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sanctuary occupied by the germ-cells, and inaccessible to the influences which modify the body-cells, appeared highly improbable.
Another step in the same direction was made when Maupas, Oscar Hertwig, and Max Verworn demonstrated how intimately are connected in every reproductive cell the protoplasm of its nucleus (the nucleo-plasm), and the protoplasm which surrounds it (the cell-plasm, or cytoplasm). Far from giving support to the idea that the transmission of material influences from the cellplasm to the nucleus should be impossible, the observations of these leading microscopists proved-as Hertwig expresses it— that the cell-plasm takes a prominent part in the whole process of fecundating evolution, at all its stages.
Altogether, the better we know the processes following fertilisation, the more we are convinced of the intimate connection that exists between the nucleo-plasm and the surrounding cellplasm. We learn also that the membrane which surrounds the nucleus is no obstacle for the intercourse, and that a constant exchange of substances elaborated in both plasms is going on in both directions during the process of development of the embryo."1 More than that. Every cell being quite a world, composed of a variety of separate physiological units, 22 all the component parts of the cell are required-we are told-in an equal degree.23
Besides, Maupas proved that in unicellular organisms (which may be considered analogous to the germ-cells of the multicellular beings) modifications in the cell-plasm, due to external influences, are continually inherited; this proves that they are transmitted to the nucleo-plasm-very probably through the intermediary of the extremely minute constituents of the cell passing through the membrane which encloses the nucleus. All taken, far from yielding support to the idea of its being inconceivable' that the germ-plasm should reflect the changes going on in the body-cells, it is the reverse that is rendered more and more probable by modern research.
Twenty-five years ago, when our knowledge of heredity was in its infancy, it was possible to concentrate our attention upon
21 For the ideas of O. Hertwig, Max Verworn, C. Rabl and Fick upon this point, see the quotations given by O. Hertwig, Der Kampf um Kernfragen der Entwicklungs und Vererbungslehre (Jena, 1909), pp. 44-45 and 107-108.
22 Some information about this last subject will be found in one of my 'Recent Science' articles (Nineteenth Century, May 1892, pp. 756 seq.).
23 See Rabl's suggestive work, Ueber Organ-bildende Substanzen und ihre Bedeutung für die Vererbung (Leipzig, 1906). See also E. Godlewski, jun., in Archiv für Entwicklungsmechanik, xxviii. 278-378.
the wonderful processes that are going on in a fertilised ovule (the 'Karyokinesis' processes), then recently revealed by the microscope. But now a broader view of the matter must be taken. Here is a mighty oak bearing hundreds of thousands of acorns, and each of these acorns contains a speck of the germplasm which is capable of reproducing, not only some sort of oak, but that special sort of oak which has been evolved since the Post-Glacial period in a given geographical region, in certain definite topographical conditions. Or, here is a pine-tree which sheds at a certain part of the year a real rain of pollen, each minutest grain of which is also a bearer of germ-plasm capable of transmitting to the offspring the aspects and properties of a given local variety of a pine-tree. More than that, each of the buds of the oak, each few inches of the cambial tissue of a willow-tree, each leaf of a begonia, are capable of reproducing both the species and the variety from which they originate. The same is equally true of the animal, for the miscroscopical particle of germ-plasm inherited from the parents, when the moment comes for sexual awakening, multiplies to such an enormous extent that it produces millions of similar particles, before one of them becomes the beginning of a new living being. One has only to think of the prodigious mass of food that is supplied every year to the germplasm for rendering possible such a prolification; one must realise that its structural materials are collected from the whole of the oak, the pine, the animal, that they are elaborated in all its parts, and then it becomes evident how untenable it is for a naturalist to maintain that the changes that are going on every year in the life of the body-cells have no effect upon the germ-plasm originating in such enormous quantities from these same cells.
