The centrosomes, at the two poles of the nucleus, are surrounded each with a halo of ray-like processes (the centrosphere), and on the sides next each other these rays penetrate the nucleus and join, forming a spindle-shaped figure with a centrosphere at each end. This spindle figure appears to be the organ by which the division is accomplished, for each of the chromosomes now splits itself in two longitudinally, as one cleaves a log of wood, and one half passes over to each centrosphere, thus making an exact division of the whole chromatin or hereditary substance. An indentation now appears in the outer wall of the cell and also in the nucleus—it deepens and deepens, and finally two cells appear instead of one, each with a nucleus, a centrosome, and a supply of chromatin, the latter now breaking up into its original condition of diffusion through the nucleus. In multicellular organisms the two new cells, of course, do not separate, but a wall is formed between them. Some plant-cells contain several nuclei; in this case division of the nucleus is not necessarily followed by that of the cell.38
Throughout the processes of cell division it is apparent that the utmost care is taken to ensure an exact partition of the chromatin between the two new cells. This partition has to be qualitative as well as quantitative; for one chromosome may, and no doubt does, differ in function and influence from another, and has various elements within itself. The longitudinal division of each chromosome, in which the elements are arranged like beads on a rosary, ensures that the different elements of the whole hereditary substance shall appear in each new cell in exactly the same relative proportion as in the parent cell; just as if two persons had to divide between them a dozen apples of different varieties, and secured perfect equality, not by taking six apples each, but by dividing every apple in two. This is the fundamental cause of the fixity of species, which means the production of offspring having the same specific characteristics as their parents. How, under these conditions, the mutability of species is brought about must be discussed later. It is first of all necessary to inquire more closely into the composition of the chromatin, and to study the special phenomena of cell-growth in connexion with conjugation, where new and extraordinary features come to light.
A chromosome is not, or is not usually, a simple body. In all but the very lowest organisms it is composed, as we have said, of a number of elements. Each of these elements is styled a ‘determinant,’ and it controls the form, colour, and function of some definite part of the future plant or animal. Weismann believes the determinants to be grouped into complex bodies called ‘ids,’ each id containing all the determinants necessary for a whole being, and each chromosome being composed of a number of ids. These ids are microscopically visible; they form the beads on the rosary already referred to; but their exact composition and potency are largely conjectural at present. How far the subdivision of determinants may go, it is, of course, impossible to ascertain. We cannot say, for instance, whether there is a determinant for every hair of the head, or one for the hirsute covering in general, or one for each of the different sections of the scalp. But the division is very minute. Each of the ids may be a very complex body, as we see by the manner in which, in some families, small physical signs like a patch of hair differing from the colour of the rest, or a tiny pit or mole on the skin of a certain part of the body, may be handed down, in that precise position, for generations. There may be, and, in fact, in the higher plants and animals there must be, a number of determinants for each part of the structure, and the final characteristics of that part must be the resultant of a blend of all these determinants, the more powerful predominating in proportion to their vitality and force. The whole body of the chromosomes may therefore be said to represent one or more complete beings in diagrammatic form, each part of the complete animal or plant being represented by some part of a chromosome, though of course not physically resembling it. And we thus strike on the very curious and startling fact that, as far as we can see, every cell in every organism throughout the world of life contains all the elements of the whole being to which it belongs, and is, potentially, that being.39 All the higher organisms possess two kinds of cells—reproductive cells which have the faculty of fusing together to reproduce their kind, and ‘somatic’ or body cells, which, although they all originate in a reproductive cell, multiply only by division, and have the function of forming the various parts of the bodily structure. Of the nucleus of a germ cell “we cannot say that it differs in any essential or definite way from the nucleus of any other cell.”40 All possess the chromatin or hereditary substance of the organism, though, according to Boveri, the germ cells alone receive all the chromatin of the parent cell, the derived somatic cells having to part with some of it.41 There may be some distinction, though on what it may be based it is at present impossible to say, between cells that are capable of developing into a complete organized creature and those that are not.
Every somatic cell is doomed to perish, but every reproductive cell now upon the globe is united, not metaphorically, not by a chain of successive originations or impulses, but by actual identity of substance, with the first beginnings 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