METAMORPHOSIS OF ORGANISMS. 489 we have of the control of the agent heat over development, than the wellascertained fact that the time of emersion of larvæ depends upon the temperature? The silk-grower, by placing the eggs of the insect in an ice-house, retards them as long as he pleases. The amputated limbs of the water-newt can only be reproduced at a temperature from 58° to 75°. The tadpole, kept in the dark, does not pass on to development as a frog. In decaying organic solutions, animalcules do not appear if light be excluded. pend on inva Upon the whole, therefore, we conclude that organisms of every kind, so far from presenting any resistance to change, are im- Changes of orpressed without any difficulty by every exterior condition; ganization deand since existing natural circumstances have been main- riable laws. tained for a long time without any apparent change, their sameness produces a sameness in the order and manner of development. But it should be borne in mind that this idea of sameness can be entertained only on an imperfect view of the state of Nature, for there is scarce one of those conditions, to the sameness of which we have been referring, which has not, in reality, undergone slow secular variations; and with those changes there have been changes in the manner of development. consequence of In truth, as I have on a former page observed, the only things which are absolutely unchangeable are the laws of Nature, such, for instance, as that of gravitation; every thing else is to be looked upon as an effect, or as a changeable phenomenon arising from the operation of those laws. So, therefore, though, in this chapter, the terms physical in- Successive metfluences and natural conditions have been repeatedly used, amorphosis a yet a higher and more philosophical view of the case brings invariable law. us inevitably at last to the idea of law; and therefore I accept the interpretation of all these facts, which has of late years been impressing itself more and more strongly and clearly on the minds of physiologists, that the development of every organism, from a primordial cell to its final condition, however elevated that condition may be, is the inevitable consequence of the operation of a universal, invariable, and eternal law. All animals, no matter what position they occupy in the scale of nature, unquestionably arise in the first instance from a cell, which, possessing the power of giving birth to other cells, a congeries at last arises, the size and form of which is determined wholly by external circumstances. In all cases, the material from which these cells are formed is obtained. from without, and, whatever the eventual shape of the structure may be, the first cell is in all instances alike. There is no perceptible difference between the primordial cell which is to produce the lowest plant and that which is to evolve itself into the most elaborate animal. The mode of growth, and the arrangement of the new cells as they come into existence, determining not only the form, but also the functions of the new 490 METAMORPHOSIS OF ORGANISMS. being, depend on the particular physical conditions under which the growth is taking place. The germ which is to produce a lichen obtains from materials around it the substances it wants as best it may; but the germ which is to end in the development of man is brought in succession under the influence of many distinct states. As a consequence of this, it gives rise in succession to a series of animated forms, which, assuming by degrees a higher complexity, end at last in the perfect human being. At one time it was believed that these metamorphoses, as they are termed, are limited to insects and frogs: the insect, which at first appears under the form of a caterpillar as it comes from the egg, and, passing through the pupa state, at last takes its true position as a winged being; the frog, which, appearing at first from the ovum as a true fish, whose respiration is carried forward by gills, and whose life is limited to the water, at last assumes a new constitution and a new organization, breathes by lungs, and becomes an amphibious reptile. But it is now known that these, so far from being exceptions, are only instances of a general rule, which is, that all organized beings shall begin existence at the bottom of the scale, and, taking on one type of life after another, in more or less rapid succession, end, finally, in assuming a size and form analogous to those of the parent which gave them birth. There is a general resemblance between the life of an individual and the life of a species. Each has its time of birth, its time of maturity, its time of decline; each also has its embryonic states. The fossil forms of the early geological ages are in many cases the embryos of existing animals. Upon each all natural agents have exerted their effects, pushing forward or retarding development; and this applies not only to animals, but also to plants: it is in accordance with the principles we are setting forth that over the whole domain of life natural forces exert their sway. Change the conditions under which growth is taking place, and you at once change resulting form and function. It is in this manner that, on a small scale, the horticulturist works in furnishing us what are called improved varieties of flowers and fruits. It is in this manner that animals, known to have been indisputably of the same original kind, assume such different forms and characters in various climates. It is true, we can not expect in an abrupt manner to bring about such striking modifications in a solitary individual, for the life of an individual is readily destroyed, but not so the life of a race; and Nature, carrying on her operations in the slow lapse of centuries, and dealing with races rather than individuals, forces them up to any point of development she may desire, but still the impress of the laws under which they have been brought to that condition is upon them, and each betrays, in the embryonic and foetal forms, a manifestation of the metamorphoses through which his race has run. RUDIMENTARY ORGANS. 491 organs and their inter Our attention might here be directed to that interesting class of phenomena known to comparative anatomists under the title of Rudimentary rudimentary organs-that is to say, organs which exist in an apparently undeveloped and useless condition, such, for in- pretation. stance, as the mammæ of the male mammalian, or the subcutaneous feet of certain snakes-for these are facts intimately connected with the subject before us. It looks as if Nature stopped short in her attempt at reaching perfection, but it proves to us the constancy of the plans on which she works. In the case of the whale, which, though apparently belonging to the fishes, is a warm-blooded mammal and suckles its young, the general type of its class is observed even down to minute particulars; it is the attribute of those belonging to it that they shall have seven cervical vertebræ, and this is equally the case with the camelopard, with its long, graceful neck, and the mole, which seems to have no neck at all. In the whale, which conforms to that general rule, the teeth are, moreover, found in the jaw, in the earlier period of