The primordial germ-cells appear in the human fetus about the twentieth day and finally mature at puberty. Then an ovum at menstruation breaks out through the surface of the ovary, and is taken by the fimbriae of the Fallopian tube into the lumen of this tube. Fecundation happens near the outer or ovarian end of the Fallopian tube, and the fecundated ovum finally is passed on to fasten on the wall of the uterus. The spermatozoön is a ciliated cell with the power of locomotion, through the movement of the tail of the cell. It can move 0.05 to 0.06 mm., or its own length, in a second. It thus passes up through the uterus and out through the Fallopian tube, against the cilary motion of the tubal cells, until it meets the ovum. A human ovum is a typical cell, but it has a covering membrane, and a minute quantity of deutoplasm or yolk, which is not alive, and is food for the growing embryo before the embryo begins to draw sustenance through the placenta. The eggs of birds have a large quantity of food stored in the yolk, since their embryos live in the ovum and draw food therefrom during the entire period which corresponds to the time of gestation in mammals. The "white" and the calcareous shell of a hen's egg are adventitious parts, added in the oviduct after the egg leaves the ovary. The spermatozoön is a complicated organism. The head is partly covered with a thin protoplasmic cap, and it contains the nucleus with the chromatin. In the neck are two centrosomes. The tail is in three parts with an axial filament throughout, which is a bundle of extremely minute fibrils. In the middle part the axial filament is surrounded by an inner sheath; outside this sheath is a spiral filament lying in a clear substance; and outside the spiral filament is a finely granular layer of protoplasm, called the Mitochondria. This organism is a living animal cell, and it can live in an incubator, or in the Fallopian tube for two or three weeks, altogether removed from the living male body that produced it. Sir John Lubbock[33] says he kept a queen ant alive for thirteen years. This ant, which died in 1888, had been fertilized in 1874, and never afterward. She laid fertile eggs for thirteen years; that is, the spermatozoa in her oviduct retained their vitality for thirteen years. [33] _Journal of the Linnean Society_, vol. xx, p. 133.