The two perfected germ-cells before fecundation are in a state of nuclear rest after the numerous mitotic changes that have taken place in the maturation of these cells. When these nuclei unite in the ovum an intense activity at once is set up. Biologists offer very many theories to explain this awakening force. Herbert Spencer, Herting, and others held that protoplasm when perfected tends to pass into a state of stable equilibrium and consequent lessened activity, but fertilization restores it to a labile state. This and similar theories are verbose amplifications of the obvious fact that the cells start to divide and the biologists do not know the cause. The soul, of course, cannot have anything to do with the matter, because you cannot smell a soul. "Senescence and rejuvenescence" is another sonorous explanation that does not explain, used by Minot, Engelmann, and Hansen. Weismann rejects these theories for his own "Fertilization as a Source of Variation." Anyhow, the fertilized cell starts to divide regardless of the biologists. Adult cells may be stimulated to divide by chemical irritation, by mechanical pressure as in the formation of calluses, traumatism, by any agency that brings about an abnormal condition of the body, but this fact does not explain the normal fission of the fecundated ovum. In about fifteen days from the date of fertilization the ovum passes through the following stages: 1. The ovum, with a full series of mitotic changes of the ordinary somatic type described above, divides, subdivides, and grows within the cell-wall until a rounded mass of cells is formed, which is called the Morula or Blastula--the original cell-wall, of course, stretches to hold these new cells. They are of unequal size, and they divide at unequal rates. 2. An albuminous fluid collects within the morula, and thus the Vesicle or Blastocyst is formed. The blastocyst is called more commonly the Cleavage Cavity or the Segmentation Cavity. As this cavity widens the cells are seen to be arranged in two groups--(_a_) an enveloping layer, the epiblast, from the outermost plate of which develops later the Trophoblast, or the nourishing and protecting covering of the embryo; (_b_) an Inner Cell Mass, made up of granular cells, attached to the epiblastic layer at the Embryonic Pole of the Vesicle. These two stages probably take place in the Fallopian tube, and thereafter the embryo is in the cavity of the uterus. 3. In the third stage the Inner Cell-Mass separates into two layers derived from the inner cell-plate of the blastula. The mass flattens and spreads peripherally, until finally it is divided into two layers. The outer is the Ectoderm and the inner is the Endoderm or Hypoblast. The three steps just described have not yet been seen in the human species by any one, but they are inferred very confidently from what is well known of the development in mammals most closely resembling man in physical formation.