More than that. As Professor Houssay points out in his elaborate and interesting work, La forme et la vie, the communication of living physiological units, at least one way-from the germ-cells to the body-cells-is already an established fact: the suppression of the reproductive glands produces a series of wellknown effects in the structure of the body, and these effects find some explanation in what we know about the thyroid glands and the suprarenal capsulæ. Although these glands and capsulæ have no excretory channels, the substances resulting from their cellular activity spread, nevertheless, through all the cells of the organism, either through osmosis or in any other way yet unknown. They are even so important for the organism's life that a complete amputation of one of these two organs in a dog produces a general impoverishment of its health, and its death, while recovery follows an injection of the products of their activity taken from another dog. As to the reverse action, in certain cases, of the body-cells upon the reproductive cells, if not yet proved,
it is rendered very probable by a number of modern researches."4 And we shall certainly learn more upon this subject, now that variation begins to be studied under its physiological and biochemical aspects. Viewed under these two aspects-the only true ones-all discussions about the 'impossibility' for internal variation to reach the material bearers of heredity in the reproductive cells appear utterly unreal.
We know already that in plants there exists an intercourse between the protoplasm of all their cells. The membranes of the cells are not impermeable, and threads of extremely fine particles of protoplasm are flowing through the cell-membranes from cell to cell. They have been well seen and described by many microscopists. 25 Besides, botanists know that a plant-including the germ-plasm of its reproductive cells-can be reproduced by a bud, a tuber, a piece of its underground stem, a piece of its cambial tissue, or even by a leaf (in begonia and several other plants). Consequently, in plants, germ-plasm, capable of reproducing a complete individual (not only the corresponding portion of it) is contained in the body-cells of the stem, the branches, the leaves. And, to say that it leads there a sleeping-beauty existence, free from the influences acting upon the body-cells amongst which and upon which it lives, feeds, grows, and multiplies, appeared so extravagant an assertion to most botanists (Nägeli, de Vries, Vines, and so on), that they repudiated it at once. In plants, at least, the tiny speck of matter which is the bearer of heredity cannot be anything but what Darwin conceived it to be: the result of all the influences which had acted formerly to produce the family, the genus, the species, and now have been acting to produce the individual with its own distinctive features.
It could be said, of course, that if the inalterability of the germplasm is highly improbable in plants, it may be a fact in animals. Here the reproductive cells may have their inaccessible sanctuary. However, modern research into the processes of regeneration in animals has induced many zoologists also to recognise the impossibility of such an isolation of the reproductive plasm.26
24 A few of them are mentioned by Houssay, La forme et la vie (Paris, 1900), pp. 834 seq.
25 See O. Hertwig, Die Zelle und die Gewebe, which appeared in its second, entirely re-written edition under the title of Allgemeine Biologie (Jena, 1906). About the continual exchange of living matter between the different portions of plants, see Prof. Jumelle, in Revue générale de botanique, 1891, xiii. 332. W. Pfeffer's Physiology of Plants (English translation by A. J. Ewart, Oxford, 1903) also contains most useful information in that direction. It was these cellbridges which gave to Nägeli the idea of his hypothesis of micelli wandering all over the body of the plant, and finally gathering in its reproductive cells.
26 The researches into regeneration promise to give us a real insight into the processes of heredity. But they are already so numerous that all I can do here is to mention only one or two points having a direct bearing upon the question
It was just mentioned that in plants extremely fine threads of protoplasm are seen to connect the cells. But the same connections were proved to exist, by Siegfried Garten's experiments upon his own arm, between the epithelial cells of man; and the existence of intercellular bridges between the epithelial cells and various others cells of the muscles and the connective tissue was proved by Hedenhain and Schuberg." It may be possible, of course, that these intercellular bridges' are only the means of transmission of nerve-currents, but it seems far more probable that minute particles of living matter travel along them, as they do in plants.28 Of course, we know yet very little about the intercellular communications in animals, but the little we know shows at any rate how cautious one must be in his assertions about the impossibility' of communication between the bodycells and the germ-cells.
That a communication exists between them may be taken now as highly probable. As to what sort of effects a modification produced in the body-cells may have upon the germ-plasm of the reproduction cells, we shall see presently, after we have cast a glance upon the facts we have learned from another vast series of investigations into regeneration, about the germ-plasm scattered all over the body.
