principles which had proved so successful in the case of the whole organ. When the improvements in the microscope opened up a new world to the anatomist, and a wholly fresh mechanical analysis of the structure of living bodies became possible, great hopes were entertained that the old method applied to the new facts would soon solve the riddles of life by showing how the mysterious operations of the living substances out of which the grosser organs were built were the outcome of structural arrangements which had hitherto remained invisible, were in fact the functions of minute component organs. A vision of a grand simplicity of organic nature dawned upon the minds of physiologists. It seemed possible to conceive of all living beings as composed of minute organic units, of units whose different actions resulted from their different structural characters, whose functions were explicable by, and could be deduced from, their anatomical features, such units being built up into a number of gross organs, the functions of each of which could in turn be explained by the direction which its mechanical build gave to the efforts of its constituent units. Such a view seemed to have touched the goal, when, in the first half of this century, the so-called "cell-theory" was enunciated as a physiological generalization. Long before, in the previous century, the genius of Caspar Wolff had led him to maintain that the bodies of living beings may be regarded as composed of minute constituent units, which, being in early life all alike and put together as an unformed mass, gradually differentiate and are ultimately arranged into the tissues and organs of the adult being. But, though Wolff was not unaware of the physiological bearing of his conception, his mind was chiefly bent towards morphological views, and his celltheory is essentially a morphological one. The cell-theory, however, which became famous in the third decade of the present century, and to which the twin names of Schwann and Schleiden will always be attached, was essentially a physiological one. The chief interest which these authors felt in the ideas that they put forth centred in the conviction that the properties of the cell as they described it were the mechanical outcome of its build; and for a time it seemed possible that all physiological phenomena could be deduced from the functions of cells, the anatomical characters of the various kinds of cells determining in turn their special functions. In the cell-theory the conception of organs and functions reached its zenith; but thenceforward its fall, which had been long prepared, was swift and great. Two movements especially hurried on its decline. It had long been a reproach to physiologists that, while to most organs of the body an appropriate function had been assigned, in respect to certain even conspicuous organs no special use or definite work could be proved to exist. Of these apparently functionless organs the most notorious instance was that of the spleen, a large and important body, whose structure, though intricate, gave no sign of what its labours were, and whose apparent uselessness was a stumbling-block to the theological speculations of Paley. While in the case of other organs a definite function could be readily enunciated in a few words, and their existence therefore easily accounted for, the spleen remained an opprobrium, existing, as it appeared to do, without purpose, and therefore without cause. The progress of discovery during the present century, oy a cruel blow, instead of pointing out. the missing use of the spleen, rudely shook the confidence with which the physiologists concluded that they had solved the riddle of an organ when they had allotted to it a special function. From very old times it had been settled that the function of the liver was to secrete bile; and the only problems left for inquiry as touching the liver seemed to be those which should show how the minute structure of the organ was adapted for carrying on this work. About the middle of this century, however, the genius of Claude Bernard led him to the discovery that the secretion of bile was by no means the chief labour of the liver. He showed that this great viscus had other work to do than that of secreting bile, had another "function" to perform, but a function which seemed to have no reference whatever to the mechanical arrangements of the organ, which could never have been deduced from any inspection however complete of its structure, even of its most hidden and minute features, and which therefore could not be called a function in the old and proper sense of that word. By a remarkable series of experiments, which might have been carried out by one knowing absolutely nothing of the structural arrangements of the liver beyond the fact that blood flowed to it along the portal vein, and from it along the hepatic vein, he proved that the liver, in addition to the task of secreting bile, was during life engaged in carrying on a chemical transformation by means of which it was able to manufacture and store up in its substance a peculiar kind of starch, to which the name of glycogen was given. Bernard himself spoke of this as the glycogenic function of the liver, but he used the word "function" in a broad indefinite sense, simply as work done, and not in the older narrower meaning as work done by an organ structurally adapted to carry on a work which was the inevitable outcome of the form and internal build of the organ. In this glycogenic function organization, save only the arrangements by means of which the blood flows on from the portal to the hepatic channels in close proximity to the minute units of the liver-substance, the so-called hepatic cells, appeared to play no part whatever; it was not a function, and in reference to it the liver was not an organ, in the old senses of the words. This discovery of Bernard's threw a great flash of light into the darkness hitherto hiding the many ties which bound