. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. OVUM. ash-coloured. In the land-newt, which is oyo- viviparous, the yolk is of considerable size, and of a dark yellow, approaching to orange. In osseous fishes, which are almost all oviparous, the ovule receives, apparently in the ovarian capsule itself, before leaving that cavity, an external covering (or chorion) of considerable firmness. This membrane ap- pears to consist of a substance deposited on the external surface of the vitelline membrane, and becomes coagulated under the action of water ; so that its density
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. OVUM. ash-coloured. In the land-newt, which is oyo- viviparous, the yolk is of considerable size, and of a dark yellow, approaching to orange. In osseous fishes, which are almost all oviparous, the ovule receives, apparently in the ovarian capsule itself, before leaving that cavity, an external covering (or chorion) of considerable firmness. This membrane ap- pears to consist of a substance deposited on the external surface of the vitelline membrane, and becomes coagulated under the action of water ; so that its density increases greatly after the ova are deposited, while it is sepa- rated at the same time from the yolk by the imbibition of water. The ova are in spawning either deposited separately, or are united in chains or bundles, and in some less common examples* in peculiar nida- mental structures, more after the manner of some of the mollusca. The structure of the ovarian ovule, or yolk, and its relation to the germ, differs somewhat from that of the batrachia ; for while in the latter animals the yolk substance consists of granules and cells of nearly uniform size, and the germinal layer covers the greater part of the surface, in osseous fishes this layer is more circum- scribed, riot extending at first over more than a third, or, at most, a half of the yolk, and the remainder of the yolk, which contains a much greater quantity of transparent fluid than in most other vertebrate animals, presents al- most invariably a peculiar heap or mass of large oil globules, which float to the upper part of the fluid below the germinal The minute ovula of mammalia, when they have reached maturity in the Graafian capsules of the ovary, are nearly spherical bodies, of from Tij7 to -^ of an inch in diameter, and consist of a mass of finely granular yolk sub- stance, more loose in the interior and more dense towards the surface, and enclosed in a thick firm and transparent vesicular envelope, t
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