. Biology; the story of living things. REPRODUCTION AND LIFE CYCLES 415 an exhaustive series of divisions, whereby each cell repeatedly becomes two (growth). The result of these successive divisions is an enormous multiplication of differentiating cells to form the entire body of the individual (development). In the present connection it is only desirable to emphasize that this complicated process of cell-division (mitosis) has been exhaus- tively studied, so that its essentials are now well known. In a word, the end result is the final distribution, to every one of the innumerable cells that
. Biology; the story of living things. REPRODUCTION AND LIFE CYCLES 415 an exhaustive series of divisions, whereby each cell repeatedly becomes two (growth). The result of these successive divisions is an enormous multiplication of differentiating cells to form the entire body of the individual (development). In the present connection it is only desirable to emphasize that this complicated process of cell-division (mitosis) has been exhaus- tively studied, so that its essentials are now well known. In a word, the end result is the final distribution, to every one of the innumerable cells that form the individual, of equal germinal contributions from the two parents in the form of gene-bearing chromosomes. azrosons. heocLJJ centrosonze middle piece- tail. sheccth- axial — filament >|^^ animal pole nacleus encL —J pieces I. vacuole vitelline, rnembroLne- ^i-cxnixle veg'eLal polt Generalized diagram of spernialozoan (left) and ovum (right) ready for fertilization. Note the two views of the spernialozoan. The head contains much nuclear material plus the acrosome. The middle piece contains two disk- like centrosomes, twisted milochondria and cytoplasm, while the tail has an outer sheath and axial filament. Eggs are always larger than spermatozoa and con- tain varying amounts of reserve food. Yolk settles toward the vegetal pole. (After McEwen.) Sexual reproduction in the vertebrates is essentially identical regardless of the group considered. In every case there is a special organ in the male called a testis, or spermary, for the production of sperm, and an ovary in the female in which eggs are elaborated. Each sperm or ovum is a single cell. Both kinds of germ cells differ in shape and size throughout the vertebrate series. The tadpole-shaped spermatozoa are always much smaller, quite active, and lack nutrient material within their bodies, as contrasted with the sedentary ova in which food is stored for the prospective embryo. Sperm may be divided morphologically
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