together distant and mechanically isolated parts of the animal body. Obviously the liver made this glycogen, not for itself, but for other parts of the body; it laboured to produce, but they made use of, the precious material, which thus became a bond of union between the two. The glycogenic labours of the simple hepatic substance carried out independently of all intricate structural arrangements, and existing in addition to the hepatic function of secreting bile, being thus revealed, men began to ask themselves the question, May not something like this be true of other organs to which we have allotted a function and thereupon rested content? And further, in the cases where we have striven in hope, and yet in vain, to complete the interpretation of the function of an organ, by finding in the minute microscopic details of its structure the mechanical arrangements which determine its work, may we not have followed throughout a false lead, and sought for organization where organization in our sense of the word does not exist? The answer to this question, and that an affirmative one, was hastened by the collapse of the cell-theory on its physiological side, very soon after it had been distinctly formulated. The "cell," according to the views of those who first propounded the cell-theory, consisted essentially of ar envelope or "cell-membrane," of a substance or substance contained within the cell-membrane, hence called cell contents, and of a central body or kernel called the "nucleus," differing in nature from the rest of the cell contents. And, when facts were rapidly accumulated, ait tending to prove that the several parts of the animal or vegetable body, diverse as they were in appearance and structure, were all built up of cells more or less modified, Protoplasia. Proto theory. as well as certain structural features, and the several kinds of material being variously arranged in the body. Each of these body-components was spoken of as a tissue, muscular tissue, nervous tissue, and the like; and the varied actions of the body were regarded as the result of the activities of the several tissues modified and directed by the circumstance that the tissues were to a great extent arranged in mechanical contrivances or organs which largely deter mined the character and scope of their actions. he hope arose that the functions of the cell might be With this anatomical change of front the physiological plasmic cell-theory was utterly destroyed. The cell was no longer a unit of organization; it was merely a limited mass of protoplasm, in which, beyond the presence of a nucleus, there was no visible distinction of parts. It was no longer possible to refer the physiological phenomena of the cell to its organization; it became evident that the work done by a "cell" was the result not of its form and cellular structure but simply of the nature and properties of the apparently structureless protoplasm which formed its body. A new idea pressed itself on men's minds, that organization was a concomitant and result of vital action, not its condition and cause; as Huxley in one of his earliest writings put it, "They [cells] are no more the producers of the vital phenomena than the shells scattered in orderly lines along the sea-beach are the instruments by which the gravitative force of the moon acts upon the ocean. Like these, the cells mark only where the vital tides have been, and how they have acted." 1 Hence arose the second of the two movements mentioned above, that which may be called the "protoplasmic" movement, a movement which, throwing overboard altogether all conceptions of life as the outcome of organization, as the mechanical result of structural conditions, attempts to put physiology on the same footing as physics and chemistry, and regards all vital phenomena as the complex products of certain fundamental properties exhibited by matter, which, either from its intrinsic nature or from its existing in peculiar conditions, is known as living matter, -mechanical contrivances in the form of organs serving only to modify in special ways the results of the exercise of these fundamental activities and in no sense determining their initial development. Long before the cell-theory had reduced to an absurdity the "organic" conception of physiology, the insight of the brilliant Bichat, so early lost to science, had led him to prepare the way for modern views by developing his doctrine of tissues." That doctrine regarded the body as made up of a number of different kinds cf living material, each kind of material having certain innate qualities proper to itself "The Cell-Theory," in Brit. and For. Med. Chir. Rev., vol. xii. 1853) p. 314. The imperfection of microscopic methods in Bichat's time, and, we may perhaps add, his early death, prevented him from carrying out an adequate analysis of the qualities or properties of the tissues themselves During the middle portion of this century, however, histological investigation, inquiry into the minute structure of the tissues, made enormous progress, and laid the basis for a physiological analysis of the properties of tissues. In a short time it became possible to lay down the generalization that all the several tissues arise, as far as structure is concerned, by a differentiation of a simple primitive living matter, and that the respective properties of each tissue are nothing more than certain of the fundamental properties of the primordial substance thrown into prominence by a division of labour running to a certain extent parallel to the differentiation of structure. Developed in a fuller manner, this modern doctrine may be expounded somewhat as follows. In its simplest form, a living being, as illustrated by some of the forms often spoken of as amoebæ, consists of a mass of substance in which there is no obvious distinction of parts. In the body of such a creature even the highest available powers of the microscope reveal nothing more than a fairly uniform network of material, a