The following quotation from Hudson, following Stephens, is interesting: "An aggregation of cells became a confederation, with its differentiation of cell functions and still further division of labor. As a result of a long process of such differentiation, the organisms of the larger animals and of man came to be composed, as we find them, of thirty or more different species of cells. For example, we have the muscle cells, whose vital energies are devoted to the office of contraction, or vigorous shortening of length; connective-tissue cells, whose office is mainly to produce and conserve a tough fibre for binding together and covering in the organism; bone cells, whose life work is to select and collocate salts of lime for the organic framework, levers and joints; hair, nail, horn and feather cells, which work in silicates for the protection, defense, and ornamentation of the organism; gland cells, whose _motif_ in living has come to be the abstraction from the blood of substances which are recombined to produce juices needed to aid the various processes or steps of digestion; blood cells, which have assumed the laborious function of general carriers, scavengers, and repairers of the organism; eye, ear, nasal and palate cells, which have become the special artificers of complicated apparatus for transmitting light, sound, odors, and flavors to the highly sentient brain cells; pulmonary cells, which elaborate a tissue for the introduction of oxygen and the elimination of carbon dioxide and other waste products; hepatic (liver) cells, which have, in response to the needs of the organism, descended to the menial office of living on the waste products and converting them into chemical reagents to facilitate digestion--these and numerous other species of cells; and lastly, most important and of greatest interest, brain and nerve cells."