Although I entirely agree with Hertwig, as far as the main question is concerned, I cannot share his opinions when he identifies Nägeli’s idioplasm with the nucleoplasm of the germ-cell. Nägeli’s idioplasm certainly includes the germ-plasm, if I may retain this expression for the sake of brevity. Nägeli in forming his hypothesis did indeed start with the germ-cells, but his idioplasm not only represents the nucleoplasm of the germ-cells, but also that of all the other cells of the organism; all these nucleoplasms taken together constitute Nägeli’s idioplasm. According to Nägeli, the idioplasm forms a network which extends through the whole body, and represents the specific molecular basis which determines its nature. Although this latter suggestion—the general part of his theory—is certainly valid, and although it is of great importance to have originated the idea of idioplasm in this general sense, in contrast to the somato-plasm (‘Nährplasma’), it is nevertheless true that we are not justified in retaining the details of his theory. In the first place the idioplasm does not form a directly continuous network throughout the entire body; and, secondly, the whole organism is not penetrated by a single substance of homogeneous constitution, but each special kind of cell must contain the specific idioplasm or nucleoplasm which determines its nature. There are therefore in each organism a multitude of different kinds of idioplasm. Thus we should be quite justified in generally speaking of Nägeli’s idioplasm as nucleoplasm, and _vice versa_. It is perfectly certain that the idioplasm cannot form a continuous network through the whole organism, if it is seated in the nucleus and not in the cell-body. Even if the bodies of cells are everywhere connected by fine processes (as has been proved in animals by Leydig and Heitzmann, and in plants by various botanists), they do not form a network of idioplasm but of somato-plasm; a substance which, according to Nägeli, stands in marked contrast to idioplasm. Strasburger has indeed already spoken of a ‘cyto-idioplasm,’ and it is certainly obvious that the cell-body often possesses a specific character, but we must in all cases assume that such a character is impressed upon it by the influence of the nucleus, or, in other words, that the direction in which the cell-substance is differentiated in the course of development is determined by the quality of its nuclear substance. So far, therefore, the determining nuclear substance corresponds to the idioplasm alone, while the substance of the cell-body must be identified with the somato-plasm (‘Nährplasma’) of Nägeli. At all events, in practice, it will be well to restrict the term idioplasm to the regulative nuclear substance alone, if we desire to retain the well-chosen terms of Nägeli’s theory.