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Situated around the vitellus, or between the vitellus and zona pellucida of an ovum.

ar body, cell, or globule (Biol.), a minute cell which separates by karyokinesis from the ovum during its maturation. In the maturation of ordinary ova two polar bodies are formed, but in parthogenetic ova only one. The first polar body formed is usually larger than the second one, and often divides into two after its…

d; the passage of more than one spermatozoön into the vitellus in the impregnation of the ovum.

d to the changes or conditions preceding fecundation, especially to the changes which the ovum undergoes before fecundation.

nd female pronuclei) which unite to form the first segmentation nucleus of an impregnated ovum.

into parts in growth; to undergo segmentation, or cleavage, as in the segmentation of the ovum.

l.), the cavity formed by the arrangement of the cells in segmentation or cleavage of the ovum; the cavity of the blastosphere. In the gastrula stage, the segmentation cavity in which the mesoblast is formed lies between the entoblast and ectoblast. See Illust. of Invagination. -- Segmentation nucleus (Biol.), the bod…

Of, pertaining to, or designating, that condition of the ovum in which there are three primary germinal layers, or in which the blastoderm splits into three layers.

The contents or substance of the ovum; egg yolk. See Illust. of Ovum.

The outer transparent layer, or envelope, of the ovum. It is a more or less elastic membrane with radiating striæ, and corresponds to the cell wall of an ordinary cell. See Ovum, and Illust. of Microscope. (b) The zona radiata. -- Zona radiata Etym: [NL.] (Biol.), a radiately striated membrane situated next the yolk o…