=Fertilization.= In fertilization, the spermatozoid (a nucleated flagellate cell) penetrates the ovum (a nucleated, encysted cell), its protoplasm mixes with that of the ovum, and its nuclear threads come into relation with the nuclear threads of the ovum; so that the fertilized ovum (a new creature, a veritable microcosm) is still a nucleated cell, but one in which the nucleus is compound, is _hermaphroditic_, in that it contains maternal and paternal threads—that is, maternal and paternal hereditary units which constitute its hereditary mass. It will be convenient to speak of the maternal and paternal hereditary units in the fertilized ovum as _ancestral hereditary units_. This hermaphroditic cell passes through complex phases, illustrated by embryology, to the adult. In doing so this hermaphroditic cell (mother cell) first divides into two smaller cells (daughter cells). The mother cell divides in such a way (by mitosis) that one-half of its nucleus and part of its protoplasm goes to one daughter cell and the other half of the nucleus, with the remainder of the protoplasm, goes to the other daughter cell. It is an interesting fact that although the amount of protoplasm which goes from the mother cell to the two daughter cells may be unequal at times, yet the amount of the nucleus in one daughter cell is always exactly equal[2] to that in the other; _so that each daughter cell contains maternal and paternal hereditary masses of equal quantity and quality_, being in fact one-half that of the fertilized ovum. In consequence of the fact that each hereditary unit in the nucleus of the daughter cell can absorb nutriment and grow, it comes about that the nucleus of each daughter cell attains to the size of that of the mother cell. The enveloping protoplasm of the nuclei also grows to a greater or less extent, so that the cells as a whole grow. These two daughter cells go through the same process and form other daughter cells, and so on through all the mitoses of development, until all the myriads of cells of a living organism are produced, _each of which contains maternal and paternal hereditary masses of equal quality and quantity, and also of the same character as that of the fertilized ovum whence they are all derived_.