network sometimes compressed, with narrow meshes, sometimes more open, with wider meshes, the intervals of the meshwork being filled, now with a fluid, now with a more solid substance or with a finer and more delicate network, and minute particles or granules of variable size being sometimes lodged in the open meshes, sometimes deposited in the strands of the network. Sometimes, however, the network is so close, or the meshes filled up with material so identical in refractive power with the bars or films of the network, and at the same time so free from granules, that the whole substance appears absolutely homogeneous, glassy or hyaline. Analysis with various staining and other re agents leads to the conclusion that the substance of the network is of a different character from the substance filling up the meshes. Similar analysis shows that at times the bars or films of the network are not homogeneous, but composed of different kinds of stuff; yet even in these cases it is difficult if not impossible to recognize any definite relation of the components to each other such as might deserve the name of structure; and certainly in what may be taken as the more typical instance, where the network seems homogeneous, no microscopic search is able to reveal to us a distinct structural arrangement in its substance. In all probability optical analysis, with all its aids, has here nearly reached its limits; and, though not wholly justified, we may perhaps claim the right to conclude that the network in such case is made up of a substance in which no distinction of parts will ever be visible, though it may vary in places or at times in what may be spoken of as molecular construction, and may carry, lodged in its own substance, a variety of matters foreign to its real self. This remarkable network is often spoken of as consisting of protoplasm, and, though that word has come to be used in several different meanings, we may for the present retain the term. The body of an amoeba, then, or of a similar organism consists of a network or framework which we may speak of as protoplasm, filled up with other matters. In most cases it is true that in the midst of this protoplasmic body there is seen a| ourselves this total change which we denote by the term peculiar body of a somewhat different and yet allied "metabolism" as consisting on the one hand of a downnature, the so-called nucleus; but this we have reason to ward series of changes (katabolic changes), a stair of many think is specially concerned with processes of division or steps, in which more complex bodies are broken down with reproduction, and may be absent, for a time at all events, the setting free of energy into simpler and simpler waste without any injury to the general properties of the proto- bodies, and on the other hand of an upward series of plasmic body. changes (anabolic changes), also a stair of many steps, by which the dead food, of varying simplicity or complexity, is, with the further assumption of energy, built up into more and more complex bodies. The summit of this double stair we call "protoplasm." Whether we have a right to speak of it as a single body, in the chemical sense of that word, or as a mixture in some way of several bodies, whether we should regard it as the very summit of the double stair, or as embracing as well the topmost steps on either side, we cannot at present tell. Even if there be a single substance forming the summit, its existence is absolutely temporary: at one instant it is made, at the next it is unmade. Matter which is passing through the phase of life rolls up the ascending steps to the top, and forthwith rolls down on the other side. But to this point we shall return later on. Further, the dead food, itself fairly but far from wholly stable in character, becomes more and more unstable as it rises into the more complex living material. It becomes more and more explosive, and when it reaches the summit its equilibrium is overthrown and it actually explodes. The whole downward stair of events seems in fact to be a series of explosions, by means of which the energy latent in the dead food and augmented by the touches through which the dead food becomes living protoplasm, is set free. Some of this freed energy is used up again within the material itself, in order to carry on this same vivification of dead, food; the rest leaves the body as heat or motion. Sometimes the explosions are, so to speak, scattered, going off as it were irregularly throughout the material, like a quantity of gunpowder sprinkled over a surface, giving rise to innumerable minute puffs, but producing no massive visible effects. Sometimes they take place in unison, many occurring together, or in such rapid sequence that a summation of their effects is possible, as in gunpowder rammed into a charge, and we are then able to recognize their result as visible movement, or as appreciable rise of temperature. Now such a body, such a mass of simple protoplasm, homogeneous save for the admixtures spoken of above, is a. living body, and all the phenomena which we sketched out at the very beginning of this article as characteristic of the living being may be recognized in it. There is the same continued chemical transformation, the same rise and fall in chemical dignity, the same rise of the dead food into the more complex living substance, the same fall of the living substance into simpler waste-products. There is the same power of active movement, a movement of one part of the body upon another giving rise to a change of form, and a series of changes of form resulting eventually in a change of place. In what may be called the condition of rest the body assumes a more or less spherical shape. By the active transference of part of the mass in this or that direction the sphere flattens itself into a disk, or takes on the shape of a pear, or of a rounded triangle, or assumes a wholly irregular, often star-shaped or branched form. Each of these