Problems of Fertilization

limited to what is necessary for understanding fertilization. I. THE RELATION OF MATURATION OF THE GERM CELLS TO FERTILIZATION After the full growth of the ovum and during or immediately before fertilization the ovum forms two small cells, known as the polar bodies, by a process of karyokinetic division (see Figs. 4f, Sc, gd, pp. 56, 62, and 66). If the first of those cells divides, as sometimes happens, four cells are produced by the fully grown ovum, three of which are rudimentary, viz., the polar bodies, and the other is the mature ovum, which is not appreciably reduced in size by the divisions. The polar bodies take no part in development, but degenerate, though they frequently remain attached to the egg for a considerable period of time. They are formed at that end of the axis, or chief developmental gradient of the ovum, which is known as the animal pole. The polar bodies are essentially abortive ova, and the divisions by which they are formed correspond precisely in their nuclear phenomena to the last twodivisions of the spermatocytes by which the four functional spermatids, or immature spermatozoa, are formed. These divisions in the history of both spermatozoon and ovum, known as the maturation divisions, are concerned in reduction of the chromosomes to one- half that characteristic of the species (haploid number), an event that always precedes fertilization; so that the union of the two germ cells in fertilization restores the species or diploid nu...