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.