transformations is simply a rearrangement of the mass, without change of bulk. When a bulging of one part of the body takes place there is an equivalent retraction of some other part or parts; and it not unfrequently happens that one part of the body is repeatedly thrust forward, bulging succeeding bulging, and each bulging accompanied by a corresponding retraction of the opposite side, so that, by a series of movements, the whole body is shifted along the line of the protuberThe tiny mass of simple living matter moves onward, and that with some rapidity, by what appears to be a repeated flux of its semi-liquid substance. The internal changes leading to these movements may begin, and the movements themselves be executed, by any part of the uniform body; and they may take place without any obvious cause. So far from being always the mere passive results of the action of extrinsic forces, they may occur spontaneously, that is, without the coincidence of any recognizable disturbance whatever in the external conditions to which the body is exposed. They appear to be analogous to what in higher animals we speak of as acts of volition. They may, however, be provoked by changes in the external conditions. A quiescent amoeba may be excited to activity by the touch of some strange body, or by some other event,-by what in the ordinary language of physiology is spoken of as a stimulus. The protoplasmic mass is not only mobile but sensitive. When a stimulus is applied to one part of the surface a movement may commence in another and quite distant part of the body; that is to say, molecular disturbances appear to be propagated along its substance without visible change, after the fashion of the nervous impulses we spoke of in the beginning of this article. The uniform protoplasmic mass of the amoeba exhibits the rudiments of those attributes or powers which in the initial sketch we described as being the fundamental characteristics of the muscular and nervous structures of the higher animals. These facts, and other considerations which might be brought forward, lead to the tentative conception of protoplasm as being a substance (if we may use that word in somewhat loose sense) not only unstable in nature but subject to incessant change, existing indeed as the expression of incessant molecular, that is, chemical and physical change, very much as a fountain is the expression of an incessant replacement of water. We may picture to These various phenomena of protoplasm may be conven- Proper iently spoken of under the designation of so many properties, ties of or attributes, or powers of protoplasm, it being understood protoplasm. that these words are used in a general and not in any definite scholastic sense. Thus we may speak of protoplasm as having the power of assimilation, i.e., of building up the dead food into its living self; of movement, or of contractility as it is called, i.e., of changing its form through internal explosive changes; and of irritability or sensitiveness, i.e., of responding to external changes, by less massive internal explosions which, spreading through its mass, are not in themselves recognizable through visible changes, though they may initiate the larger visible changes of movement. These and other fundamental characters, all associated with the double upward and downward series of chemical changes, of constructive and destructive metabolism, are present in protoplasm wherever found; but a very brief survey soon teaches us that specimens of protoplasm existing in different beings or in different parts of the same being differ widely in the relative prominence of one or another of these fundamental characters. On the one hand, in one specimen of protoplasm the energy which is set free by the series of explosions constituting the downward changes of destructive metabolism may be so directed as to leave the mass almost wholly in the form of heat, thus producing very little visible massive change of form. Such a protoplasm consequently, however irritable and Endo ectoderm cells. The cell to cell over the surface of the whole body. explosive, exhibits little power of contractility or move- | foreign body, or from intrinsic events, may sweep from Some of the simpler and earlier features of such a division and differentiation may be brought out by comparing with the life of such a being as the amoeba that of a more complex and yet simple organism as the hydra or freshwater polyp. Leaving out certain details of structure, which need not concern us now, we may say that the hydra consists of a large number of units or cells firmly attached to each other, each cell being-composed of protoplasm, and in its broad features resembling an amoeba. The polyp is in fact a group or crowd of amoeba-like cells so associated together that, not only may the material of each cell, within limits, be interchanged with that of neighbouring cells, but also the dynamic events taking place in one cell, and leading to exhibitions of energy, may be similarly communicated to neighbouring cells, also within limits. These cells are arranged in a particular way to form the walls of a tube, of which the body of the hydra practically consists. They form two layers in appoderm and sition, one an internal layer called the endoderm, lining the tube, the other an external layer called the ectoderm, forming the outside of the tube. And, putting aside minor details, the differences in structure and function observable in the organism are confined to differences between the ectoderm on the one hand, all the constituent cells of which are practically alike, and the endoderm on the other, all the cells of which are in turn similarly alike. The protoplasm of the ectoderm cells is so constituted as to exhibit in a marked degree the phenomena of which we spoke above as irritability and contractility, whereas in the endoderm these phenomena are in abeyance, those of assimilation being prominent. The movements of the hydra are chiefly brought