The ideas developed in this essay were first expressed during the past winter in a lecture delivered to the students of this University (Freiburg), and they were shortly afterwards—in February and the beginning of March—written in their present form. I mention this, because I might otherwise be reproached for a somewhat partial use of the most recent publications on related subjects. Thus I did not receive Oscar Hertwig’s paper—‘Contributions to the Theory of Heredity’ (Zur Theorie der Vererbung), until after I had finished writing my essay, and I could not therefore make as much use of it as I should otherwise have done. Furthermore, the paper by Kölliker on ‘The Significance of the Nucleus in the Phenomena of Heredity’ (Die Bedeutung der Zellkerne für die Vorgänge der Vererbung), did not appear until after the completion of my manuscript. The essential treatment of the subject would not, however, have been altered if I had received the papers at an earlier date, for as far as the most important point—the significance of the nucleus—is concerned, my views are in accordance with those of both the above-named investigators; while the points upon which our views do not coincide had already received attention in the manuscript. A. W. Freiburg I. Breisgau, June 16, 1885. ------------------------------------------------------------------------ CONTINUITY OF THE GERM-PLASM, &c. CONTENTS. Introduction 165 I. The Germ-Plasm 174 1. Historical development of the theory as to the 174 localization of the germ-plasm in the nucleus 2. Nägeli’s ‘idioplasm’ is not identical with Weismann’s 180 ‘germ-plasm’ 3. A retransformation of somatic idioplasm into germ- 183 idioplasm does not take place 4. Confirmation of the theory as to the significance of the 185 nuclear substance afforded by Nussbaum’s and Gruber’s experiments on regeneration in Infusoria 5. The nucleoplasm changes during ontogeny according to a 186 certain law 6. The identity of the daughter-nuclei produced by indirect 187 nuclear division, as assumed by Strasburger, is not necessary for my theory 7. The gradual decrease in complexity of the structure of 190 the nucleus during ontogeny 8. Nägeli’s view on the germs (‘Anlagen’) in the idioplasm 192 9. The manner in which germ-cells arise from somatic cells 194 10. ’Embryonic’ cells in the mature organism 196 11. The rule of probability is against a retransformation of 198 somatic idioplasm into germ-plasm