s 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
which is to result from the further division of the egg. This process of
the formation of the polar cells is thus simply a device for getting rid
of some of the chromatin material in the egg cell, so that it may unite
with a second cell without doubling the normal number of chromosomes.

[Illustration: FIG. 38--First division complete and first polar cell
formed, _pc'_.]

[Illustration: FIG. 39.--Formation of the second polar cell, _pc"_.]

[Illustration: FIG. 40.--Completion of the process of extrusion of the
chromatic material; _fn_ sho