Hence, the nuclear contents (nucleo-plasm) of every cell, whether germinal or somatic, present two substances, which we may, in the absence of any better terms supplied by Weismann himself, respectively designate “idio-plasm-A” and “idio-plasm-B.” Idio-plasm-A is the substance which has to do only with the formation and control of the individual cell in which it resides, like a mollusc in its shell. Idio-plasm-B is the substance out of which future cells are to be formed and controlled, when in due course either of ontogeny or phylogeny this idio-plasm-B becomes converted into idio-plasm-A,—i.e., into each subsequently developing tissue or organism, as the case may be. I say ontogeny or phylogeny, and tissue or organism, because, where a _germ-cell_ is concerned, idio-plasm-B is capable of reproducing entire organisms of its own and of subsequent generations; whereas, in the case of all _somatic_-cells, idio-plasm-B is capable only of reproducing, stage by stage, some greater or less number of the cells which are to construct the single organism of which they form a part. Or, otherwise expressed, in the particular case of a germ-cell idio-plasm-B is germ-plasm, and therefore is alone capable of producing an entire organism of somatic-cells, while it is likewise alone capable of reproducing successive organisms; for it alone contains the carriers of heredity[7]. Thus, idio-plasm-B of an unsegmented germ-nucleus is germ-plasm. But as soon as the germ-nucleus has undergone its first nuclear division, its nucleo-plasm is no longer germ-plasm, inasmuch as each of the half-portions is now no longer capable of reproducing an entire organism—unless it be in the case of identical twins. Similarly in the second nuclear division, each of the four resulting idio-plasms-B is still further removed from the pristine character of germ-plasm; and so on through all successive stages of segmentation. Hence these successive nuclear divisions must indicate a partitioning and re-partitioning of the original idio-plasm-B (germ-plasm) into the idio-plasms-B severally distinctive of all the various cells of the soma.