life, uncut, precisely as we find them at birth in the human infant. In this last instance we think we see a wise provision and foresight of nature, which does not give to man these masticating organs before the time they are wanted; but what are we to make of the former case? Man is not always a true interpreter of the works of God. Shut up, as they are, in the interior of the bony mass of the jaw, never to be developed and never to be used, does not that look to a careless observer something like a work of supererogation? Or, in the case of such snakes as the anguis, typhlops, and amphisbana, why is it that Nature has placed under the skin the bony representatives of the extremities: the mode of progression of those animals is by the use of the ribs, and organs such as feet are never wanted. We may also turn to the other department of physiology, the vegetable world, and what do we there see? Rudimentary organs and excess of development are every where presented. An attentive examination of any flower proves that we may with truth regard it as a transformed branch,. the law of development being such that that which might have passed forward to the condition of a branch has turned to the condition of a flower; or, in still minuter particulars, we witness the same principle: that which might have evolved into a leaf turns indifferently, as circumstances may direct, into a sepal, a petal, or a stamen. But is it possible that there is all this confusion and want of precision in the works of Nature? Not so. If we consider rightly, Appearance of we shall come to the conclusion that Nature never works rudimentary organs the concontingently, nor resorts to a sudden contrivance to meet an sequence of exigency. All her operations are carried forward under far- law. reaching and universal laws. These rudimentary and perhaps useless organs come into existence through a general plan, of which they are 492 OF THE ORGANIC CELL. witnesses to us, if they subserve no other duty. They tell the same great fact which is so loudly proclaimed by all the phenomena of the restoration of parts and renovation of tissues, that the grouping of organized matter into definite and special forms is not a wanton or chance effect, but is the direct and inevitable consequence of invariable physical laws. Expedients are for the vacillating and weak, law is for the strong. It takes from the merit of any human contrivance if the engineer has to be constantly tampering with it to keep it going; we admire the machine that continues its movements without variation after it has left its maker's hand. I think we can have no nobler conception of the great Author of the wonderful forms around us than to regard them all, the vegetable and animal, the living and lifeless, the earth, and the stars, and the numberless worlds that are beyond our vision, as the offspring of one primitive idea, and the consequences of one primordial law. CHAPTER III. OF THE ORGANIC CELL: ITS DEVELOPMENT, REPRODUCTION, AND DIFFERENTIATION OF STRUCTURE AND FUNCTION. Simple and Nucleated Cells.-The Simple Cell: its Parts and Functions.-The Nucleated Cell: Differentiation of Cells.—Acquisition of new Functions.-Differentiation of the Animal Cell.— THE organic cell, which is the starting-point of every organism, vegSimple and nu- etable or animal, consists of a vesicle or shell, with included cleated cells. contents. If the vesicle be of uniform thickness all over, the cell is a simple one; but if there be upon some portion of it a thickened granular spot, the cell is said to be nucleated. its wall, utri cle, and endo chrome. The vesicle of the SIMPLE VEGETABLE CELL, more closely examined, The simple is found to be composed of different laminæ or strata. The vegetable cell: innermost, designated the primordial utricle, consists of an azotized substance, a member of the protein group. On the exterior of this pellicle, and, as it were, arising from its surface, lies the cell wall, which serves to give protection to the parts within. The cell wall is not a mere extension by thickening of the primordial utricle, as is proved by its chemical constitution; for, though it may vary in physical condition from a mere glairy mucus to a firm woody THE NUCLEATED CELL. 493 texture, it uniformly consists of a non-nitrogenized body, gummy, amylaceous, or ligneous. Indeed, though the vegetable cell is usually said to have two concentric investitures, the nitrogenized primordial utricle and the non-nitrogenized wall, it is more exact to describe the latter as consisting of several pellicles, which have been generated in succession from the outside surface of the utricle, and these differ from one another in their physical qualities, according as they are nearer to the surface of the utricle or nearer to the general exterior, recalling, in this respect, the analogous condition of the cuticle under circumstances that are somewhat parallel. Within the primordial utricle, the cell contents present themselves of a different nature and different form, according as the species of the cell may be. In different cases they are colored of various tints, and are of various consistency, more solid or more liquid. To the cell-contents the convenient designation of endochrome is given. This interior content is not to be understood as having a homogenous constitution, since sometimes even its colored portions are separated out and arranged in dots or spiral lines, which are very distinct from the remaining uncolored material. The active portion of such a cell consists of the utricle and endochrome conjointly, the cell wall only discharging a mechanical office. In the simple cell, all parts of the utricle appear to be endowed with equal power for carrying on the functions of the organism. Nucleated cell: But in those cells which possess a nucleus, the energy is no longer diffused with uniformity, the nucleus concentrating much of the power in itself, and serving as a centre of activity. Its activity of its nitrogenized constitution indicates that it is in relation with nucleus. the primordial utricle, and not with the cell wall; a conclusion which is corroborated by its physiological activity, as also by the fact that in those nucleated cells which exhibit currents, the nucleus appears to be the starting-point from which they diverge in various directions. There are subordinate species of cells, as the spiral and the dotted. These exhibit points of re-enforcement or thickening, such Subordinate as the appearance of a thread wound spirally, or in dots here forms of cells. and there on the interior of the wall. There would seem to be a tendency during the development of a cell for these parts to assume a spiral arrangement. Even the endochrome shows this peculiarity, the green material being often arranged in a spiral course on the interior of the cell. Thus constituted, each cell runs through a definite cycle or career, having its moment of birth, its period of maturity, its time of death. During its mature life it discharges with activity the special function to which it is devoted, but in so doing becomes eventually worn out and old. The period of activity of cells of different species is very different, some passing away quickly, and others having a longer duration. |