. The cell in development and inheritance. Cells; Cells. THE OVUM 97 spermatozoon touches it.^ In other forms (insects, birds) the vitelline membrane may be present before fertilization, and in such cases the Qgg is often surrounded by a chorion as well. The latter is usually very thick and firm and may have a shell-like consistency, its surface sometimes showing various peculiar markings, prominences, or sculpt- ured patterns characteristic of the species (insects).^ The accessory envelopes are too varied to be more than touched upon here. They include not only the products of the oviduct or
. The cell in development and inheritance. Cells; Cells. THE OVUM 97 spermatozoon touches it.^ In other forms (insects, birds) the vitelline membrane may be present before fertilization, and in such cases the Qgg is often surrounded by a chorion as well. The latter is usually very thick and firm and may have a shell-like consistency, its surface sometimes showing various peculiar markings, prominences, or sculpt- ured patterns characteristic of the species (insects).^ The accessory envelopes are too varied to be more than touched upon here. They include not only the products of the oviduct or uterus, such as the albumin, shell-membrane, and shell of birds and reptiles, the gelatinous mass investing amphibian ova, the capsules of molluscan ova and the like, but also nutritive fluids and capsules secreted by the external surface of the body, as in leeches and earth- worms. When the is surrounded by a membrane before fertilization it is often perforated by one or more openings known as micropyles, through which the spermatozoa ^ make their entrance (Figs. 44, 45). Where there is but one micro- pyle, it is usually situated very near the upper or anterior pole (fishes, many insects), but it may be at the opposite pole (some in- sects and mollusks), or even on Fig. 45. — Upper pole of the egg of r^r^^- the side (insects). In many insects ^atita. [Ussow.] .1 • r I. ir J The egg is surrounded by a very thiclc there is a group of half a dozen or .membrane perforated at m by the 'funnel- more micropyles near the upper shaped micropyle; below the latter lies the pole of the ^ZZ^ and perhaps cor- egg-nucleus in the peri-vitelline layer of proto- . plasm; the polar bodies. related with this is the fact that several spermatozoa enter the Q^^, though only one is concerned with the actual process of fertilization. The plant ovum, which is usually known as the o'dsphere (Figs. 46, 80), shows the same general features as that of animals, being a relatively large, quiesc
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectcells, bookyear1896
