We have already seen that the young egg-cell multiplies itself by simple binary division, after the manner of 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 spermatozoön, 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, spermatozoön; _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 mucilaginous coat of the ovum; a prominence is rising from the surface of the ovum towards a spermatozoön; B, they have almost met; C, they have met; D, the spermatozoön enters the ovum through a distinct opening; H, the entire ovum, showing extruded polar bodies on its upper surface, and the moving together of the male and female pronuclei; E, F, G, meeting and coalescence of the pronuclei.]