At the same time a nuclear division apparatus has formed in the space between the two nuclei--the so-called male and female 'pronuclei' (♂ _k_, ♀ _k_)--two centrospheres (_csph_) become visible, at first lying close together, but afterwards moving apart (_D_) to form the poles of a nuclear spindle, in the equatorial plane of which the four chromosomes of the male and female pronuclei are now arranged. The nuclear membranes disappear, and the two nuclei now unite to form one, the segmentation nucleus (_D_). A dividing spindle then develops and brings about the first embryonic cell-division (_E_), and thus the beginning of the 'segmentation' of the ovum; each of the four chromatin loops splits longitudinally, and each of the split halves migrates, one to one, the other to the other daughter-nucleus (_F_). As this same method of distribution of the chromatin substance is repeated at every successive cell-division throughout embryogenesis, and indeed through the whole of development, it follows that the result of fertilization is, that all the cells of the body of the new animal which develops from the ovum contain an equal quantity of paternal and of maternal chromatin. If we are right in regarding the chromatin substance as the hereditary substance, it becomes immediately apparent that this equal division is of the most far-reaching importance, for it shows us that the so-called process of fertilization is the union of equal quantities of hereditary substance of paternal and maternal origin. The process of fertilization is now known in all its details in a great number of animals in the most diverse groups; it is everywhere the same in its essential features; there is always only one sperm-cell which normally enters into conjugation with the ovum-nucleus, and in every case the sperm-cell, however minute it may be to begin with, forms a nucleus nearly or exactly as large as the nucleus of the ovum, and in all cases it contains the same number of chromosomes as the nucleus of the ovum. Of special interest, however, is the fact that this number is always half the number of the chromosomes exhibited by the somatic cells of the particular animal in question, and that the reduction of the number of chromosomes to half the normal is effected in both male and female germ-cells by the last divisions of these cells, which take place before they have attained to a state of maturity. In the ovum the reduction occurs in the directive divisions, to which we must therefore turn our attention once more, with special reference to the number of chromosomes.