Moreover, throughout the vegetable kingdom divisions often occur in connexion with the origin of the sex-cells which can be compared, in occurrence and result, with the maturation divisions of animal germ-cells. In the lichen, Basidiobolus, we have already seen that an abortive cell separates itself off from the sex-cell before the latter becomes capable of reproduction ([Fig. 81], C). Similar cell-divisions occur in many if not in all groups of plants. In the marine algæ of the genus Fucus it has even been proved that the division of the first primordial cell of the ovary into the so-called 'stalk-cell' and the primitive egg-cell is a reducing division, and brings down the number of chromosomes from thirty-two to sixteen. In vascular plants the reduction is not postponed until the formation of the sex-cells, but occurs earlier in the formation of the spores, as Calkins has demonstrated for ferns; in the Conifers and other Gymnosperms several so-called 'preparatory' divisions precede the formation of the germ-cells, and we know by comparison with the alternation of generations in vascular plants that these are related to the gradual waning of the strictly sexual generation. As the 'polar bodies' or 'directive corpuscles' of the animal ovum are rudimentary egg-cells, so the cells which, in the pollen-grains, separate themselves from the sex-cells proper are rudimentary Prothallium-cells, and, like the animal cells, they perish without playing any further physiological rôle. I will not assert that it is precisely in these divisions that the reducing divisions are concealed, for the analogy with the spore-formation of ferns leads us rather to suppose that it may lie further back; but in any case there is no lack of opportunity in the ontogeny of phanerogamic plants for the interpolation of a reducing division, and as long as it remains unproved that a reduction of the chromosomes can take place directly, that is, without the help of nuclear division, we shall continue to expect with confidence that the reducing divisions of phanerogams will be discovered in the future. Processes of a similar kind are known among unicellular organisms, and there, too, they are associated with nuclear divisions.
In passing to the so-called 'sexual reproduction' of unicellular organisms, I should like first to call attention to the fact that the expression 'reproduction' is not very suitable in this case, for the process in question does not always effect an increase in the number of individuals as reproduction ought to do, but leads, in fact, in many cases, even to a decrease, when two individuals unite to form one. Even if the phenomena of sexual 'reproduction' among higher organisms, which we have already studied, had not made it clear to us that there are two associated processes, quite different in nature, the conjugation of unicellular organisms would have led us to that conclusion. It has long been known that two unicellular plants or animals occasionally become closely apposed and fuse; and this process of 'conjugation' was many years ago regarded as an analogue to 'fertilization,' although it is only through the laborious investigations of the last two or three decades that this supposition has been proved to be correct. We now know that a process quite analogous to that which we have learnt to know as 'fertilization' takes place among unicellulars, only in this case it is not directly connected with reproduction and multiplication, but occurs independently of them, and, in its most primitive form, it results, not in an increase but—for a short time at least—in a diminution of the number of individuals. This occurrence of the process independently of reproduction appears to me of inestimable value theoretically, for it frees us completely from the old deep-rooted preconceptions in the interpretation of fertilization.
Fig. 83. Conjugation of Noctiluca, after Ischikawa.
A, two Noctilucas beginning to coalesce; pr, the protoplasm
drawn out into processes which traverse the
gelatinous substance of the cell; k, the nucleus.
B, the cells and their gelatinous substance have fused;
the nuclei, in which the chromosomes are visible, are
closely apposed; CK, centrospheres. C, the two nuclei
are united in one nuclear spindle; beginning of
division. D, completion of the division. Highly
magnified.
First let us briefly sketch the process itself in the main forms of its occurrence.
The most primitive form of conjugation is undoubtedly the complete fusion of two unicellular organisms of the same species, as we see it to-day in unicellular plants, and also among the lowest unicellular animals, such as the flagellate Infusorians, Gregarines, and Rhizopods. It is well seen, for instance, in the Noctilucæ, those unicellular flagellate organisms which cause the familiar marine phosphorescence extending uniformly over wide surfaces of water (Fig. 83). In these forms Prof. Ischikawa of Tokio was able to trace the whole process of conjugation. To begin with, two Noctilucas range themselves side by side (Fig. 83) and coalesce at the surfaces in contact, both as to the spherical gelatinous envelope (A, G) and the protoplasm (pr) itself, which branches in amœboid fashion into the jelly. The union becomes gradually complete, and the two animals form a single sphere (B) with one cell-body. But the two nuclei (K) also place themselves side by side (B), and though they do not actually fuse, they form together, under the guidance of two centrospheres (C), a single nuclear division-figure, which is obviously analogous to the segmentation spindle of the fertilized egg. Then follows a division, by means of which the chromatin substance of the nuclei of both animals is divided between the two daughter-nuclei, and after this has been accomplished the united individual again separates into two independent Noctilucas (D). Although I have spoken here—that is, in referring to the Protozoa—of chromosomes, I must immediately add that these have not yet been seen with full clearness in Noctiluca itself; nothing more has been recognized than deeply staining thickenings of the spindle fibrils, which move from the equator of the nuclear spindle towards the pole. Since, however, in other Protozoa, as, for instance, in the beautiful freshwater Rhizopod (Euglypha alveolata), these thickenings of the nuclear spindle fibrils have been clearly recognized as chromosomes, doubt on this point is hardly justifiable. Apart from this, the assumption that each of the two daughter-nuclei receives half the chromosomes of each of the conjugated nuclei rests on a secure basis, not only because otherwise the whole process would have no meaning, but because the position of the mitotic figure conditions this. Even the fact that the two conjugation-nuclei lying side by side remain apart during nuclear division is not without parallel; Häcker and Rückert observed it also in the segmentation-nucleus of much higher animals, the Copepods, and it has no effect in altering the process of division, but only proves that the chromosomes of maternal and those of paternal origin in the combination-nucleus remain independent—a fact the significance of which I shall discuss later on.
The process of conjugation occurs, in the same manner as in Noctiluca, in a freshwater Rhizopod, the well-known Sun-animalcule, Actinophrys sol (Fig. 84), but in this case complete fusion of the two nuclei takes place (Fig. 84, V) before the formation of the division-spindle (VI, sp), which, with the simultaneous division of the cell-body, gives rise to two new individuals. The process in this case is especially interesting, because Schaudinn has succeeded in observing a maturation division (III, Rsp, directive spindle) as well as in demonstrating polar bodies (IV, Rk). Thus the analogy with the process of fertilization in the Metazoa and the Metaphyta is almost complete.
But that the conjugation of unicellular organisms, like the fertilization of multicellular organisms, is essentially a matter of nuclear conjugation is shown more distinctly still by the ciliated Infusorians, the most highly organized of the Protozoa.
Fig. 84. Conjugation and polar body formation in the Sun-animalcule, Actinophrys sol, after Schaudinn. I, two free-swimming conjugated individuals, which in II have become surrounded by a transparent gelatinous cyst. III, formation of the directive spindles (RSp). IV, the polar bodies are formed (RK); K, the two sex-nuclei. V, these are fused to form the conjugation-nucleus (K). VI, the conjugation-nucleus is transformed into the division-spindle; the polar bodies (RK) have penetrated the internal cyst-wall, and are in process of degeneration.