The maturation of the egg, as already intimated, follows the same plan as that of the sperm, and the reduction of the chromosomes is the same. Such modifications as occur are related to the fact that the egg is usually a relatively large passive cell stored with nutritive materials for use during the developmental process, while the sperm is among the smallest of cells essentially a nucleus surrounded with a delicate envelope of cytoplasm. Accordingly it is only necessary to emphasize that the growth period of egg formation , in which the Oogonium becomes transformed into the PRIMARY OOCYTE, is characterized by a much greater increase in size than is the case in the corresponding period in spermatogenesis; and that both of the ensuing cell divisions (one a reduction and the other an equation division) involve very unequal divisions of the cytoplasm. Thus one OOCYTE is very large, while the other is a tiny cell termed the FIRST POLAR BODY. Both the large secondary oocyte and first polar body now divide again; the former giving rise to a large cell, the mature EGG, and a tiny SECOND POLAR BODY; while the first polar body divides equally to form two polar bodies.
In this way arise the four cells, comparable to the four sperm in spermatogenesis, each with half the somatic number of chromosomes. But only one of these, the egg, functions as a gamete. The three polar bodies, although possessing a similar chromosome complex, are sacrificed in providing one cell with its special cytoplasmic equipment.
The polar bodies get just enough cytoplasm to be regarded as cells, and soon degenerate and disappear.
Such is the outline of the essentials of spermatogenesis and oogenesis in animals; processes which involve at one stage a modification of ordinary mitosis to give each gamete half the somatic number of chromosomes characteristic of the species.
It is clear that this is not merely a mass reduction of chromatin material, but is a separating of definite chromatin entities, the chromosomes, so that the gametes receive the reduced number.