass which subsequently becomes the “vertebral plates,” a group of cells is very early differentiated87: this is the rudimentary spinal ganglion, which becomes enveloped88 in247 a membrane, and then pursues a widely different course from that of the other cells surrounding it, so that “the same cell which was formerly89 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 Cosmos90, typical forms once attained91 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 scattered92 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 appropriation60 acquiring different248 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 modification67 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 Hydra93, recorded by Kleinenberg, and of neuro-muscular fibres in the Bero?, 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 stimulus95. The first morphological change is one whereby a part of the surface is bent94 inwards, and forms the lining249 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 insignificant96 compared with that of the inner surface, which not only absorbs but secretes97 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—K?lliker, 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 inevitable98: 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
250 111. 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 merged99 in nerve actions.
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1
stimulation
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| n.刺激，激励，鼓舞 | |
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glands
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| n.腺( gland的名词复数 ) | |
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neural
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| adj.神经的，神经系统的 | |
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stimulated
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| a.刺激的 | |
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intervention
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| n.介入，干涉，干预 | |
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underneath
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| adj.在...下面，在...底下；adv.在下面 | |
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awakened
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| v.（使）醒( awaken的过去式和过去分词 )；（使）觉醒；弄醒；（使）意识到 | |
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interpretation
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| n.解释，说明，描述；艺术处理 | |
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phenomena
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| n.现象 | |
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unity
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| n.团结，联合，统一；和睦，协调 | |
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embryo
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| n.胚胎，萌芽的事物 | |
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anatomy
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| n.解剖学，解剖；功能，结构，组织 | |
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physiology
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| n.生理学，生理机能 | |
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beak
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| n.鸟嘴，茶壶嘴，钩形鼻 | |
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differentiation
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| n.区别，区分 | |
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molecular
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| adj.分子的；克分子的 | |
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agitation
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| n.搅动；搅拌；鼓动，煽动 | |
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varnished
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| 浸渍过的，涂漆的 | |
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deformed
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| adj.畸形的；变形的；丑的，破相了的 | |
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labors
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| v.努力争取(for)( labor的第三人称单数 )；苦干；详细分析；（指引擎）缓慢而困难地运转 | |
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wondrous
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| adj.令人惊奇的，奇妙的；adv.惊人地；异乎寻常地；令人惊叹地 | |
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incessant
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| adj.不停的，连续的 | |
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emergence
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| n.浮现，显现，出现，（植物）突出体 | |
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pointed
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| adj.尖的，直截了当的 | |
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membrane
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| n.薄膜，膜皮，羊皮纸 | |
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capability
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| n.能力；才能；(pl)可发展的能力或特性等 | |
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serially
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| adv.连续地，连续刊载地 | |
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insufficient
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| adj.(for，of)不足的，不够的 | |
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microscopic
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| adj.微小的，细微的，极小的，显微的 | |
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nucleus
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| n.核，核心，原子核 | |
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subdivides
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| 再分，细分( subdivide的第三人称单数 ) | |
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subdivide
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| vt.细分（细区分，再划分，重分，叠分，分小类） | |
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appreciation
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| n.评价；欣赏；感谢；领会，理解；价格上涨 | |
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progeny
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| n.后代，子孙；结果 | |
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yolk
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| n.蛋黄，卵黄 | |
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downwards
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| adj./adv.向下的（地），下行的（地） | |
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precisely
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| adv.恰好，正好，精确地，细致地 | |
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dermal
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| adj.皮肤的，真皮的 | |
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mucous
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| adj. 黏液的，似黏液的 | |
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analogous
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| adj.相似的；类似的 | |
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nutrient
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| adj.营养的，滋养的；n.营养物，营养品 | |
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disposition
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| n.性情，性格；意向，倾向；排列，部署 | |
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fig
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| n.无花果(树) | |
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epidermis
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| n.表皮 | |
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dermis
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| n.真皮 | |
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alimentary
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| adj.饮食的，营养的 | |
