Apart from their activity in absorbing nutriment and growing, the great majority of the hereditary units in the nuclei of the forming cells remain latent. But some of the hereditary units in each cell produced are active. They multiply, grow and migrate out of the nucleus, and get among the units of the enveloping protoplasm. During this activity they undergo physical and chemical changes and effect corresponding differentiations in the protoplasm of the cell. Thus, through many mitoses and many differentiations of the protoplasm of cells, we finally derive from the fertilized ovum all the cells that constitute the adult body, such as muscle cells, glandular cells (as liver cells, kidney cells, etc.), nerve cells, skeletal cells (as bone, cartilage, and connective-tissue cells), and the ova and spermatozoids. According to this theory, the _nucleus_ of each cell in the adult animal or plant is _pure hereditary mass_, exactly like that in the nucleus of the fertilized ovum; but the _protoplasm_ of each adult cell that envelopes the nucleus may differ greatly in different cell groups, as in muscle, nerve, cartilage, and the like. Of course, the protoplasm of those cells that develop into the ova and the spermatozoids has differentiated along such lines as to become like that of the ovum and spermatozoid, the junction of which formed the fertilized ovum. These statements hold true for plants and the lower animal organisms, although they cannot be verified for the higher animals. More than likely the pure hereditary masses are present in the body (somatic) cells of the higher organism in latent conditions, but are unable ever to be developed owing to the greater specialization of these cells. It is thus seen that all the cells of many animals and plants can perform their own special functions and at the same time contain all of the hereditary units of the complex organism in a state ready to develop under favoring conditions. Since the ova and spermatozoids are cells specially differentiated for the purpose of propagating the species by sexual generation, and since their conjugation produces the germ of a new creature, they may very appropriately be spoken of as =germ cells=. Since all the other cells of the adult form the great bulk of the body that envelopes and protects the germ cells, they may be termed the =body cells=, or =somatic cells=. Suppose that ova, containing maternal and paternal groups of hereditary units, are fertilized by similarly complex spermatozoids, and the process is repeated generation after generation. There will come a time when the fertilized ovum will have a highly complex nucleus composed of _many different ancestral groups of hereditary units_. One often hears the expression that a child is a chip of the old block; but this is only a very partial truth, for a child is preëminently a composite chip of many old blocks.