The entire vertebrate body is the product of developmental changes taking place after fertilization in a single primitive CELL, the EGG or OVUM (Fig. 1).
[Illustration: FIG. 1.--Human ovum, from a mature follicle, a sphere of about 0.2 mm. diameter. × 25. (Kollmann.)]
In structure the ovum corresponds to other animal cells. On account of their special significance during development the different component parts of the egg-cell have received special distinctive names. The _cell-body_ is known as the _vitellus_ or _yolk_. It is composed of two substances, the _protoplasm_ or formative yolk and the _deuteroplasm_ or nutritive yolk, which vary in their relative proportions in the ova of different animals.
The protoplasm represents the material from which in the course of development the cells forming the body of the individual are derived, while the deuteroplasm serves for the nutrition of the ovum during the earliest stages of development.
The _nucleus_ of the egg-cell is distinguished as the _germinal vesicle_, and its _nucleolus_ as the _germinal spot_.
The _cell-body_ or _vitellus_ is surrounded by a condensed portion of the cell contents to which the name of the vitelline membrane has been applied, which in turn is enclosed by a transparent and elastic cover, the _zona pellucida_, presenting a radially striated appearance.
The ovum is contained in the cortical portion of the ovary, enclosed in the _Graafian follicle_, a vesicle 4-8 mm. in diameter, whose fibrous walls are lined by several layers of epithelial cells, which surround the ovum, forming the _discus proligerus_.
After impregnation the egg-cell, by a process of repeated division or cleavage, undergoes _segmentation_, the cell-body being divided successively into two, four, eight, sixteen, thirty-two, etc., _cells_, called _blastomeres_ (Figs. 2 and 3). The mass of cells finally resulting from this process of segmentation forms the ground work of the future body. A vertebrate ovum in this stage of complete segmentation is called the _morula_ from its resemblance to a mulberry (Fig. 4).
[Illustration: FIG. 2.--Segmentation of mammalian ovum (bat). (After E. von Beneden.) Two blastomeres, each with a nucleus, shown in lighter color. The dark bodies are yolk-granules.]
