The ripe _spermatozoid_ (a flagellate sexual cell of the male) corresponds to the ovum of the female. It also contains a pronucleus having only half the number of chromosomes that the other cells of the adult body possess. =Fertilization.= The male sexual cell (_spermatozoid_) is vastly smaller than the female sexual cell (ovum). Having a _flagellum_ (Fig. 9, B, s), it moves about, like a tadpole in water, and seeks the ovum. When it comes in contact with the ovum it penetrates into its interior (usually only one doing so), as indicated at (Fig. 9, B, s). The tail or _flagellum_ of the spermatozoid fuses with the protoplasm of the ovum, and disappears from view. Its _pronucleus_ (C, m. pr.), accompanied by its _centrosome_ (C, m. c.), approaches the _female pronucleus_ (f. pr.) of the ovum (Fig. 9, C, D). Finally the male and female pronuclei coalesce to form a single nucleus (Fig. 9, E, n. os.). The centrosome of the ovum persists for awhile and then disappears; that of the spermatozoid remains in the ovum, and is the agency by which cell multiplication, through _mitosis_, takes place. [Illustration: FIG. 9.—Diagram illustrating the maturation and fertilization of the human ovum. A, one polar body is formed and a second is in process of formation; B, both polar bodies are formed and a spermatozoid is penetrating the ovum; C and D represent the approach of the male pronucleus towards the female pronucleus; E indicates the amalgamation of the two pronuclei to form the nucleus of the oösperm (segmentation nucleus); pol. b, polar bodies; pol. c, centrosome of the polar body; chr. p, chromatin of the polar body; f pr, female pronucleus; p, protoplasm; p p, peripheral protoplasm (but _not_ cell wall); f c, female centrosome; m c, male centrosome; m pr, male pronucleus; n, os, nucleus of the oösperm (first stage of a human being).] The cell resulting from the coalescence of the male pronucleus with the pronucleus of the ovum, and which is only one fifth of a millimeter in size, is the first stage in the existence of a human being. Man thus starts his career as a Protozoan-like creature,—as a unicellular animal. The fertilized ovum is called the sperm-egg (_oösperm_), and contains now the normal number of _hereditary threads_ (chromosomes); for those of the male have been added to those of the female.