The human egg is, like that of all other mammals, a small globular bladder, which contains all the constituent parts of a simple organic cell (Fig. 5). The most essential parts of it are the mucous cell-substance, or the protoplasma (_c_), which in an egg is called the “yolk,” and the cell-kernel, or nucleus (_b_), surrounded by it, which is here called by the special name of the “germinal vesicle.” The latter is a delicate, clear, glassy globule of albumen, of about 1-600th part of an inch in diameter, and surrounds, a still smaller, sharply-marked, rounded granule (_a_), the kernel-speck, or the nucleolus of the cell (in the egg it is called the “germinal spot”). The outside of the globular egg-cell of a mammal is surrounded by a thick pellucid membrane, the cell-membrane or yolk-membrane, which here bears the special name of zona pellucida (_d_). The eggs of many lower animals (for example of many Medusæ) differ from this in being _naked_ cells, as the outer covering, or cell-membrane, is wanting. As soon as the egg (ovulum) of the mammal has attained its full maturity, it leaves the ovary of the female, in which it originates, and passes into the oviduct, and through this narrow passage into the wider pouch or womb (uterus). If, meanwhile, the egg is fructified by the male seed (sperm), it develops itself in this pouch into an embryo, and does not leave it until perfectly developed and capable of coming into the world at birth as a young mammal. The variations of form and transformations which the fructified egg must go through within the uterus before it assumes the form of the mammal are exceedingly remarkable, and proceed from the beginning in man, in precisely the same way as in the other mammals. At first the fructified egg of the mammal acts as a single-celled organism, which is about to propagate independently and increase itself; for example, an Amœba (compare Fig. 2, p. 188). In point of fact the simple egg-cell becomes two, by the process of cell-division which I have previously described. There arise from the single germinal spot (the small kernel-speck of the original simple egg-cell) two new kernel-specks, and then in like manner, out of the germinal vesicle (the nucleus), two new cell-kernels. Then, and not until then, does the globular protoplasma first separate itself by an equatorial furrow into two halves, in such a manner that each half encloses one of the two kernels, together with its kernel-speck. Thus the simple egg-cell, within the original cellular membrane, has become two naked cells, each possessing its own kernel (Fig. 6).