. The cell in development and inheritance. Cells; Cells. 150 FERTTUZATION OF THE OVUM where it is very large and conspicuous,, and I have since observed it also in the sea-urchin (Fig. 69). The most profound change in the ovurai is, however, the migration; 'of the germinal vesicle to the periphery, and the formation of the polar bodies. In many cases either or both these processes may occur ibefore contact with the spermatozoon (echinoderms, some vertebrates). In others, however, the egg awaits the entrance of the spermatozoon (annelids, gasteropods, etc.), which gives it the necessary stimulu
. The cell in development and inheritance. Cells; Cells. 150 FERTTUZATION OF THE OVUM where it is very large and conspicuous,, and I have since observed it also in the sea-urchin (Fig. 69). The most profound change in the ovurai is, however, the migration; 'of the germinal vesicle to the periphery, and the formation of the polar bodies. In many cases either or both these processes may occur ibefore contact with the spermatozoon (echinoderms, some vertebrates). In others, however, the egg awaits the entrance of the spermatozoon (annelids, gasteropods, etc.), which gives it the necessary stimulus. This is well illustrated by the egg of Nereis. In the newly-dis- charged €^g^ the germinal vesicle occupies a central position, the yolk, consisting of deutoplasm- spheres and oil-globules, is uni- formly distributed, and at the periphery of the egg is a zone of clear perivitelline protoplasm (Fig. 43). Soon after entrance of the spermatozoon the germinal vesicle moves towards the periphery, its membrane fades away, and a radi- ally directed mitotic figure appears,, by means of which the first polar body is formed (Fig. 71). Mean- while the protoplasm flows towards the upper pole, the perivitelline zone disappears, and the Q.^g now shows a sharply marked polar differentiation. A remarkable phe- nomenon, described by Whitman in the leech ('78), and later by Foot in the earthworm ('94), is the formation of " polar rings," a process which follows the entrance of the spermatozoon and accompanies the formation of the polar bodies. These are two ring-shaped cytoplasmic masses which form at the periphery of the ^gg near either pole and advance thence towards the poles, the upper one surrounding the point at which the polar bodies are formed (Fig. y6). Their meaning is unknown, but Foot ('96) has made the interesting discovery that they are probably of the same nature as the yolk-nuclei (p. 121).. Fig. 76. — Egg of the leech Clepsine, dur- ing fertihzation. [Whitman.]
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectcells, bookyear1896
