Then sets in what is known as the segmentation or cleavage of the ovum. First the nucleus and then the cell itself divides into two equal halves (B.), each of these shortly afterwards again dividing into two. We may call the points of intersection of these two planes of division the "poles," and the planes "vertical planes." We thus have four cells produced by two vertical planes (C.). The next plane of division is equatorial, midway between the poles. By this plane the four cells are subdivided into eight (D.). Then follow two more vertical planes intermediate between the first two. By them the eight cells are divided into sixteen. These are succeeded by two more horizontal planes midway between the equator and the poles. Thus we get thirty-two cells. So the process continues until, by fresh vertical and horizontal planes of division, the ovum is divided into a great number of cells. But meanwhile a cavity has formed in the midst of the ovum. This makes its appearance at about the eight-cell stage, the eight cells not quite meeting in the centre of the ovum. The central cavity so formed is thus surrounded by a single layer of cells, and it remains as a single layer throughout the process of segmentation, so that there results a hollow vesicle composed of a membrane constituted by a single layer of cells (E.). The cells on one side of the vesicle are rather larger than the others, and the next step in the process is the apparent pushing in of this part of the hollow sphere; just as one might take a hollow squash indiarubber ball, and push in one side so as to form a hollow, two-layered cup (F.). The vesicle, then, is converted into a cup, the mouth of which gradually closes in and becomes smaller, while the cup itself elongates (G.).[I] Thus a hollow, two-layered, stumpy, worm-like embryo is produced, the outer layer of which may be ciliated, so that by the lashing of these cilia it is enabled to swim freely in the water. The inner cavity is the primitive digestive cavity.