This is a spherical cell, just visible to the naked eye (about 1/120 inch), composed of the following parts: - A transparent finely striated outer investment, the "zona pellucida"; within this is the "vitellus", or "yolk", made up of albuminous and fatty granules constituting the greater part of the contents of the cell. Embedded in the yolk, generally near the circumference, is a large spherical nucleus, the "germinal vesicle", within which is a nucleolus or "germinal spot". When fully developed, or ripe, the Graafian follicles project from the surface of the ovary, and sooner or later burst, at which time the ovum enters the Fallopian tube and is conveyed to the uterus, where, if fertilized by the male element, it develops into a foetus. Should fertilization not take place then, the egg dies and undergoes disintegration.

The Ovum.

Fig. 233. - The Ovum.

1, Ovum ruptured. 2. Entire ovum. a, Zona Pellucida. B, Space left by retraction of Yolk. C, Vitellus or Yolk. D, Germinal Vesicle. E, Germinal Spot.

The small cavity in the substance of the ovary, from which the egg has been discharged, becomes filled with blood, which ultimately disappears and is succeeded by a small quantity of fibro-gelatinous matter, the " corpus luteum".

Should pregnancy follow the discharge of the ovum, the corpus luteum becomes much' larger, and the changes occurring in it go on much more slowly, so that it persists for a longer time.