LITMIR.BIZ

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is a hypothetical one; it has not, I believe, been actually observed in unicellular organisms, but it is exactly what we find to be taking place when we reach the stage of sexual reproduction among multicellulars. Multicellular organisms of more or less elaborate structure plainly cannot, without breaking up, fuse together like single cells. How, then, are they, as a species, to gain the advantages of the temporary union and interchange of elements which we have observed in the low unicellular organisms? Only in one way—by producing special cells for this purpose. These cells must represent the whole parent, they must be capable of shedding half their chromatin, and, when they have fused, must be capable of growing into a complete organism like the parent. When these specialized cells have been formed, the others, the somatic cells, will at the same time have been specialized for other functions, and will thus naturally lose the original capacity for interchanging chromatin with other cells, i.e. for conjugation. We see the significance, then, of Weismann’s remark, “germ cells made their appearance along with the multicellular body.”43 They are an instance of that differentiation of structure and function which takes place in all highly organized life. We must note also that the benefits of conjugation which are realized individually by the lowest unicellular forms are only realized as a species by the multicellulars. A species must, then, be regarded as in some sense an organic whole, and not as a mere aggregate of individuals.

      In some very curious cases which stand on the borderland between sexual and non-sexual reproduction, the same organism is capable of employing both methods. Thus, among the lower seaweeds (Algæ), the genus Pandorina consists of a colony of sixteen green cells contained in a kind of gelatinous matrix which the cells excrete. Each cell is ordinarily capable of recreating the whole organism by division. But after this process has gone on for some time, the need of conjugation is felt, the colony breaks up and cells begin to fuse with each other, though never with those of the same colony. In Pandorina the two conjugating cells are similar in appearance, but in the genus Volvox we begin to see a difference in the appearance of the two kinds of conjugating cells. What may be called the ‘female’ cells (germ cells) are large and quiescent; the ‘male’ (sperm cells) are smaller and active. The primary meaning of this is that the larger cells have stored up a supply of nutriment for the young organism, and are therefore bulkier and less active, while the others contain only the bare elements of cell-structure and are therefore able, as they are obliged, to be active in order to search out their quiescent mates. A strictly vegetable organism, in this stage, may therefore possess organs of locomotion, and be as free-moving as a fish. A remarkable fact has come to light respecting those organisms (like some Algæ among vegetables and Infusorians among animals), which are capable both of conjugation and of reproduction by division, namely, that the supply of nutriment often determines which method shall be followed. If nutriment is abundant, division is practised; if it becomes scanty, an impulse appears to be given to conjugation. Infusorians, which ordinarily conjugate at pretty regular intervals, can be kept indefinitely from doing so, and confined to division, by the simple process of supplying abundance of nutritive matter in the water in which they live.

      “As far as we can see from an a priori point of view,” writes Dr. E. B. Wilson in his great work on cell structure and cell phenomena, “there is no reason why, barring accident, cell-division should not follow cell-division in endless succession in the stream of life. It is possible, indeed probable, that such may be the fact in some of the lower and simpler forms of life where no form of sexual reproduction is known to occur. In the vast majority of living forms, however, the series of cell-divisions tends to run in cycles in each of which the energy of division gradually comes to an end and is only restored by an admixture of living matter derived from another cell. This operation, known as fertilization, or fecundation, is the essence of sexual reproduction, and in it we behold a process by which, on the one hand, the energy of division is restored, and by which, on the other hand, two independent lines of descent are blended into one. Why this dual process should take place we are as yet unable to say.”44

      The actual mechanism of sexual reproduction is essentially the same wherever it occurs, whether in a seaweed or a human being. Two cells have to play their part in it, the Germ cell and the Sperm cell, and these, in the higher orders of organized beings, come to be located respectively in distinct classes or sexes of individuals. Reproduction begins by the fusion of a sperm, or male cell with a germ, or female cell.

      These cells originally resemble the other cells of the same species, containing the same number of chromosomes. If this number was, say, sixteen, which is believed to be the number in man, then a fusion of two complete cells, if it were possible, would produce a cell with thirty-two chromosomes, and that would mean a different species of animal. What happens is that each of the reproductive cells, male and female, prepares itself for conjugation by getting rid of half its chromosomes. Two divisions of the nucleus take place, not as in the ordinary fashion of cell-division, when the chromosomes split longitudinally, but in such a way that, in each division, four of the sixteen chromosomes are bodily expelled from the nucleus and from the cell, when they either perish or, in some cases, appear to help in forming an envelope of nutritive matter round the germ cell. These divisions are called ‘maturation divisions,’ and until they are accomplished, fecundation is impossible. When a sperm cell after maturation comes into the neighbourhood of a germ cell, it penetrates into its substance, using the long flagellum, or tail-like process, with which it is equipped as an organ of locomotion. The two nuclei come into contact and coalesce, and we have thus a new cell with its sixteen chromosomes complete. This cell is the origin of the new being. It divides in two, and each part divides and redivides, different cells gradually differentiating themselves as muscular tissue, cartilage, blood-corpuscles, nerves, reproductive cells, and so forth, until the whole animal is built up and is ready for birth. One point of cardinal importance must here be noted. The originating cell, as we have seen, has eight of its sixteen chromosomes from one parent and eight from another. When division takes place, these chromosomes, as we have seen, split longitudinally, and the result is that each new cell gets exactly the same mixture of chromatin as that of the originating cell—half from each parent. This principle of division is carried on throughout the whole process of building up the new being—every cell of the latter, down to the minutest details of its structure, containing an exactly equal quantity of hereditary elements from each of its parents.

      It will be seen from the above account that the old conception of the germ-cell as a passive body, incapable of a change till ‘fertilized’ by a male or sperm cell, was altogether wrong. Both male and female cells prepare themselves for conjugation long before it takes place, and neither of them can be said to be a more active agent in fertilization than the other. Not ‘fertilization’ but ‘fusion’ is the keyword of the process. The mystical conception, as old as Plato, of the male and female as representing respectively the two halves of a complete being, turns out to be no poetic metaphor. As regards the essential features of reproduction, it is a literal fact.

      If we now ask why and by what mysterious law all these exact and elaborate choric movements take place Weismann and his school refer us to “chemotactic forces,” the nature of which is yet unknown. Chemotaxis means simply the effect of the presence of certain substances on vital organisms without specific chemical action. The really essential fact is that these special chemotactic forces are working in living protoplasm. Life is not the product or the slave of any chemotactic forces, but their maker and steersman.

      The following passage from a work of the late Prof. Geo. Rolleston may be pertinently quoted here:—

      “There exists, as is well known, a tendency to resolve all physiological into physico-chemical phenomena: undoubtedly many have been, and some more may still remain to be, so resolved; but the public may rest assured that in the kingdom of Biology no desire for a rectification of frontiers will ever be called out by any such attempts at, or successes in the way of, encroachment; and that where physics and chemistry can show that physico-chemical agencies are sufficient to account for the phenomena, there their claim upon the territory will be acceded to, as in the cases we have been glancing at [certain animal poisons], and where such claims cannot be established and fail to come up to the quantitative requirements of strict science, as in the cases of continuous and of discontinuous development or self-multiplication of a contagious germ, and in some others, they

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