From our point of view all this cannot surprise us, since all these organisms, though only single cells, possess great complexity of structure; we need only call to mind the extremely fine differentiation of structure in numerous ciliated Infusorians, such as _Stentor_, which has already been mentioned, or the bell-animalcule (_Vorticella_) with its long and peculiarly ciliated gullet, its retractile ciliated disk, its muscular or myophane layer, its spirally retractile stalk with the ribbon-like, rapidly acting muscular axis; or the regular geometrically constructed flinty skeleton of the Radiolarians, with their radially disposed sword-like or rod-like needles and their complex interlacing lattice-work shells. In the latter case the complexity of the living substance becomes visible only through its product, the shell, for the protoplasm itself does not show any visible intricacy, and the same is true of the Coccidium whose life-history we have just been tracing, for in each of its stages it seems to be of very simple organization, though the succession of numerous different forms shows that its germ-substance must be composed of numerous determinants. We cannot doubt, however, that, in all unicellular organisms, the protoplasm can be hardly less complicated as regards its minute invisible structure, since otherwise it would be impossible that the delicate vital processes which we observe in them should run their course. In this I agree, at least in principle, with the beautiful picture drawn by Ludwig Zehnder in his recent book[22] already mentioned, though he reached it in quite a different way, namely, by a purely synthetic method. He made the daring attempt to build up the organic world from below, starting from atoms and molecules, and ascending from these to the lowest vital units, our biophors, to which he attributes a tubular shape and therefore calls fistellæ. He imagines the cell to be made up of a large number, perhaps millions, of different kinds of fistellæ, of which one presides over the power of turgidity, another over endosmosis, a third over contraction, a fourth over the conduction of stimuli, &c., so that there results a high degree of cellular complexity, a composition out of numerous kinds of biophors arranged on a definite architectural plan. All this corresponds perfectly with the views I have so long championed, and which alone make the existence of a nucleus intelligible, if it is composed--as I assume--essentially of an accumulation of determinants, that is, of hereditary substances. And that such a high degree of complexity of structure is not a mere fanciful picture we see occasionally even in the case of unicellular organisms. Thus, for instance, in _Coccidium proprium_, parasitic in the newt (_Triton_), the macrogamete or egg-cell (Fig. 123, _Ma_) before fertilization by the sperm-cell or microgamete (Fig. 123, _Mi_) surrounds itself with a capsule, at one pole of which a minute opening, the micropyle, remains