Eggor Ovum

Egg, or Ovum, the germ or female element in the process of sexual reproduction. Such a process and such a structure occurs in most groups of both plants and animals, and the latter in its earliest stage, before impregnation, consists of a single minute primordial cell or but slightly differentiated rounded mass of protoplasm. Perhaps its most rudimentary type is the "zygospore" (q.v.) of the common grey mould Mucor or of the algal Mesoearpus, in which two filaments approach one another and put out processes which become coalescent, the spore forming between the two filaments, with no appreciable distinction of sex. Such algae as Spiroyyra, where the entire protoplasm of one cell (male) passes into the other (female), mark a, step in advance. The bladderwrack (Facus), in which eight spherical" oosph'eres" are liberated by the bursting of their mother-cell and are fertilised in the water by numerous relatively minute ciliated antherozoids (q.v.), present a case more resembling that of most of the animal series. Higher in the plant-world the egg or oosphere is fertilised whilst still within a cavity at the base of a trichogyne or of an archegcnium; and in the highest group of all, the flowering plants, the fertilised oosphere does not give rise directly by its division to the embryo (q.v.), but forms a chain of cells or suspensor, from the end of which the embryo originates.

Among animals the ovum or germ-cell commonly remains unicellular until impregnation, and then undergoes "segmentation," which gives, rise to the yolk; and it may also become surrounded by albumen or "white," and by a "shell," either membranous or calcareous.

The number of eggs produced at one time is very variable - from 30 to 50 in the case of the snail, to tens of thousands in that of the whelk; 60 a minute or 80,000 a day in that of the queen termite, continuing for two years; nearly 2,000,000 in the oyster, and nearly 5,000,000 in the cod. The common snake lays about 20 eggs, the crocodile 25, and the turtle 150 to 200. Domestic poultry lay one egg daily for 120 to 150 days; but wild birds lay but a small number in each season.

In size, the eggs of insects, from which the larvae emerge in a very immature state, are minute, as also are those of crustaceans, fish, and mammals. Most mollusks have minute eggs, but those of some species of whelk (Buccinum) and others are larger than those of many birds. In this latter group, the young attain an advanced stage of development before leaving the egg, and here we have the largest and most highly organised eggs, with yolk, albumen, and shell. The egg of the ostrich weighs as much as three dozen fowl's eggs; that of the moa (Dtnornis) is still larger, and that of extinct AEpyornis of Madagascar contained as much as 148 fowl's eggs.