o'vum, gen. o'vi, pi. o'va

[L. egg.] The egg or female sexual cell, from which, when fecundated by union with the male element, a new individual is developed. It is a simple cell, but its parts have received special names; the protoplasm is called the vitellus or yolk, the ectoplasm is the zona pellucid a or zona radiaia, and the cell membrane is the vitelline membrane; the nucleus is the germinal vesicle and the nucleolus the germinal spot. In the Ovipara the yolk consists very largely of nutritive material which does not enter into the segmentation proce of development but is reserved for the nourishment of the embryo; such an ovum is called meroblasiic or lecithal. In the Vivipara, the embryo receives its nourishment through the placenta and the yolk is formative only, containing little or no nutritive material; such an ovum is called holoblastic or alecithal. alec'ithal 0,, an o. in which the foodyolk is absent or consists of a few particles uniformly distributed, centrolec'ithal o,, one in which the food-yolk is large and central, forming the bulk of the ovum, as in a bird's egg. holoblas'- tic o,, one in which the yolk is largely or entirely formative, lec'ithal o., meroblastic o., one containing a large food-yolk, as in the bird's egg. meroblas'tic 0,, lecithal o. ova'rian 0., oocyte. o. gallina'ceum (N.F.), hen's egg. prim'itive o., ooblast, telolec'ithal o., an o. in which there is a large food-yolk situated near one end.