In the cells of the body of the animal to which this description applies there are four chromosomes This is true of all the cells of the animal except the sexual cells. The eggs arise from the other cells of the body, but during their growth the chromatin splits in such a way that the egg contains double the number of chromosomes, i.e., eight (Fig. 34). If this egg should now unite with the other reproductive cell from the male, the resulting fertilized egg would plainly contain a number of chromosomes larger than that normal for this species of animal. As a result the next generation would have a larger number of chromosomes in each cell than the last generation, since the division of the egg in development is like that already described and always results in producing new cells with the same number of chromosomes as the starting cell. Hence, if the number of chromosomes in the next generation is to be kept equal to that in the last generation, this egg cell must get rid of a part of its chromatin material. This is done by a process shown in Fig. 35. The centrosome divides as in ordinary cell division (Fig. 35), and after rotating on its axis it approaches the surface of the egg (Figs. 36 and 37). The egg now divides (Fig. 38), but the division is of a peculiar kind. Although the chromosomes divide equally the egg itself divides into two very unequal parts, one part still appearing as the egg and the other as a minute protuberance called the polar cell (_pc'_ in Fig. 38). The chromosomes do not split as they do in the cell division already described, but each of these two cells, the egg and the polar body, receives four chromosomes (Fig. 38). The result is that the egg has now the normal number of chromosomes for the ordinary cells of the animal in question. But this is still too many, for the egg is soon to unite with the male cell; and this male cell, as we shall see, is to bring in its own quota of chromosomes. Hence the egg must get rid of still more of its chromatin material. Consequently, the first division is followed by a second (Fig. 39), in which there is again produced a large and a small cell. This division, like the first, occurs without any splitting of the chromosomes, one half of the remaining chromosomes being ejected in this new cell, the second polar cell (_pc"_) leaving the larger cell, the egg, with just one half the number of chromosomes normal for the cells of the animal in question. Meantime the first pole cell has also divided, so that we have now, as shown in Fig. 40, four cells, three small and one large, but each containing one half the normal number of chromosomes. In the example figured, four is the normal number for the cells of the animal. The egg at the beginning of the process contained eight, but has now been reduced to two. In the further history of the egg the smaller cells, called _polar cells_, take no part, since they soon disappear and have nothing to do with the animal