[4] Amer. Journ. Physiol. vols. iii. and iv. 1900.
[5] According to Delage (Arch. Zool. exp., 3 sér. 10, 1902), it is indifferent for the realisation of artificial parthenogenesis, whether but one, or both, or neither of the “polar bodies” has been formed. But the egg must be in the first stages of maturation to the extent that the “nuclear membrane” must be already dissolved.
[6] The older theories, attributing to fertilisation (or to “conjugation,” i.e. its equivalent in Protozoa), some sort of “renovation” or “rejuvenescence” of the race, have been almost completely given up. (See Calkins, Arch. für Entwickelungsmechanik, xv. 1902). R. Hertwig recently has advocated the view, that abnormal relations between the amounts of nuclear and of protoplasmatic material are rectified in some way by those processes. Teleologically, sexual reproduction has been considered as a means of variability (Weismann), but also as a means of preserving the type!
[7] The phrase “ceteris paribus” has to be added of course, as the duration of each single elementary morphogenetic process is liable to vary with the temperature and many other conditions of the medium.
[8] We shall not avoid in these lectures the word “explain”—so much out of fashion nowadays. To “explain” means to subsume under known concepts, or rules, or laws, or principles, whether the laws or concepts themselves be “explained” or not. Explaining, therefore, is always relative: what is elemental, of course, is only to be described, or rather to be stated.
[9] Das Keimplasma, Jena, 1892.
[10] Die Bedeutung der Kernteilungsfiguren, Leipzig, 1883.
[11] Unsere Körperform, Leipzig, 1875.
[12] Die Entwickelungsgeschichte der Unke, Leipzig, 1875.
[13] Gesammelte Abhandlungen, Leipzig, 1895. Most important theoretical papers:—Zeitschr. Biolog. 21, 1885; Die Entwickelungsmechanik der Organismen, Wien, 1890; Vorträge und Aufsätze über Entwickelungsmechanik, Heft i., Leipzig, 1905.