I think it necessary to emphasise the fundamental importance of this simple, but often unappreciated, feature in order to have a correct and clear idea of conception. With that end, I have given a special name to the new cell from which the child develops, and which is generally loosely called “the fertilised ovum,” or “the first segmentation sphere.” I call it “the stem-cell” (_cytula_). The name “stem-cell” seems to me the simplest and most suitable, because all the other cells of the body are derived from it, and because it is, in the strictest sense, the stem-father and stem-mother of all the countless generations of cells of which the multicellular organism is to be composed. That complicated molecular movement of the protoplasm which we call “life” is, naturally, something quite different in this stem-cell from what we find in the two parent-cells, from the coalescence of which it has issued. _The life of the stem-cell or cytula is the product or resultant of the paternal life-movement that is conveyed in the spermatozoon and the maternal life-movement that is contributed by the ovum._ The admirable work done by recent observers has shown that the individual development, in man and the other animals, commences with the formation of a simple “stem-cell” of this character, and that this then passes, by repeated segmentation (or cleavage), into a cluster of cells, known as “the segmentation sphere” or “segmentation cells.” The process is most clearly observed in the ova of the echinoderms (star-fishes, sea-urchins, etc.). The investigations of Oscar and Richard Hertwig were chiefly directed to these. The main results may be summed up as follows:— Conception is preceded by certain preliminary changes, which are very necessary—in fact, usually indispensable—for its occurrence. They are comprised under the general heading of “Changes prior to impregnation.” In these the original nucleus of the ovum, the germinal vesicle, is lost. Part of it is extruded, and part dissolved in the cell contents; only a very small part of it is left to form the basis of a fresh nucleus, the _pronucleus femininus._ It is the latter alone that combines in conception with the invading nucleus of the fertilising spermatozoon (the _pronucleus masculinus_).