. The cell in development and inheritance. Cells; Cells. 96 THE GERM-CELLS eii Nereis (Fig. 43). Often associated with the proper deutoplasm- spheres are drops of oil, either scattered through the yolk (Fig. 43) or united to form a single large drop, as in many pelagic fish-eggs. The deutoplasm is as a rule heavier than the protoplasm; and in such cases, if the yolk is accumulated in one hemisphere, the egg assumes a constant position with respect to gravity, the egg-axis standing vertically with the animal pole turned upward, as in the frog, the bird, and many other cases. There are, however,
. The cell in development and inheritance. Cells; Cells. 96 THE GERM-CELLS eii Nereis (Fig. 43). Often associated with the proper deutoplasm- spheres are drops of oil, either scattered through the yolk (Fig. 43) or united to form a single large drop, as in many pelagic fish-eggs. The deutoplasm is as a rule heavier than the protoplasm; and in such cases, if the yolk is accumulated in one hemisphere, the egg assumes a constant position with respect to gravity, the egg-axis standing vertically with the animal pole turned upward, as in the frog, the bird, and many other cases. There are, however, many cases in which the Q,^^ may lie in any position. When fat-drops are present they usually lie in the vegetative hemisphere, and since they are lighter than the other constitu- ents they usually cause the Q,^g to lie with the animal pole turned downwards, as is the case with some annelids (^Nereis) and many pelagic fish-eggs. 3. TJie Egg-envelopes The egg-envelopes fall under three categories. These are : — {a) The vitelline membrane, secreted by the ovum itself. {U) The chorion, formed outside the ovum by the activity of the maternal follicle-cells. (6-) Accessory envelopes, secreted by the walls of the oviduct or other maternal structures after the ovum has left the ovary. Only the iirst of these properly be- longs to the ovum, the second and third being purely maternal products. There are some eggs, such as those of certain coelenterates { Renilld), that are naked throughout their whole develop- ment. In many others, of which the sea-urchin is a type, the fresh-laid &%,% is naked but forms a vitelline mem- brane almost instantaneously after the. Fig. 44. — Schematic figure of a median longitudinal section of the egg of a fiy {Mtisca), showing axes of the bilateral egg, and the mem- branes. [From KORSCHELT and Heider, after Henking and Bloch- MANN.] The germ-nuclei uniting; ot., micropyle; the polar bodies. The flat side of the egg is the dorsal, th
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectcells, bookyear1896
