[9] Notwithstanding the objections raised by Bergh, Verworn, and Haacke, I abide by the supposition that the nucleus of reproductive cells contains the hereditary mass or germinal material. My reasons may be found in my text-book on _The Cell_ (English edit., p. 274). Briefly they are: 1. The equivalence of the male and female hereditary masses. 2. The equal distribution of the growing nuclear mass of the primary egg-cell among the daughter-cells that, arising from it, build up the organism. 3. The preservation of a constancy of bulk of the hereditary mass when fertilization occurs. 4. The isotropism of protoplasm. Following Pflüger, I mean by isotropism that the protoplasm of the egg does not contain local areas for the formation of different organs; but that, according to the conditions, any part of the protoplasm may be employed in the formation of any organ. Isotropism is merely the negation of His' doctrine of the presence of local areas for definite organs, and without losing its meaning, is compatible with the fact that many eggs have their poles different, and that others have a bilateral symmetry which determines the plane of the first division. 5. The fact that the first stages of many embryonic developments consist in the multiplication of the nuclear material and its distribution in the yolk, following which the yolk-mass cleaves into cells. [10] English edition, p. 32. [11] English edition, p. 34. [12] In this section upon heteromorphosis I rely upon the following treatises, which have appeared recently. Loeb, _Untersuchungen zur physiologischen Morphologie der Thiere. Organbildung und Wachsthum_. Heft, 1 and 2 (1891-1892). H. de Vries, _Intracellulare Pangenesis_ (1889). H. Driesch, _Entwicklungsmechamische Studien_, i.-vi.; _Zeitschrift f. wissenschaft, Zool._, vol. liii.-lv. The same, _Zur Theorie der thierischen Formbildung._ _Biol. Centralblatt_, vol. xiii., 1893. Chabry, _Contribution à l'embryologie normale et tératologique des Ascidies simples. Jour. de l'Anat. et de Physiol._ (1887). Wilson, _Amphioxus and the Mosaic Theory. Journal of Morph._ (1893). See also _Anatomischer Anzeiger_ (1892).