The biophores are his smallest material units, and to them are due the fundamental qualities of life--assimilation, metabolism, and reproduction by division. Thus, they correspond to Herbert Spencer's physiological units, Darwin's gemmules, De Vries' pangenes, and Hertwig's idioblasts. They are the bearers of the various characters of cells, and there are present in the germplasm a very large multitude of different kinds of them, corresponding to the number of cells with different characters. The determinants are units of the rank next higher; they have qualities of their own, but are composed of groups of several kinds of biophores. They, too, have the power of division which is associated with, and comes about by, multiplication of the coherent company of biophores which lies within them. The histological character of every cell in a multicellular organism is determined by a single determinant (cell-determinants). Weismann has framed his conception of determinants so as to avoid the supposition that every single cell is represented in the germplasm by its own biophores. There are small parts in the body in which the cells are all alike, and for these parts a single determinant suffices, afterwards multiplying by division. On the other hand, each cell or cell-group in the body, that is independently variable, must have its special determinant in the germplasm. And so the germplasm of a species must possess as many determinants, or guiding particles, as there are in the organism cells or cell-groups that are independently variable in the germ or in later stages (hereditary pieces or determinates). As every cell or group of cells which corresponds to determinants has a definite position in the body, Weismann infers that the determinants are definitely placed in the germplasm, and form an ordered, complicated community. He has given the name id to these communities, which are higher units with definite constitution and with complicated architecture. These _ids_ are bodies containing all the determinants necessary to build up the individual of a species, and correspond to what Weismann previously called ancestral plasms. Every id must be able to grow and multiply, for it is by their multiplication that the germplasm for new individuals is formed. A single _id_ would suffice for the conduct of a single life-history; Weismann, however, in the pursuit of a chain of thought connected with the relation of sexual reproduction to heredity, and which I shall not discuss here, regards the germplasm as being still more complicated, and consisting of many, sometimes more than a hundred, ancestral plasms or _ids_, which have been derived from near or distant ancestors, the peculiarities of whose structures they retain, and may at some time actually produce (explanation of atavism).