We now know that they are cells, and that their origin depends on a twice repeated division of the egg-cell; but it is a very unequal division, for these 'directive cells' or 'polar bodies' are always much smaller than the ovum, and indeed are usually so small that it is easy to understand why their cellular nature was for so long overlooked. Yet they have always a cell-body, and in many ova, for instance those of certain marine Nudibranchs, this is quite considerable; and they have likewise always a nucleus, which, notwithstanding the smallness of the cell-body, is in all cases exactly of the same size as the sister nucleus which remains behind in the ovum after division--a fact which is in itself enough to indicate that we have here to do essentially with readjustments and changes in the nucleus of the ovum. Long before the polar or directive divisions were recognized as divisions of the egg-cell it was known that the nucleus of the ovum disappeared as soon as the latter attained to its full size within the ovary. It was also known that this nucleus--the large so-called 'germinal vesicle' lying in the middle of the ovum--left its central position and moved to the upper surface of the ovum, there to become paler and paler, and ultimately to disappear altogether from the sight of the observer. By many it was believed that it broke up, and that the 'segmentation nucleus,' which is afterwards obvious, is a new formation. The truth is that the germinal vesicle, at the time of its disappearance, is transformed into a division figure which is invisible without the aid of artificial staining. The nuclear membrane breaks up; the centrosome of the ovum, which, although hardly visible, had previously lain beside the germinal vesicle, divides into two centrosomes and their centrospheres, and these now form the 'mitotic figure' by moving away from each other and sending out their protoplasmic rays. This nuclear spindle soon ranges itself at right angles to the surface of the egg, which at the same time arches itself into a protuberance, and soon two daughter-nuclei are formed, one of them lying within the protuberance (Fig. 75, _A_, _Rk1_). This soon separates itself off from the ovum, surrounded by a small quantity of cell-substance. The other daughter-nucleus remains within the ovum, but neither of them remains in a state of rest; both are again transformed into a spindle and divide once more; the minute first 'polar body' dividing into two 'secondary polar bodies' of half the size (_B_, _Rk1_), while the nuclear spindle within the egg brings about a second division of the ovum (_B_, _Rk2_) whose unequal products are the second polar cell and the mature ovum--that is, the ovum ready for fertilization. The process is now complete; the egg-cell, which has lost very little plasmic material through the 'polar bodies' and has not become visibly smaller, has now a nucleus (_B_, _Eik_) which has become considerably smaller through the two rapidly successive