. Outlines of zoology. Zoology. 58 REPRODUCTION AND LIFE HISTORY. spermatogonium divides into spermatocytes, which usually divide again into spermatides or young spermatozoa. Maturation of ovum.—When the egg cell attains its definite size or limit of growth, it bursts from the ovary or from its place of formation, and in favourable conditions meets either within or outside the body with a spermatozoon from another animal. Before the union between ovum and spermatozoon is effected, generally indeed before it has begun, the nucleus or germinal vesicle of the ovum moves to the periphery and divid
. Outlines of zoology. Zoology. 58 REPRODUCTION AND LIFE HISTORY. spermatogonium divides into spermatocytes, which usually divide again into spermatides or young spermatozoa. Maturation of ovum.—When the egg cell attains its definite size or limit of growth, it bursts from the ovary or from its place of formation, and in favourable conditions meets either within or outside the body with a spermatozoon from another animal. Before the union between ovum and spermatozoon is effected, generally indeed before it has begun, the nucleus or germinal vesicle of the ovum moves to the periphery and divides twice. This division results in the formation and extrusion of two minute cells or polar. Fig. 26.—Diagram of maturation and fertilisation. (From " Evolution of ;) A. Primitive sex cell, supposed to be amceboid. B. Ovum; C. formation of first polar body (1. ); D. formation of second polar body (2. p.£). B'. Mother sperm cell; C. the same divided (sperm-morula). iy. Ball of immature spermatozoa ; s/>., liberated spermatozoa. E. Process of fertilisation ; F. approach of male and female nuclei within the ovum. bodies, the first containing half, the second a quarter of the nuclear material which composed the germinal vesicle. The second division follows the first without the inter- vention of the "resting stage" which usually succeeds a nuclear division. Moreover, there is this important differ- ence between the formation of polar bodies and ordinary cell division, that the number of nuclear rods or chromosomes suffers reduction, whereas in ordinary karyokinesis the daughter nuclei have as many nuclear rods as the original cell. The extruded polar bodies come to nothing, though they may linger for a time in the precincts of the ovum, and may even divide. The extrusion of polar globules from. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of
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