Ovum

the In biology,(o'-vum) the female reproductive an,animal egg. It consists of protoplasm (the yolk), and a nucleus (the germinal vesicle) in the unfertilized ovum. It is either naked or covered by one or more membranes and envelops, which either originate from the egg itself, constituting real primary yolk or egg-integuments, or from the surrounding parietal ti ues, constituting secondary or acce ory egg-envelops. Ova arise in special organs of the metazoan body called ovaria or germaria. The nucleus or germinal vesicle (yesicula germinativa) is of relatively great size, and central or peripheral in situation. It consists of a distinct membrane and an enclosed nuclear fluid, in which lie one or more solid nucleoliox germinal spots (macula germinativa:), which may be connected by an intranuclear network of fine threads. The protoplasm consists of fine, firm particles, forming the threads and meshes of a fine network, the spongioplasm, within which is the more fluid hyaloplasm. This constitutes the formative yolk, besides which there is frequently present nutrient material in the form of drops of fat or oil, and small plates or spheres which form the deuteroplasm, or nutritive yolk. This is rarely absent; usually it is present in considerable quantity. Eggs are distinguished as (A) Alecithal, in which the food-yolk is entirely absent, or present only in very small quantity. (B) Telolecithal, in which the foodyolk and the formative yolk divide the egg into two hemispheres, called respectively the vegetative and the animal; or in which the quantity of the nutritive yolk is greatly in exce , the formative yolk being but a segment or ma at the animal pole, in which lies the germinal vesicle. In both cases a thin layer of protoplasm spreads over the entire surface of the ovum. (C) Centrolecithal, in which the formative yolk is arranged in a regular layer around the whole ovum, as well as in a ma at the center of the ovum in which lies the germinal vesicle. The region between the central and the peripheral formative yolk is more or le completely occupied by deuteroplasm. The distribution of formative yolk and food-yolk affects the germinative phenomena.