Thus the beginning of embryonic development in the ovum depends not on a definite number of chromosomes, but on the presence of an apparatus for division. Upon what the awakening of this to activity just at that time depends cannot as yet be exactly stated; we can only indicate that all parts of the cell have interrelations with each other, and that, therefore, the division mechanism is dependent on the condition of the rest of the cell-parts at the moment, and on the substances which they contain or produce. From what we know experimentally in regard to artificial parthenogenesis it is not difficult to imagine that some sort of chemical substances are necessary to stimulate the central corpuscle to activity. In any case, the whole nutrition of the central corpuscle depends on the cell in which it lies, as is shown by the fact that the sperm-nucleus, whose centrosome before the entrance of the sperm into the ovum was inactive and scarcely recognizable, grows rapidly after entrance and forms a large aster round itself--is, in short, in the highest degree active (Fig. 80). As the chromosomes certainly play an important part in the life of the cell, and materially help to determine its various phases, it cannot be disputed that they also may share in awakening the activity of the central corpuscle. But this influence is only indirect; it is not the mere number of chromosomes that decides whether the central corpuscle is to become active or remain inactive. This cannot be assumed, because we have in the maturation divisions a proof that division may take place with a double number of chromosomes as well as with the undoubled number; while in the divisions of the mother-egg-cells and the mother-sperm-cells we have proof that a doubled number of chromosomes does not in itself compel to division. The exceptional and artificially produced cases of parthenogenesis which we have discussed above are probably to be interpreted thus: through slight differences in the constitution of the ovum, or through certain mechanical or chemical stimuli, the metabolic processes in the ovum are so altered that the centrosome of the ovum, instead of breaking up, is stimulated to growth, and thus produces the active dividing apparatus which is otherwise only brought into it by the sperm. This is a more exact definition of the interpretation I gave earlier (1891) of the 'chance' parthenogenesis of the silk-moth, which was then the only case known, when I said 'the nucleoplasm of some ova must possess the power of growth in a greater degree than the majority.' But we are not yet in a position to go further, or to define more exactly the nature of the processes of metabolism which are involved. LECTURE XVI FERTILIZATION IN PLANTS AND UNICELLULAR ORGANISMS, AND ITS IMMEDIATE SIGNIFICANCE