unicellular organisms in
general--thereby indicating, as also by its amoebiform movements, its
fundamental identity with such organisms in kind. But, as we have
likewise seen, when the ovum ceases to resemble these organisms, by
taking on its higher degree of functional capacity, it is no longer able
to multiply itself in this manner. On the contrary, its cell-divisions
are now of an endogenous character, and result in the formation of many
different kinds of cells, in the order required for constructing the
multicellular organism to which the whole series of processes eventually
give rise. We have now to consider these processes _seriatim_.

[Illustration: FIG. 33.--Stages in the formation of the polar bodies
in the ovum of a star-fish. (After Hertwig.) _g.v._, germinal
vesicle transformed into a spindle-shaped system of fibres; _p.'_,
the first polar body becoming extruded; _p._, _p._, both polar
bodies fully extruded; _f. pn._, female pronucleus, or residue of
the germinal vesicle.]

First of all the nucleus discharges its polar bodies, as previously
mentioned, and in the manner here depicted on the previous page. (Fig.
33.) It will be observed that the nucleus of the ovum, or the germinal
vesicle as it is called, gets rid first of one and afterwards of the
other polar body by an "indirect," or karyokinetic, process of division.
(Fig. 33.) Extrusion of these bodies from the ovum (or it may be only
from the nucleus) having been accomplished, what remains of the nucleus
retires from the circumference of the ovum, and is called the female
pronucleus. (Fig. 33. _f. pn._) The ovum is now ready for fertilization.
A similar emission of nuclear substance is said by some good observers
to take place also from the male germ-cell, or spermatozooen, at or about
the close of _its_ development. The theories to which these facts have
given rise will be considered in future chapters on Heredity.

Turning now to the mechanism of fertilization, the diagrams (Figs. 34,
35) represent what happens in the case of star-fish.

[Illustration: FIG. 34.--Fertilization of the ovum of an echinoderm.
(From _Quain's Anatomy_, after Selenka.) S, spermatozooen; _m. pr._,
male pronucleus; _f. pr._, female pronucleus. 1 to 4 correspond to D
to G in the next figure.]

[Illustration: FIG. 35.--Fertilization of the ovum of a star-fish.
(From the _Encycl. Brit._ after Fol.) A, spermatozoa in the
muc