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various names, endowed with various particular properties, and attributed, as to their origin, to varying sources. In the seventeenth century and early part of the eighteenth century, before the time of the microscope, many naturalists and physicians believed that in each germ cell (or, according to some, in each egg cell, according to others, in each sperm cell) there existed, preformed and almost complete, a new organism in miniature, and that development was simply the expanding and growing up of this tiny embryo man, or monkey, or chick. Also they were forced to believe, if this first assumption were true, that in each preformed embryo still smaller replicas of their particular kind must exist to be the children of this child, and so on, ad infinitum. Like the nests of Japanese boxes, the outer one encasing a smaller and this still a smaller, and this yet a smaller and so on, the young and future young of any kind of organism were, according to this encasement theory of the germ cell structure, nested in the egg and sperm cells of any organism. But the invention and use of the microscope soon put this theory aside. The germ cells were found to contain no preformed embryo. Indeed, they seemed to the earlier microscopists to be utterly homogeneous little specks or masses of protoplasm, and the pendulum of speculative explanation tended to swing well away from any preformation theory toward the speedily formulated epigenetic theories, which assumed that all germ cells were practically alike except as to their paternity and maternity, and that the development of these homogeneous specks of protoplasm must be determined chiefly by external conditions and influences. However, it was obvious that there was no logical or even fair reason for believing that the lack of structural differentiation in the germ plasm revealed by the microscope was a proof of the actual absence of such organization. The first microscope magnified but a few hundred diameters, revealing structure invisible to the unaided eye; but later microscopes, magnifying objects a thousand and more diameters, revealed structure and organization which were quite invisible to the lower-powered instruments. And so, although to-day we examine germ plasm with lenses magnifying three thousand times, and yet fail to discover more than threads, rods, grains, or droplets in a viscous
ground substance, we do not believe at all that this structural differentiation is the ultimate physical make-up of the mysterious substance protoplasm. We readily believe there may exist an ultramicroscopic structure of great complexity.
Buffon suggested that the living stuff is composed ultimately of tiny structural units, which he called organic molecules; these molecules are universal and indestructible; they do not increase in number or decrease; when united in groups they form organisms; when an organism dies its organic molecules are freed but not destroyed, and later may help compose other organisms. Bechamp believed in similar living micromeric units called microzymes, created directly by the Supreme Being, indestructible and strewed everywhere in earth, air, and water.
Herbert Spencer postulated the existence of so-called physiological units: living units all of the same structure, active because of their polarity of form and of molecular vibrations, in size and character midway between molecules and cells, small but complex and possessed of a delicate and precise polarity analogous to that of the molecules of crystalline substances, a polarity which gives them the capacity to group themselves into organic parts and wholes. Other theories similar to Spencer's assume a special physicochemical endowment of the chemical molecules in the organic body (Berthold), or a special electrical endowment of the life units (Fol), or a special chemical one (Altmann and Maggi), or, finally, a special vital one (Wiesner).
Darwin proposed a theory to explain how the germ plasm could unfold into the whole body, called the theory of the pangenesis of gemmules. Darwin postulated the existence in the body of a host of life units called gemmules to be found in all the various body cells, capable of rapid self-multiplication and of a migratory movement through the body, the direction and goal of which movement is determined by delicate affinities existing among the various gemmules. When a gemmule enters an undifferentiated or developing cell, as yet gemmuleless, it controls the development of that cell. Thanks to the delicate and precise affinities of the gemmules, they always get to just where they should, to produce harmonious development; but in the germ cells lodge gemmules from all over the body, so the development of these cells results in a new whole body.
Nägeli, a philosophical botanist, proposed a theory of germplasm structure and behavior which may be called the theory of micella, nutritive plasm and idioplasm. When the complex, life-characterizing albuminous substances took their birth in an aqueous liquid, they were precipitated as tiny particles called micella', which attracted other micell:e to themselves and thus produced aggregates of primitive life stuff, or protoplasm. The micell:e are all separated from each other by thin envelopes of water, thus making water an integral part of protoplasm, and making growth by intercalation of new micell:e possible; this primitive protoplasm becomes arranged in two ways, resulting in producing two kinds, one called nutritive protoplasm, and the other idioplasm or germ plasm, extending all through the nutritive protoplasm as a fine network.
Finally, the most recent micromeric theory of germ-plasm structure is that of Weismann, the modern champion of natural selection. According to him the protoplasm of the nucleus is made up of units called biophors, which are the bearers of the individual characters of the cell; the biophors are complex groups of molecules, capable of assimilating food, growing, and reproducing; the number of biophors is enormous, as it must equal the possibilities of cell variety. The biophors are united into fixed groups called determinants, each determinant containing all the biophors necessary to determine the whole character of any one cell; in each specialized cell there need be but one determinant, but in the germ cells every kind of determinant must be represented.
