OVUM

(Lat. an egg.) In Anatomy, the body formed by the female in which, after impregnation, the development of the foetus takes place. It is generally formed in a special organ, called the ovarium: but in some of the simplest animals, as the Polypes, the common cellular parenchyme of the body seems to have the unlimited faculty of producing the ova. The e ential, and, apparently, first formed, part of an ovum, is a minute pellucid cell, called the "germinal vesicle," which is characterized by an opaque speck or nucleus called the "germinal spot." The vesicle i< immediately surrounded by a stratum of granules or nucleated cells, which form the "germinal disk." These parts flo;U in a greater or le quantity of fluid and granules, called the yolk, which is generally of some well-marked colour, as yellow, green, violet, red, through the presence of a minutely diffused oil. The yolk is inclosed in a thin, delicate, structurele coat, called the " vitelline membrane," and this is finally surrounded by an outer tunic, called the "chorion." Between the chorion and vitelline membrane there is commonly a greater or le quantity of albumen. In the birds, this fluid, which is called the " white," and the "yolk," is in great quantity; the chorion is laminated, and the outer layer is combined with earthy salts to give due firmne , and preserve the shape of the egg while subject to the weight of the parent during incubation. Two twisted strings of firm albumen called "chalazse," are continued from each end of the yolk, a little below the poles, and serve to keep uppermost the " cicatricula or "tread," formed by the impregnated germinal vesicle and disk. A space intercepted between two of the layers of the chorion, or "membrana pntaminis," at the great end of the egg, contains a small quantity of gas, containing more oxygen than atmospheric air: this space is called the "Eoq.)vesica aerea." For the chemical properties of ovum, see Ovum. In Architecture, the same as Ovolo; which see. [s. 884]