Our lives, then, have their origin from two cells of intensely vitalized protoplasm which unite to form a single cell. The saying of Linnaeus, "Omne vivum ex ovo," is now known to be true, for all animal life springs from a cell which has all the true characteristics of an egg. The ova of all animals higher in the scale of life than the protozoa, i.e., from the Porifera, or sponges, up through the animal kingdom, including man, are scarcely distinguishable from one another in their essential characteristics and their structure, though varying much in size in the different animals.
The ovum, like most cells, is usually of microscopic size, though sometimes it is of enormous bulk, as in the bird tribe, ostrich's eggs averaging three pounds in weight, and holding about three pints. An ostrich egg is an example of one of the largest cells known to physiologists, but morphologically it differs in no degree from the human ovum. The largest known eggs are those of the gigantic fossil bird of Madagascar, maximus, being twelve to fourteen inches in length, six times the bulk of an ostrich's egg, and equivalent to twelve dozen hen's eggs. And yet these eggs are single cells!
Other examples of enormous cells are the eggs of all birds, of most fish, of some batrachians, and moet reptiles. In some animals the ovum is encased in a hard, chalky egg shell, while in others it is protected by a more or less tough envelope.
But yet these ova are all morphologically similar; in some of them there is an enormous adventitious addition of the albuminous part, or "white" of the egg, which serves to nourish the developing embryo, while the egg shell is merely a protective envelope of calcareous matter derived during the passage of the ovum down the oviduct or Fallopian tube of the bird. A hen, like a woman, may "lay" an egg which is incapable of developing an embryo, for this is only possible if the ovum has been fertilized by a spermatozoon.
At each menstrual period, then, a woman discharges one of these ova, or eggs, similar in every detail morphologically to the ova of all other metazoic animals, i.e., all animals higher in the scale of life than the protozoa, from sponges up and unless it is vivified by the male cell it is soon discharged from her body, successive ova continually ripening, and continuously preparing themselves, as it were, for a possible fecundation.
All animals which have feminine sex "lay" eggs, some being hatched outside the body, oviparous; some hatching within the mother's body without having vascular connection with the parent, ovovivipa rous ; and others, viviparous, establishing a vital connection within the mother by means of a placenta and umbilical cord.
All mammals are viviparous with the single exception of the curious Orniihorhynchus, or "duck billed mole" of Australia, which lays eggs like the birds and is oviparous.
Most eggs are spherical in shape, but some are cylindrical ; some are ovoid, as in birds; while others are conical or elliptical.
With the exception of the anomalous Ornithorhynchus, the ova of mammals are exceedingly minute spherical cells; but it must be distinctly remembered that they are structurally the same as all other eggs, the "white" and the "egg shell" of the latter being merely modifications of homologous parts in the former which serve for the nutrition and protection of the embryo.
The human ovum has thus been compared with the ova of other metazoic animals, and especially to the ova of birds, because of the familiar acquaintance with the latter, and because they are structually identical.