seen that this is in most unripe ova a large, transparent, round
vesicle. This germinal vesicle contains a viscous fluid (the
caryolymph). The firm nuclear frame (caryobasis) is formed of the
enveloping membrane and a mesh-work of nuclear threads running across
the interior, which is filled with the nuclear sap. In a knot of the
network is contained the dark, stiff, opaque nuclear corpuscle or
nucleolus. When the impregnation of the ovum sets in, the greater part
of the germinal vesicle is dissolved in the cell; the nuclear membrane
and mesh-work disappear; the nuclear sap is distributed in the
protoplasm; a small portion of the nuclear base is extruded; another
small portion is left, and is converted into the secondary nucleus, or
the female pro-nucleus (Figure 1.24 e k).

The small portion of the nuclear base which is extruded from the
impregnated ovum is known as the "directive bodies" or "polar cells";
there are many disputes as to their origin and significance, but we
are as yet imperfectly acquainted with them. As a rule, they are two
small round granules, of the same size and appearance as the remaining
pro-nucleus. They are detached cell-buds; their separation from the
large mother-cell takes place in the same way as in ordinary "indirect
cell-division." Hence, the polar cells are probably to be conceived as
"abortive ova," or "rudimentary ova," which proceed from a simple
original ovum by cleavage in the same way that several sperm-cells
arise from one "sperm-mother-cell," in reproduction from sperm. The
male sperm-cells in the testicles must undergo similar changes in view
of the coming impregnation as the ova in the female ovary. In this
maturing of the sperm each of the original seed-cells divides by
double segmentation into four daughter-cells, each furnished with a
fourth of the original nuclear matter (the hereditary chromatin); and
each of these four descendant cells becomes a spermatozoon, ready for