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.