The _centrosome_ (Fig. 1), with its enveloping attraction-sphere, constitutes another fundamentally important part of the cell. It is especially concerned with the phenomena of cell division and multiplication. Just as the living body consists of an infinitely complex figure of living mosaics termed cells, so the cell itself consists of an infinitely complex figure of still smaller living mosaics called, by Spencer, _Physiological Units_. These units have been given different names by various writers, viz.: by Darwin, _gemmæ_ (gemmules); by de Vries, _pangennæ_; by Hertwig, _idioblasts_; by Weismann, _biophors_, etc., etc. Like the atom of the chemist and the molecule of the physicist, the physiological unit of the biologist is merely at present an intellectual conception, yet it is, at the same time, an intellectual necessity and plays a very important part as the theoretical component of many vital questions. Just as the cells are the _visible_ units of life, so the physiological units are the _invisible_ units. The physiological activities of cells are those that pertain to their _nutrition_ and _reproduction_. The nutrition of cells includes all processes that are subservient to their life and well-being, such as irritability, contractility, absorption of food, its digestion and assimilation, secretion, etc. In consequence of the wonderful _nutritive_ activities of cells, we may well speak of them as marvelous magicians. Hertwig, following Haeckel, speaks of many cells as being _builders_. In the same spirit, we can say that multitudes of cells are expert chemists, artists, sculptors, mathematicians, and so on, in that they make all the myriad chemical products of organic nature, such as spices, pigments, sugars, starches, acids, perfumes, and numerous other substances; they paint in colors that rival the hues of the rainbow; they construct all of the beautiful forms in the animal and plant worlds; and they draw lines as straight and curves as graceful as the most expert mathematician.