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gastric
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| adj.胃的 | |
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specially
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| adv.特定地；特殊地；明确地 | |
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vascular
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| adj.血管的，脉管的 | |
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controverted
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| v.争论，反驳，否定( controvert的过去式和过去分词 ) | |
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germinative
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| adj.发芽的，有发育力的 | |
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glandular
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| adj.腺体的 | |
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derive
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| v.取得；导出；引申；来自；源自；出自 | |
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vessels
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| n.血管( vessel的名词复数 )；船；容器；（具有特殊品质或接受特殊品质的）人 | |
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lining
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| n.衬里，衬料 | |
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briefly
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| adv.简单地，简短地 | |
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membranes
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| n.（动物或植物体内的）薄膜( membrane的名词复数 )；隔膜；（可起防水、防风等作用的）膜状物 | |
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differentiates
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| 区分，区别，辨别( differentiate的第三人称单数 )； 区别对待； 表明…间的差别，构成…间差别的特征 | |
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appropriations
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| n.挪用（appropriation的复数形式） | |
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appropriation
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| n.拨款，批准支出 | |
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inverted
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| adj.反向的，倒转的v.使倒置，使反转( invert的过去式和过去分词 ) | |
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mechanism
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| n.机械装置；机构，结构 | |
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spinal
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| adj.针的，尖刺的，尖刺状突起的；adj.脊骨的，脊髓的 | |
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axis
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| n.轴，轴线，中心线；坐标轴，基准线 | |
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converged
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| v.（线条、运动的物体等）会于一点( converge的过去式 )；（趋于）相似或相同；人或车辆汇集；聚集 | |
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modifications
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| n.缓和( modification的名词复数 )；限制；更改；改变 | |
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modification
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| n.修改，改进，缓和，减轻 | |
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stimuli
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| n.刺激（物） | |
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perceptive
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| adj.知觉的，有洞察力的，感知的 | |
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sensory
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| adj.知觉的，感觉的，知觉器官的 | |
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penetrate
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| v.透(渗)入；刺入，刺穿；洞察，了解 | |
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physiological
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| adj.生理学的，生理学上的 | |
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groove
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| n.沟，槽；凹线，（刻出的）线条，习惯 | |
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reptile
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| n.爬行动物；两栖动物 | |
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justified
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| a.正当的，有理的 | |
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specify
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| vt.指定，详细说明 | |
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luminous
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| adj.发光的，发亮的；光明的；明白易懂的；有启发的 | |
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complexity
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| n.复杂(性)，复杂的事物 | |
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metaphor
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| n.隐喻，暗喻 | |
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laborers
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| n.体力劳动者，工人( laborer的名词复数 )；（熟练工人的）辅助工 | |
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laborer
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| n.劳动者，劳工 | |
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prawn
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| n.对虾，明虾 | |
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lobster
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| n.龙虾，龙虾肉 | |
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transformation
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| n.变化；改造；转变 | |
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crustacean
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| n.甲壳动物；adj.甲壳纲的 | |
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cellular
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| adj.移动的；细胞的，由细胞组成的 | |
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differentiated
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| 区分，区别，辨别( differentiate的过去式和过去分词 )； 区别对待； 表明…间的差别，构成…间差别的特征 | |
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enveloped
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| v.包围，笼罩，包住( envelop的过去式和过去分词 ) | |
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formerly
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| adv.从前，以前 | |
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cosmos
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| n.宇宙；秩序，和谐 | |
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attained
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| （通常经过努力）实现( attain的过去式和过去分词 )； 达到； 获得； 达到（某年龄、水平、状况） | |
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scattered
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| adj.分散的，稀疏的；散步的；疏疏落落的 | |
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hydra
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| n.水螅；难于根除的祸患 | |
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bent
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| n.爱好，癖好；adj.弯的；决心的，一心的 | |
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stimulus
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| n.刺激，刺激物，促进因素，引起兴奋的事物 | |
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insignificant
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| adj.无关紧要的，可忽略的，无意义的 | |
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secretes
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| v.（尤指动物或植物器官）分泌( secrete的第三人称单数 )；隐匿，隐藏 | |
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inevitable
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| adj.不可避免的，必然发生的 | |
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merged
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| （使）混合( merge的过去式和过去分词 )； 相融； 融入； 渐渐消失在某物中 | |
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