Elements of comparative zoology (1904) Elements of comparative zoology elementsofcompar1904king Year: 1904 THE CELL. 141 c, spects. The nucleus has a different refractive index, so that it stands out. under the microscope, like a drop of oil in water. It has a great affinity for certain stains, and hence in microscopical preparations it is brightly col- ored, while the protoplasm is usually but slightly stained. W- N~x '•J^jk'-a^£* \ •'•''' •'•-•>C~'>'-' A Physiological experiments show ^^fS^^ft>f5^%%%%%%%%%%%%%%%% that the nucleus controls the action of the cell; while there is a
Elements of comparative zoology (1904) Elements of comparative zoology elementsofcompar1904king Year: 1904 THE CELL. 141 c, spects. The nucleus has a different refractive index, so that it stands out. under the microscope, like a drop of oil in water. It has a great affinity for certain stains, and hence in microscopical preparations it is brightly col- ored, while the protoplasm is usually but slightly stained. W- N~x '•J^jk'-a^£* \ •'•''' •'•-•>C~'>'-' A Physiological experiments show ^^fS^^ft>f5^%%%%%%%%%%%%%%%% that the nucleus controls the action of the cell; while there is all but conclusive evidence that it includes the mechanism of heredity. In a few minute forms, both ani- mals (Protozoa) and plants, the FlG of a ceii. whole organism consists of but a single cell, which consequently carries on all the functions of life, but all the rest are multicellular. In the multicellular animals (Metazoa) the cells are not all alike, but may be divided into groups or layers specialized in different directions. This differentia- tion is both of form and of function. The cells of the separate layers differ in shape and have different work to perform, there being what is termed a division of labor among the groups of cells. All animals above the Protozoa reproduce by eggs. These eggs, when carefully studied, are found to agree in their essential characteristics. Each, in fact, is a cell (p. 126) containing a nucleus; but to these essentials other structures—shell, white, yolk, etc.—may be added. Each egg, under proper conditions, is capable of growing into a form like that which produced it. The essential condition is that a peculiarly modified cell, the spermato-
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