of protoplasmic life in the abyss of time; and it has before it a potential immortality commensurate with life itself. It is not, as used to be thought, a physiological product of the organism in which it dwells; it is a part of the original reproductive cell from which that organism sprang.
To understand these conceptions we must now study the phenomena of reproduction in the light of recent discoveries.
The lowest form of the reproductive process is, of course, by simple division and redivision. This is characteristic of many of those organisms which consist only of a single cell, and it may co-exist, even in these, with a considerable degree of structural complexity, as in the ‘trumpet animalcule,’ Stentor raselii. But among the lowest of these unicellular organisms a curious process is sometimes observed to take place, in which we may doubtless recognize the origin of sexual reproduction. Two, three, or more Amœbæ42 approach each other, partially coalesce, and remain united for some time. They then separate again. No new creatures are formed by this contact; there are no visible results at all. But that something which is for the advantage of the organisms takes place during this period of union is certain, and in the light of what is known of processes in other organisms we can make a very good guess at what this something is. Each Amœba parts with some of its chromatin to some other and receives an equivalent in exchange. The creature is thus reconstituted. The element of change, which always provides so marked a stimulus to vital processes, has been obtained. The process has actually been observed in a certain Infusorian, Noctiluca. Two Noctilucas coalesce, and then proceed to divide at right angles to the plane of contact. This necessarily has the effect of giving to each of the two new Noctilucas which result from the division half the nucleus and chromatin of one parent and half of the other. There is, however, no actual new birth or multiplication of beings; there are only two Noctilucas as before.
We can now imagine that if a certain class of unicellular organisms are in the habit of approaching each other for the purpose of this interchange of portions of their chromatin, they might occasionally, under the influence of the approaching conjugation, expel those portions of chromatin before another cell was in a position to receive it. What would happen if two cells, each of which had thus got rid of half its chromatin, were to come into contact? Plainly, they would fuse together; they would not separate again; they would become a new organism. Each would have supplied just what the other lacked.
This process, forming the bridge from mere cell division to sexual reproduction, 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
