Fig. 122. Conjugation of a Coccidium (Adelea ovata), after Schaudinn and Siedlecki. A, the microgamete (sperm-cell) (Mi) has become closely apposed to the macrogamete (Ma). B, the reduction division of the nucleus of the macrogamete has been effected; Rk, directive corpuscles. In the microgamete the first division of the nucleus has begun. C, four nuclei in the microgamete, of which three come to nought. D, the fourth microgamete-nucleus (♂K) has become apposed to the nucleus of the ovum, in which distinct chromosomes are seen.
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 for the entrance of the male cell. This proves, it seems to me, that this particular spot of the capsule is hereditarily determined, just as much and just as definitely as the ray of the flint-skeleton of a Radiolarian. But if any spot of the capsule can vary by itself alone, may not numerous other points in the animal also be hereditarily determinable? With such complexity of the invisible structure it would not greatly surprise us if we should find amphimixis occurring in all unicellular organisms, and in many of them at a high level of elaboration. These apparently lowly and simple organisms are obviously very far from being the lowliest and simplest, as we shall discover later in a different connexion. But that amphimixis is found as a periodically recurring process even among these, must depend upon the fact that here too the preservation of the best-adapted structure, as well as adaptability to new conditions, requires that the best variants of many different parts of the cell should be brought together, and since the hereditary substance lies in the ids of the nucleus, the union of the ids of two unicellulars will make harmonious and many-sided adaptation materially easier. It will thus give an advantage in the struggle for existence, and we may therefore expect to find that the nuclear substance in all unicellular organism is made up of ids.
[22] Zehnder, Die Entstehung des Lebens, Freiburg-i.-Br., 1899.
Fig. 123. Conjugation of Coccidium proprium, a cellular parasite of the newt (Triton), after Siedlecki. A, a microgamete (Mi) in the act of penetrating the shell of a macrogamete (Ma) through the micropyle. B, the male and the female nuclear constituents are uniting (♂ chr and ♀ chr).
The observations hitherto made do not, however, appear to bear this out, for in the lower Flagellata and Algæ the nuclear substance does indeed consist of chromatin, but—as far as it can be made out—of a compact unarranged mass of it. But even though deeper investigations should succeed in demonstrating chromosomes in many of these, the nucleus must have arisen at some time, and we must assume that it did so through a more intimate union of previously loose aggregates of determinants, which were gradually arranged and bound together by the combining forces (affinities) we have assumed to obtain among them, thus giving rise to the first chromosomes or ids which were complete in themselves. Then came the multiplication of these ids by the process of division, and only then was the state arrived at from which amphimixis, as we now know it, could have arisen, namely, the existence of a considerable number of identical ids, half of which could be exchanged for the identical ids of another individual in conjugation.
But as to our question, In what organisms did amphimixis first arise, and how? there seems, from what we have already learned with regard to the Coccidia, little prospect of our being able to give a definite answer, for if amphimixis occurs even in these lowly organisms, and occurs, too, in the same manner as in the higher unicellular organisms, and not very much more simply than among the highest multicellular organisms, we may conclude that the preliminary stages will now be very difficult or impossible to detect, either because they are extinct, or because they occur only in ultra-microscopic organisms.
Nevertheless there do appear to be preliminary stages, and they are exactly those which we should have assumed if we had been obliged to construct them theoretically.
The first phenomenon of this kind is the mere juxtaposition of two or more unicellular organisms, without the occurrence of fusion. This was probably first observed by Gruber in Amœbæ, and it was theoretically interpreted at a later date by Rhumbler. As many as fifty Amœbæ gather together to form a 'nest,' and remain closely apposed to each other for a fortnight. Although no fusion took place, and there were no visible results of this juxtaposition, it may be concluded that the animals had some sort of attractive effect upon each other, and it may be supposed that some sort of advantage must have been associated with this state of quiet, close apposition against one another. Cytotropism, the mutual attraction of similar cells, which Wilhelm Roux first observed in the segmentation-cells of the frog's egg, seems to occur also in unicellular organisms, and this may help us to understand how a fusion of cell-bodies may have come about.