about by changes of form of the ectoderm cells, especially of tail-like processes of these cells, which, arranged as a longitudinal wrapping of the tubular body, draw it together when they shorten, and lengthen it out when they elongate, and it is by the alternate lengthening and shortening of its body, and of the several parts of its body, that the hydra changes its form and moves from place to place. Inaugurating these changes of form, the products of contractility, are the more hidden changes of irritability; these also are especially developed in the ectoderm cells, and travel readily from cell to cell, so that a disturbance originating in one cell, either from some extrinsic cause, such as contact with a Thus the total labour of the organism is divided between these two membranes. The endoderm cells receive food, transmute it, and prepare it in such a way that it only needs a few final touches to become living material, these same cells getting rid at the same time of useless ingredients and waste matter. Of the food thus prepared the endoderm cells, however, themselves use but little; the waste of substance involved in the explosions which carry out movement and feeling is reduced in them to a minimum; they are able to pass on the greater part of the elaborated nourishment to their brethren the ectoderm cells. And these, thus amply supplied with material which it needs but little expenditure of energy on their part to convert into their living selves, thus relieved of the greater part of nutritive labour, are able to devote nearly the whole of their energies to movement and to feeling. Microscopic examination further shows that these two kinds of cells differ from each other to some extent in visible characters; and, though, as we have seen, the differences in activity appear to be dependent on differences in invisible molecular arrangement rather than on gross visible differences such as may be called structural, still the invisible differences involve or entail, or are accompanied by, visible differences, and such differences as can be recognized between endoderm and ectoderm, even with our present knowledge, may be correlated to differences in their work; future inquiry will probably render the correlation still more distinct. The ectoderm cells together constitute what we have spoken of above as a tissue, whose function in the modern sense of the word is movement and feeling, and the endoderm cells constitute a second tissue, whose function is assimilation; and the phenomena of the whole being result from the concurrent working of these two functions. Of organs, in the old sense of the word, of mechanica contrivances, there is hardly a trace.1 The performances of the being are, it is true, conditioned by its being moulded in the form of a long tubular sac with a crown of like tubular arms, but beyond this the explanation of every act of the hydra's life is first to be sought in the characters of the endoderm and ectoderm. The physiology of the hydra is, for the most part, a series of problems, dealing on the one hand with the intimate nature of the ectodermic protoplasm and the changes in that protoplasm which give rise to movement and feeling, as well as with the laws whereby those changes are so regulated that movement and feeling come and go as the needs of the organism may require, and on the other hand with the intimate nature of the endodermic protoplasm and the changes in that protoplasm whereby the dead food is, also according to the needs of the economy, transformed into living substance. Whereas the older physiology dealt almost exclusively with mechanical problems, the physiology of to-day is chiefly busied with what may be called molecular problems. The physiology of the higher animals, including man, is merely a development of the simpler physiology of the hydra, which has been rendered more complex by a greater division of physiological labour, entailing greater differen 1 The existence of certain minute mechanisms called urticating organs lodged in the ectoderm cells does not affect the present argument. tiation of structure, and been varied by the intercalation | the like tissues of mechanical virtues, manufactured by of numerous mechanical contrivances. an active protoplasm, but themselves passive, no longer active), into various mechanical contrivances. Similarly the sensory cells, as notably those of the eye and the ear, set apart to be acted upon by special agents, are provided with special mechanisms in order that the agent may act with more complete precision. Thus the sensory cells constituting the retina of the eye, in which alone sensory, visual impulses are generated, are provided with an intricate dioptric mechanism, formed partly of inert tissues such as the lens, partly of peculiarly arranged muscular and nervous elements. In the hydra each ectoderm cell-for, broadly speaking, they are all alike-serves three chief purposes of the body. (1) It is sensitive, that is, it is thrown into peculiar molecular agitations, with expenditure of energy, when acted upon by external agents. In man and the higher animals certain cells of the original ectoderm of the embryo are differentiated from their fellows (which, losing to a large extent this sensitiveness, remain as a mechanical covering to the body) by a more exquisite development of this power of reaction, and moreover are differentiated from each other in their relative sensitiveness to different agents, so that In this way the simple ectoderm of the hydra is replaced one set of cells becomes peculiarly susceptible to light, by a complicated system composed of organs, some of them another set to pressure, and the like. Thus the uniform of extremest intricacy. But the whole system may be reectoderm of the hydra, uniformly susceptible to all agencies, duced to two sets of factors. On the one hand there are is replaced by a series of special groups of cells forming organs in the old sense of the word, that is, mechanical