. Principles of modern biology. Biology. FERTILIZED EGG (diploid). ;UES OflDAUtft Fig. 3-9. Fertilization: origin of diploid cells. so forth, take part in both kinds of cell divi- sion. The meiotic divisions take place in the ovaries or the testes, in the case of animals, and in the spore capsules, in the case of plants. At present it will suffice to describe meiosis as it occurs in the testis of a male animal. Later, in connection with the study of heredity, the similar development of eggs in the ovary, and of spores in the sporan- gium, will also be described. The testes of man and ot
. Principles of modern biology. Biology. FERTILIZED EGG (diploid). ;UES OflDAUtft Fig. 3-9. Fertilization: origin of diploid cells. so forth, take part in both kinds of cell divi- sion. The meiotic divisions take place in the ovaries or the testes, in the case of animals, and in the spore capsules, in the case of plants. At present it will suffice to describe meiosis as it occurs in the testis of a male animal. Later, in connection with the study of heredity, the similar development of eggs in the ovary, and of spores in the sporan- gium, will also be described. The testes of man and other multicellular animals contain a group of numerious special cells, called the spermatogonia (Fig. 3-10). Sooner or later these cells eventually give rise to the sperm cells. The spermatogonia are diploid cells. They have arisen, along with all other cells of the organism, from the repeated mitoses of the original diploid fertilized egg. Furthermore, the spermato- gonia in the testes may continue to multiply by mitosis, giving rise to a steady supply of more spermatogonia, all diploid. Finally, a few at a time, the germ cells in the testes stop multiplying by mitosis. Now each begins to grow and can be recognized as a primary spermatocyte (Fig. 3-10). Each of the primal) spermatocytes is destined to undergo only two further divisions and these will be the meiotic divisions. Conse- quently each primary spermatocyte will form only lour sperm cells (Fig. 3-10). The prophase ol the first meiotic division commences almost as soon as the primary spermatocyte begins to enlarge. This pro- phase differs in two ways horn that of an ordinary mitosis. First it endures much longer, usually for days, rather than for hours or minutes, and during all this period the elongate chromosomes are definitely vis-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfe
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