As may be seen in his essays, _On Life and Death_, _On the Duration of Life_, etc., Weismann believes himself to have established a fundamental distinction between unicellular and multicellular organisms. Unicellular organisms (he would have it) do not undergo natural death, but, since they are able to reproduce themselves continuously by a process of simple division, are immortal. Multicellular organisms, on the other hand, must perish after a definite duration of life, and so are mortal. He makes an exception of the sexual cells, which, like unicellular organisms, are able to multiply indefinitely, and so are immortal. Thus Weismann came to make a distinction between the mortal (somatic) cells and the immortal (germ) cells of multicellular organisms. The latter he regarded as arising directly from the egg-cell, and never from somatic cells. Nussbaum has given utterance to similar views, holding that the dividing egg at a very early period cleaves into the cells from which the individual grows and the cells for the maintenance of the species. He has enunciated the proposition that, when the sexual cells have been separated from the cells of the young embryo, the material of the germ has been divided into shares for the individual and shares for the species; that the sexual cells take no part in the formation of the body, and that body-cells never give rise to ova or spermatozoa. Weismann differs from Nussbaum in one important point. He lays no stress on the direct origin of the sexual cells, as cells, from the egg at the beginning of its development. He found, for instance, that, in the case of hydroids, the sexual cells did not arise in such a fashion. He considers, therefore, that the chain of events is as follows: The whole of the protoplasm of an egg-cell is not required to build up the new being, and the superfluous part remains unaltered to form the sexual cells of the new generation. Unlike Nussbaum, then, he asserts a continuity, not for the sexual cells, but for the germinal protoplasm which he believes to pass along definite cell-tracks until it forms the sexual cells. From this germinal protoplasm, which makes the germ-cells, he distinguishes the somatic protoplasm which makes the mortal, somatic cells. The germplasm theory entered a new phase in the year 1885, after the independent appearance in 1884 of essays by Strasburger and by me, in which we gave reason for thinking that the cell nucleus was, as I expressed it, the bearer of the characters which were transmitted by parents to their offspring; that, in fact, the nucleus was the material basis of heredity.