[Illustration: FIG. 82. Fertilization in the Lily, _Lilium martagon_, after Guignard. _A_, the embryo-sac before fertilization; _sy_, synergidæ; _eiz_, ovum; _op_ and _up_, upper and lower 'polar nuclei'; _ap_, antipodal cells. _B_, the upper part of the embryo-sac, into which the pollen-tube (_pschl_) has penetrated with the male sex-nucleus (♂_k_) and its centrosphere; below that is the ovum with its (also doubled) centrosphere (_csph_). _C_, remains of the pollen-tube (_pschl_); the two sex-nuclei are closely apposed. Highly magnified.] When a pollen-grain has reached the stigma it sends out a tube, usually after a few hours, which penetrates into the soft tissue of the style, and grows deep down into the interior of the ovule, ultimately penetrating as far as the embryo-sac through a special little opening in the covering of the ovule, the so-called 'micropyle' (Fig. 82_B_, _pschl_). Its blunt end is now closely apposed to this, so that the true sperm-nucleus (_B_, ♂_k_), surrounded by some protoplasm, can leave the pollen-tube and wander in among the cells of the embryo-sac. Later on we shall see that two generative nuclei migrate from the pollen-tube, but in the meantime we shall devote our attention only to one of them, the fertilizing nucleus, which immediately moves towards the ovum-nucleus and apposes itself closely to it. Then follows the fusion or conjugation of the two nuclei, which are alike in size and appearance, just as in the fertilization of the animal ovum (_C_, ♂ _k_ and ♀ _k_). Whether in this case, too, the sperm-nucleus brings with it a central corpuscle, or whether, as Guignard believed he observed, the ovum retains its central corpuscle (_C_, _csph_), or finally, whether both modes occur, is not yet known with certainty. The fact that, as a rule, seeds capable of reproduction only form in an ovule when the stigma has been previously dusted with pollen, leads us to suppose that, in this case, as among animals, the ovum lacks something that is necessary to induce embryonic development, only retaining this power in very exceptional cases, namely, when adapted for parthenogenesis. And this something may very well be the dividing apparatus of the cell, the centrosome with the centrosphere. But whether this supposition prove correct or not, a nuclear spindle always forms simultaneously with the fusion of the two sex-nuclei into a segmentation nucleus, and this spindle is the starting-point of the young plant, thus exactly corresponding to the first segmentation of the animal ovum. It agrees with it also in the important respect that it again contains the full number of chromosomes--twenty-four in the lily--while the two nuclei, male and female, only exhibit half the number each, that is, twelve.