The human ovum, like that of all other animals, is a single cell, and this tiny globular egg cell (about the 120th of an inch in diameter) has just the same characteristic appearance as that of all other viviparous organisms. The little ball of protoplasm is surrounded by a thick, transparent, finely reticulated membrane, called the _zona pellucida_; even the little, globular, germinal vesicle (the cell-nucleus), which is enclosed in the protoplasm (the cell-body), is of the same size and the same qualities as in the rest of the mammals. The same applies to the active spermatozoa of the male, the minute, threadlike, ciliated cells of which millions are found in every drop of the seminal fluid; on account of their lifelike movements they were previously taken to be forms of life, as the name indicates (spermatozoa--sperm animals). Moreover, the origin of both these important sexual cells in their respective organs is the same in man as in the other mammals; both the ova in the ovary of the female and the spermatozoa in the spermarium of the male arise in the same fashion--they always come from cells, which are originally derived from the coelous epithelium, the layer of cells which clothes the cavity of the body. The most important moment in the life of every man, as in that of all other complex animals, is the moment in which he begins his individual existence; it is the moment when the sexual cells of both parents meet and coalesce for the formation of a single simple cell. This new cell, the impregnated egg cell, is the individual stem cell (the _cytula_), the continued segmentation of which produces the cells of the germinal layers and the gastrula. With the formation of this cytula, hence in the process of conception itself, the existence of the personality, the independent individual, commences. This ontogenic fact is supremely important, for the most far-reaching conclusions may be drawn from it. In the first place, we have a clear perception that man, like all the other complex animals, inherits all his personal characteristics, bodily and mental, from his parents; and, further, we come to the momentous conclusion that the new personality which arises thus can lay no claim to "immortality."