In connection with the postulation concerning the ultimate make-up of the plasm of the germ cell, Weismann has formulated a theory of germinal selection to account for the obvious fact that a certain cumulation of variation of a certain kind or along fixed lines may take place without the aid of natural selection: this variation cumulation often being indeed of a degree too slight to give any opportunity for interference by natural selection. To account for this fact, which has been much used by adverse critics of natural selection, Weismann assumes a competition of the determinants in the germ cells for food, hence for opportunity to grow, to be vigorous, and to multiply; the initially slightly stronger or more favorably situated determinants will get the most food, lessening, at the same time, the food supply of others. Now, when the germ cell
begins development the kind of cells or tissues or organs will be best developed whose determinants happen to be the better fed, stronger ones, while other parts of the body may be made smaller or even not appear at all on account of the starvation of their determinants; also the stronger determinants in the better developed parts of the body will produce by multiplication more and stronger daughter determinants for the germ cells of the new individual than the weak determinants in the ill-developed body parts, and thus this disparity in development of body parts will be passed on, cumulatively, to successive generations: which is nothing more nor less than determinate variation.
All the speculations about the ultimate structure of the germ plasm are interesting, but none of them of course is really convincing. As Delage has well said, the chances are too many to one against the probability of anyone's guessing correctly the actual facts concerning the complex structural detail of the protoplasmic make-up. The structural or inherent factors in ontogeny, then, are to be understood only in so far as obvious results or effects may reveal them. Now there is one set of phenomena in ontogeny, to which we have not as yet called attention, which does seem to throw some light on certain essential features or facts of germ-cell structure which otherwise would not be obvious to us. This set of phenomena is that called mitosis or karyokinesis, and occurs in connection with each division or cleavage of the egg cells, and of their daughter cells or blastomeres. It occurs also in the division or multiplication of cells in all the tissues of the body, and is a phenomenon normal to cell increase anywhere in the body at any time in the life of the organism.
Direct or amitotic cell division is much less common and seems to be restricted to certain kinds of tissues or to certain periods in the history of the life of certain tissues. However, the recent investigations of Child and others show that cell division without mitosis is more common than is usually thought. In this kind of division, the process consists simply of the constriction and equal (or unequal) splitting of the cell body into two parts, the dividing of the nucleus usually being slightly in advance of that of the cytoplasm. Each half of the parent cell has then but to increase in size to become the counterpart of its progenitor. In the mitotic or indirect division, on the contrary, the process is more complex. It has been described by F. M. McFarland 1 as follows:
"One of the earliest results of the study of cell multiplication was the discovery that division of the nucleus precedes the division of the cell body. Furthermore, a careful examination of the different phases of the process offers the strongest proof that the most important feature of this division, an end to which all the other processes are subsidiary, is the exact halving of a certain nuclear substance, the chromatin, between the two daughter cells which result from the division. To gain a clear conception of this process of indirect cell division, called 'mitosis' or 'karyokinesis,' let us consider the changes which take place in typical cell multiplication. Two parallel series of changes occur nearly simultaneously, the one affecting the nucleus, the other the cytoplasm. In the so-called 'resting' nucleus—i. e., the nucleus not in active division—the chromatin, as we have seen, exists usually in the form of scattered granules arranged along the linin network, and does not color readily with nuclear stains. As division approaches, these chromatin granules become aggregated together in certain definite areas, forming usually a convoluted thread or skein, which now readily takes up the nuclear stains which may be used. In some nuclei this skein is in the form of a single long filament, in others the chromatin is divided up from the first into a series of segments, a condition which soon follows in the case of a single filament. By transverse fission the latter breaks up into a series of segments, the 'chromosomes,' the number of which is constant for each species of animal or plant. Thus in the common mouse there are twenty-four, in the onion sixteen, in the sea urchin eighteen, and in certain sharks thirty-six. The number may be quite small, as, for example, in Ascaris, a cylindrical parasitic worm inhabiting the alimentary canal of the horse. Here the number is either two or four, depending upon the variety examined. In other forms the number may be so large as to render counting exceedingly difficult or impossible. In all cases, however, one fact is to be especially noted, viz., the number is always an even one, a striking fact which finds its explanation in the phenomena of fertilization to be discussed later on.
"While the chromatin is collecting into the form of the chromo
Most of the discussion in the following twenty pages, whether indicated by quotation marks or not, is taken from McFarland's essay on “The Physical Basis of Heredity" in Jordan's “Footnotes to Evolution" (1902).