Dr. Oscar Hertwig is a scientific naturalist of the very first rank, and his name is peculiarly associated with many of the most important advances in our knowledge of cells and of embryology. To him chiefly, for instance, is due the discovery of the intimate nature of fertilisation--that it consists in the union of the nuclear matter of a cell from the male with the nuclear matter of a cell from the female. With the exception of Francis Balfour, no man has laboured more patiently, or achieved more wonderful results, in the investigation of the origin and marshalling of cells by which the egg changes into the adult. From his own experience, and from his study of the observations made by others, he has been led to doubt the validity of apparently fundamental parts of Weismann's conception. In the first place, he thinks that there is no evidence for the existence of differentiating as opposed to doubling divisions, and that there is evidence that divisions always are doubling divisions. He thinks, in fact, that when a portion of germplasm divides, the daughter-cells receive portions of germplasm exactly alike and exactly like the original portion in the parent-cell. The cells, indeed, become different from each other as the organism grows, some becoming muscle-cells, others nerve-cells, others digestive-cells, and so forth. Weismann thinks that the differences occur because, in the disintegration of the germplasm-microcosms, according to a prearranged plan, only the determinants for nerve-cells are marshalled into nerve-cells, only those for muscle-cells into muscle-cells, and so forth. The development is an evolution, an unfolding or unwrapping of little rudiments that lie in the germplasm. Hertwig insists that every cell receives the same kind of germplasm, but that, according to the situations in which they come to lie, different characters are impressed upon them. The development is an epigenesis, or impressing on identical material of different characters by different surrounding forces. His second line of argument against Weismann leads to a similar conclusion. A large number of the characters that arise in an organism during its development are due to the combination of many cells. They cannot come into existence until the multiplication of cells has made their existence possible, and he thinks, therefore, that they cannot have rudiments inside a single cell as their determining cause.