Before quitting this somewhat complicated addition to the original theory of germ-plasm, I must briefly allude to the descriptions and illustrations of karyokinesis which were given in Part I of _Darwin and after Darwin_, for the prospective benefit of any general readers who might afterwards be sufficiently interested in Weismann’s speculations to desire a statement of the main facts on which this further development of his theory rests. It seemed undesirable to burden the present volume with an account of recent investigations so well known to naturalists, while, on the other hand, it was clearly desirable that such an account should be given somewhere, if the speculations in question were to be rendered intelligible to anybody else. Therefore I must here request those of my readers who are not already acquainted with the matter to consult pp. 128-134 of Part I. It will there be seen how enormously complex are the visible processes which take place in the nucleus of a germ-cell (and likewise of a somatic-cell), preparatory to its division; and therefore, supposing that the nucleus alone contains the material concerned in the phenomena of heredity, it appears that no small corroboration is lent to Weismann’s views by these histological observations. And, more particularly, if we suppose with him that the material in question is restricted to that portion of the segregating nuclear matter which is called the “nuclear thread[9],” in the formation of the “loops” or “rods” of this substance we seem to have presented a visible expression of the marshalling of “the carriers of heredity,” and the successive passage of the originally generalized “germ-plasm” of the germ-cell into the ever more and more specialized “nucleo-plasms” of the somatic-cells. Indeed, the new theory of heredity, when thus brought into relation with the new results of histological observation, appears so well to fit the latter, that one would be sorry to find the coincidence unmeaning, or the theory false. But, without passing any criticism, it is sufficient to note that the question whether or not the theory is true—and therefore correctly interprets the phenomena of karyokinesis,—must depend chiefly on whether it be eventually proved that the “nuclear thread” is indeed the only part of a germ-cell, or even the only part of a tissue-cell, which is concerned in controlling the phenomena of heredity on the one hand, and of ontogeny on the other. Into this question, however, I do not propose to enter. It will be enough to assume, for the sake of argument, that Weismann’s view of the matter will eventually prove to be true. At the same time, we must remember that at present this view as to the nuclear thread being the sole repository of the material of heredity is merely hypothetical. * * * * *