the basis of sensory organs, each group being specially arrangements, some connected with the muscles and others sensitive to one agent, and having the nature of its con- connected with the sensory cells, organs whose functions stituent cells correspondingly modified. (2) In each ecto- have. for the most part to be interpreted on mechanical derm cell of the hydra the agitations primarily induced principles, since their most important factors, putting aside by the exciting agent become so modified by changes intervening muscular and nervous elements, are the inert taking place in the cell that the outcome is not always products of protoplasm doing simple mechanical work. the same. According to processes taking place in the On the other hand there are organs in the later sense of cell, movement of one kind or another, or no movement the word, namely, sensory cells differentiated to be sensiat all, may result, and such movement as results may take tive to special influences, central nervous cells differentiated place immediately or at some other time; it may be at a to to carry on the inner nervous work, muscles differentiated time so distant that the connexion between the exciting to contract, and nerves differentiated to bind together these disturbance is lost, and the movement appears to be spon- three other factors, The work of these latter organs is taneous. In man and the higher animals these more dependent on the nature of their protoplasm; mechanical complex. "neural" processes are carried on, not by the arrangements play but little part in them; and the results simple sensory cells which receive the primary impression, of their activity can in no way be explained on simple but by a group of cells set apart for the purpose. These mechanical principles. cells constitute a central nervous system, in which a still further division of labour and differentiation of structure takes place, the simple neurotic processes of the hydra, with its dim volition and limited scope of action, being developed in a complex manner into processes which range from simple elaboration of the initial additional agitation of the sensory cell into what we speak of as intelligence and thought. (3) Each octoderm cell, by its tail-like prolongation, or by its whole body, contributes to the movement of the animal while still carrying on the two other actions just described. In man and the higher animals the material of the sensory cell and of the central nervous cells is too precious to be wasted in movements; these accordingly are carried out by groups of cells constituting the muscular tissue, in which both the sensitiveness and the higher neurotic processes of the primitive cell are held in abeyance; indeed, the latter have almost disappeared in order that the energy of the protoplasm may be more completely directed to producing those changes of form which mine the movements of the animal. endo derm. Corresponding with this differentiation of the ectoderm Differcells runs a somewhat similar differentiation of the endo- entiation of derm cells. In the hydra each endoderm cell appears to receive some of the food bodily into itself and there to elaborate it into what may be spoken of as prepared nutritive material. Some of this material the cell retains within itself in order to renew its own protoplasm; the rest oozes out to the ectoderm cells, the replenishment of whose protoplasm is thereby effected with a saving of labour. In the higher animals the preparation of food is far more com plicated. The endodermic sheet of the alimentary canal is folded and arranged into organs called glands, with the mechanical advantage that a large amount of surface is secured within a small bulk; and the constituent endodermic cells of their glands pour out, or secrete, as is said, divers fluids into the cavity of the canal, so that much preliminary preparation of digestion of the food takes place before the food really enters the body. Further, these deter-secreting glandular cells are so differentiated as to pour out special juices acting on special constituents of a meal, Further, the separation in space of these three groups and the food subjected in turn to the action of these several of cells or tissues necessitates the introduction of elements juices becomes thoroughly prepared for reception into the whereby the agitations set up in the sensory cell should body. This reception is carried out by other endoderm be communicated to the central nervous cells, where these cells, which in receiving the digested food probably act agitations are further elaborated, as well as of elements upon it so as still further to heighten its nutritive value; whereby the muscular tissue may receive vibrations and the absorbed food, before it is presented to the mus from the central nervous cells, so that the movements of cular and nervous tissues, for whose use it is largely, though the body may be determined by these. Hence strands of of course not exclusively, intended, is subjected to the irritable protoplasm whose energy is not spent in move-action of other cells, such as those forming the lymphatic ment, but wholly given up to the rapid and easy transmission of molecular vibrations, unite, as sensory nerves, the sensory cells with the central nervous cells, and, as motor nerves, these with the muscles. Lastly, for the adequate carrying out of complex movements, the contractile cells, elongated into specially constructed fibres and constituting the muscles, are arranged, with inert tissues such as bones, cartilages, tendons, and glands and the liver, in order that it may be still further elaborated, still further prepared for the final conversion into living protoplasm. As in the case of the tissues and organs of ectodermic origin, so also here, the wide separation in space of the masses of differentiated cells constituting tissues necessitates the introduction of mechanical contrivances for the carriage of material from place to place. In the simple |