. The cell in development and inheritance. Cells; Cells. GENERAL SKETCH I.^I earlier observers, from the time of Martin Barry ('43) onwards, had seen the spermatozoa inside the egg-envelopes, or asserted its entrance into the ^. In many cases the entire spermatozoon enters the o^g^ (mollusks, insects, nematodes, some annelids, Petroniyzon, axolotl, etc.), and in such cases the long flagellum may sometimes be seen coiled within the (Fig. 64). Only the nucleus and middle-piece, however, are concerned in the actual fertilization; and there are some cases (echinoderms) in which the tail is
. The cell in development and inheritance. Cells; Cells. GENERAL SKETCH I.^I earlier observers, from the time of Martin Barry ('43) onwards, had seen the spermatozoa inside the egg-envelopes, or asserted its entrance into the ^. In many cases the entire spermatozoon enters the o^g^ (mollusks, insects, nematodes, some annelids, Petroniyzon, axolotl, etc.), and in such cases the long flagellum may sometimes be seen coiled within the (Fig. 64). Only the nucleus and middle-piece, however, are concerned in the actual fertilization; and there are some cases (echinoderms) in which the tail is left outside the ^gg. At or near. Fig. 64. — Fertilization of the egg of the snail Physa. [Kostanecki and WiERZEjSKi.] A. The entire spermatozoon lies in the &gg, its nucleus at the right, flagellum at the left, while the minute sperm-amphiaster occupies the position of the middle-piece. The first polar body has been formed, the second is forming. B. The enlarged sperm-nucleus and sperm-amphiaster lie near the centre; second polar body forming and the first dividing. The egg-centrosomes and asters afterwards disappear, their place being taken by those of the spermatozoon. the time of fertilization, the successively segments off at the upper pole two minute cells, known as the polar bodies (Figs. 64, 65, 89) or directive corpuscles, which degenerate and take no part in the subse- quent development. This phenomenon takes place, as a rule, imme- diately after entrance of the spermatozoon. It may, however, occur before the spermatozoon enters, and it forms no part of the process of fertilization proper. It is merely the final act in the process of matjiration, by which the is prepared for fertilization, and we may defer its consideration to the following 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 perfectly resemble the
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectcells, bookyear1896
