Now, it is evident that not _all_ the idio-plasm-B of a germ-cell which thus passes over into the nuclei of somatic-cells can be represented by the idio-plasm-B of those cells. At every stage of successive cell-formation a certain part of the original idio-plasm-B of the germ-cell must become the idio-plasm-A of somatic-cells distinctive of that stage. For, supposing that at its differentiation stage 99 the original germ-plasm (now somatic-idio-plasm-B of 99th stage) has reached a phase of ontogeny where the formation of tissue _m_ has next to be followed by the formation of tissue _n_, then there still remain the further differentiation stages 101, 102, 103, &c., to be provided for, which, when their time arrives, will go to form the still later tissues _o_, _p_, _q_, &c. Consequently the idio-plasm-B of stage 100 cannot be _all_ consumed in making the tissue _n_. There must be a residual portion which will afterwards be called upon to form successively the idio-plasm-A of _o_, _p_, _q_, &c. Where, then, is this residual portion of idio-plasm posited? Clearly it must be posited in the nuclei of _n_. Thus it is that, as we began by stating, all the nuclei of any given tissue _n_ really contain two kinds of substance,—(1) their own idio-plasm-A, which was part of idio-plasm-B of the preceding tissue, _m_; and (2) the idio-plasm-B, which is destined to become idio-plasms-A of succeeding tissues _o_, _p_, _q_, &c. Thus it follows also that the more the original idio-plasm-B is differentiated into these successive formations of idio-plasms-A the less of it remains for further differentiation, till, at the last stage of ontogeny, all the original idio-plasm-B (germ-plasm) has been thus changed into idio-plasms-A severally distinctive of all the somatic-tissues _a_, _b_, _c_—_x_, _y_, _z_,—save only the portion of it which has been carried through all these ontogenetic stages in a wholly _un_differentiated condition, for the purpose of securing the _phylogenetic_ production of the next generation. And this, of course, is secured by the portion of undifferentiated germ-plasm in question being deposited in the nuclei of germ-cells, at whatever stage of the ontogeny these may be formed. Finally, it is evident that, _at each stage_ of the differentiation of idio-plasm-B into idio-plasms-A, the portion concerned must be capable of self-multiplication to an almost incalculable extent,—yet this only as idio-plasm-B of the particular kind required for constructing the idio-plasm-A which is appropriate to the particular stage. Such is a necessary deduction from the terms of Weismann’s theory, inasmuch as we know that at each of the ontogenetic stages there is an incalculable multiplication of cells belonging to that stage—cells, the “cytoplasm” of which necessarily presupposes for its formation its own appropriate idio-plasm in both kinds, and this in similarly increased quantities. * * * * *