The main point established by these investigations is that while both in plants and animals there is germ-plasm scattered all over the body, this germ-plasm is capable of reproducing not only those cells in which it is lodged, but also the cells of quite different parts of the organism. The extraordinary powers of regenerating, not only parts of tissues and amputated members, but even, in some divisions of the animal kingdom, the whole animal out of a small piece of it, alter many of our previous conceptions about heredity. It is well known that a piece, a few square millimetres in size, will do to regenerate a whole Hydra. And when a Planaria was cut crossways into nine pieces, seven of them regenerated the whole animal.29 A worm, the Lumbriculus,
of inherited modifications, and to refer to such works as Prof. Th. H. Morgan's Regeneration, 1907 (completed and rewritten in collaboration with the German translator, Max Moszkowski, 2nd edition, Leipzig, 1908), and to the original memoirs appearing in great numbers in Roux's Archiv, the Zoologischer Anzeiger, in the publications of different zoological laboratories, and so on.
27 S. Garten, in Archiv für Anatomie und Physiologie (1895), pp. 407-409; Heidenhain, in Anatomischer Anzeiger, viii. (1893), 404-410; Schuberg, in Sitzungsberichte of the Würzburg Physico-Medical Society; all quoted in Eugenio Rignano, La transmissibilité des caractères acquis (Paris, 1908, pp. 34-41.
28 The transmission of irritations along these 'bridges'-O. Hertwig remarks-is slower than that of a nerve-current.
29 See for a rich array of facts Th. H. Morgan's Regeneration
which lives in the mud of small lakes and ponds, having been cut into twenty-seven pieces, each of only two millimetres in length, every piece reproduced the whole creature. Even the much higher organised Tritons have an astounding power of regeneration thus it is known long since that Spallanzani saw one of them regenerating an amputated leg six times in succession.
The most striking fact with regeneration, recently discovered, is that an organ may be regenerated by tissues quite different from those from which it originates in the embryo.30 Besides, in most cases the regenerating power does not come from the surface of amputation it comes from cells lying far from it, much deeper in the body, and of a quite different character. We see it very well in the Planaria, which regenerates its head, with all its organs, after the whole of the anterior part of the body has been cut off.
Only two possible explanations of these facts can be given: either the whole of the germ-plasm scattered in the body-cells is capable of supplying the elements necessary for the regeneration of all parts of the body, as we have it in plants; or particles of germ-plasm, capable of regenerating such an important part as the head, with its brain, its eyes, its mouth, etc., are scattered in certain parts of the body, far away from the germ-cells, and they wander to the necessary spot whenever their constructive powers are required. In both cases it follows that germ-plasm, capable of reproducing other parts than those it is lodged in, stands in continual intercourse with whatever is going on in the body; and whatever hypothesis of heredity we adopt-a hypothesis of representative elementary units (Darwin's 'gemmules,' Galton's stirp,' Nägeli's 'micelli,' De Vries's 'pangenes,' Weismann's ' determinants'), or some hypothesis of a central ruling body (the ' epigenetic' hypotheses of Hertwig, Delage, and several others), it must account for the just-mentioned facts.
In this respect the recent researches of Jósef Nusbaum and Mieczyslaw Oxner on regeneration in Nemertinae offer a special interest, as they seem to open a new field of research. The experiments were made on the small Nemertina, Lineus ruber, which was cut into two parts at different distances from its front end, and the regeneration process was studied under the microscope on
30 Thus O. Hertwig, in the Introduction to his Handbuch der Vergleichenden und Experimentalen Entwicklungslehre der Wirbelthiere, Bd. i. (1906), speaks of the astonishment produced in anatomical circles by the discovery made by Colucci and Wolff, confirmed by Erik Müller and others, that in the eye of a Triton, after a complete extraction of the lens, a quite normal new lens was re-developed-not from the original mother-cells, but from the epithelium of the upper rim of the iris, which stands in no connexion with the lens at the time of its embryonal development.