These two branches are the male and female—the active, energy-spending, hungry, food-obtaining branch; and the sessile, non-active, assimilative and reproductive branch. And by the division of labor consequent on the formation of these two branches the whole race is benefited; but only of course on condition that the diverse elements are reunited from time to time. It is in the fusion of these elements that the real quality and character of the race is restored; and it is by their fusion that development and reproduction are secured.
In some of the Infusorians[4] there seems to be a beginning of sex-differentiation, and fusion takes place between two individuals slightly differing from each other; but as we have already seen, in most of the Protozoa the union is a union of similars—that is, as far as can at present be observed, though of course there is a great probability that here also there is generally some difference which supplies the attraction and the value of union.[5]
It is in the Metazoa generally, and those forms of life which consist of co-operative colonies of cells, that sex-differentiation into male and female begins to decisively assert itself. Here—since it is obviously impossible for all the cells of one individual to fuse with all the cells of another—certain special cells are set apart in each organism for the purpose of union or conjugation; and it seems quite natural that in the course of time the differentiation spoken of above, into male and female, should set in—each individual tending to become decisively either masculine or feminine—both in the sex-cells or sex-apparatus, and (though in a less marked degree) in the general ‘body’ and structure.
In the lower forms of life, generally, as among the amphibia, fishes, molluscs, &c., the male and female sex-cells—the sperm and the germ—do not conjugate within either of the parent bodies, but are expelled from each, in order to meet and fuse in some surrounding medium, like water. There the double cell, so formed, develops into the new individual. But in higher forms the meeting takes place, and the first stages of development ensue, within one of the bodies. And, as one might expect, this occurs within the body of the female. For the female, as we have said, represents quiescence, growth, assimilation. The germ or ovum is large compared with the spermatozoon; it is also sessile in habit. The spermatozoon, on the other hand, is exceedingly active. And so it seems natural that the latter should seek out the germ within the body of the female. Just as, in general, the female animal remains impassive and quiescent, and is sought out by the male, so the female germ remains at home within the female body, and receives its visitor or visitors there. And the whole apparatus of connection is symbolical of this relation. The body of the female is the temple in which the sacred mystery of the union or fusion of two individuals is completed, as a means to the birth or creation of a new individual.
Yet though the female is thus privileged to be the receptacle and sanctum of the life-giving power, it must not be thought that this argues superiority of the female, as such, over the male. The process of conjunction is sometimes spoken of as a fertilization merely, implying the idea that the ovum or female element is the main thing, and that this only requires a slight impulse or stimulus from the male side for its powers of development to be started and set in operation. But though it is true that the ovum can in many cases of the lower forms of life be started developing by the administration of a chemical solution or even a mechanical needle-prick, this development does not seem to continue; and modern investigation shows that in normal fecundation an absolute equality reigns, as far as we can see, between the two contracting parties and their contributions to the new being that has to be formed.
Nothing is more astounding than the results of these investigations; and they not only show us that the protozoic cells (and sex-cells), instead of being very simple in structure, are already extremely complex, and that their changes in the act of fertilization or fusion are strangely elaborate and systematic; but they suggest that though to us these cells may represent the microscopic beginnings of life in its most primitive stages, in reality they stand for the first visible results of long antecedent operations, and indicate highly organized and, we may say, intelligent forces at work within them.
The mere process by which a primitive cell divides and reproduces itself has an air of demonic intelligence about it. Roughly, the process may be described as follows. The nucleus appears to be the most important portion of a cell. Certainly it is so as regards the supply of hereditary and formative material—the surrounding protoplasm fulfilling more of a nutritive and protective function. Within and through the liquid of the nucleus there spreads an irregular network of a substance which is (for a purely accidental reason) called chromatin. As long as the nucleus is at rest, this network is fairly evenly distributed through it; but the first oncoming of division is signalled by the break-up of the chromatin into a limited and definite number of short, threadlike bodies—to which the name chromosomes has been given. These chromosomes, after some curious evolutions, finally arrange themselves in a line across the middle of the nucleus; and they are apparently governed in this operation, and the whole splitting of the cell is governed, by a minute, starlike and radiating centre (called centrosome), which first appearing outside the nucleus and in the general protoplasm of the cell, seems to play a dominant part in the whole process. This centrosome, when the time comes for the cell-division, itself divides in two, and the two starlike centres so formed (which are to become centrosomes of the two new cells), slowly move to opposite ends or poles of the original cell—all the time, as they do so, throwing out raylike threads or fibrils which connect them somehow with the chromosomes and which seem to regulate the movements of the latter, till, as described, the latter form themselves in a line across the centre of the cell, transversely to the line joining the poles. At this stage, then, we have a tiny, starlike centrosome at each end of the cell, and a transverse line of chromosomes between. (Also, during the process the wall or enclosing membrane of the nucleus has disappeared and the general contents of cell and nucleus have become undivided.) It is at this moment that the real division begins. The chromosomes—of which it is said that there are always a definite and invariable number for every species of plant or animal,[6] and which are now generally supposed to contain the hereditary elements or determinants of the future individual—these chromosomes have already arranged themselves longitudinally and end-on to each other across the middle of the cell. They now, apparently under the influence of the radiating points at each pole, split longitudinally (as one splits a log of wood)—so that each chromosome, dividing throughout its length, contributes one half of itself to one pole and one half to the other. The halves so formed separate, and approach their respective poles; and at the same time the cell-wall constricting itself along the equatorial line, or line of separation, soon divides the original cell into two. Meanwhile the chromosomes in each new division group themselves (not round but) near their respective poles or centrosomes, and a new nucleus membrane forming, encloses each group, so that finally we have two cells of exactly the same constitution as the original one, and with exactly the same number and quality of chromosomes as the original.[7]
The whole process seems very strange and wonderful. No military evolutions and formations, no complex and mystic dance of initiates in a temple, with advances and retreats, and combinations and separations, and exchanges of partners, could seem more fraught with intelligence.[8] Yet this is what takes place among some of the very lowest forms of life, on the division of a single cell into two. And it is exactly the same, apparently, which takes place in the higher forms of life when the single cell which is the result of the fusion together of the sperm-cell and the germ-cell, divides and subdivides to form the ‘body’ of the creature. As is well known, the joint cell divides first into two; then each of the cells so formed divides into two, making four in all; then each of these divides into two, making eight; then each into two again, making 16, 32, 64, and so on—till they number the thousands, hundreds