Here there is usually no complete union of the cell-bodies of the two animals, but only an adhering of the apposed surfaces. In the relatively large Paramœcium caudatum the process of conjugation is very exactly known through the beautiful investigations of Maupas and R. Hertwig. In this case the mouth-surfaces of the two animals come together and unite over a short area, and then the two animals swim about together in this conjugated state. During this time very remarkable changes take place in their nuclei.
It is well known that these Infusorians have a double nucleus, a large one, the macronucleus ([Fig. 85], ma), and one which is usually very small, the micronucleus (mi). We may ascribe to the former of these the guidance and regulation of the everyday processes of life, that is, briefly, of metabolism, and the preservation of the integrity of the whole animal. The small nucleus has often been designated the 'reproductive nucleus,' but as it plays no other part in reproduction, as far as can be recognized, than that of dividing into two daughter-nuclei, I cannot regard this designation as suitable; it obviously originated in the mistaken interpretation, prevalent till very lately, of conjugation as a 'kind of reproduction,' and this in its turn depends on the conception, transferred from multicellular organisms, of fertilization as a 'sexual reproduction.' We shall immediately see that the micronucleus plays the main part in conjugation, and from this we may suppose that it otherwise fills no rôle in the life of the animal, and therefore it may best be designated the 'supplementary' or reserve nucleus. In every conjugation the macronucleus, which has hitherto been active, breaks up and becomes completely absorbed, very much like a ball of food. This of course takes place slowly; the large nucleus elongates, becomes indented, falls into several pieces, and these are so gradually absorbed that, even after the act of conjugation has been accomplished, irregular fragments of the macronucleus often lie about in the animal ([Fig. 85], 9).
But while the macronucleus falls to pieces the previously minute micronucleus grows enormously and forms a distinct longitudinally striated spindle (1, mi). About the same time these divide in both animals, and each of the daughter-nuclei immediately divides again, so that after these two divisions four spindle-shaped descendants of the micronucleus are to be seen in each animal ([Fig. 85], 4). We have previously noted that the apparatus for nuclear division in unicellular organisms was similar to that in multicellular organisms, and yet was different from it. In these ciliated infusorians we see an essential difference, for the striated spindle, after the division into daughter-chromosomes has taken place, lengthens out enormously, and becomes so thin in the middle of its length (2) that the two daughter-nuclei at the ends of this long stalk suggest the appearance of a very long and thin dumb-bell, or of a long silk purse. Of asters (centrospheres) there is nothing to be seen, and the mechanism of division is still very obscure; it almost seems as if a rapidly growing substance forced the two groups of chromosomes apart.
Hardly have these four descendants of the micronucleus arisen when three of them begin to break up and very shortly disappear; only the fourth is of any further importance, and it divides once more (5), and so gives rise to the two nuclei which play the chief part in the process of conjugation—the copulation-nuclei, exactly analogous to the male and female pronuclei in the fertilized ovum (5, mi4). But in this case each of the two animals functions doubly, that is, both as male and female, for each sends one of the two copulation-nuclei across the bridge formed by the union of the apposed surfaces into the other animal (6, mi ♂), so that it may form, by union with the nucleus which has remained there, a double nucleus (7), a structure which corresponds to the segmentation nucleus of the ovum (copk). From it there then arises by division a new macronucleus and a new micronucleus, not usually directly, however, that is, not by a single division, but through several successive nuclear divisions, into the meaning of which I cannot here enter. Immediately after the union of the two sex-nuclei the two animals sever their connexion with each other; each begins again to feed, and is subject to multiplication by division just as it was before conjugation took place (8 and 9).
Fig. 85. Diagram of the conjugation of an Infusorian, Paramœcium, after R. Hertwig and Maupas. 1, two animals with the mouth-openings apposed; ma, the macronucleus beginning to degenerate; mi1, the micronucleus has already increased considerably in size and is beginning to divide. 2. each micronucleus has divided into two daughter-nuclei (mi2), which are connected only by the division-strand (ts). 3, to the left each of the daughter-micronuclei (mi2) is beginning to divide; to the right this division is already completed and the grand-daughter-nuclei of the original micronucleus hang together by their division-strands (ts). 4, in each of the animals there are now four grand-daughter-micronuclei (mi3). 5, three of these are in process of dissolution, the fourth is dividing into two great-grand-daughter-nuclei (mi4), which are the two sex-nuclei. 6, one (the male) sex-nucleus (mi ♂) migrates into the other animal, and there unites with the remaining (female) sex-nucleus. 7, the conjugation-nucleus (copk) being formed. 8, the animals have separated; the conjugation-nucleus divides into (9) the new macronucleus (n ma) and the new micronucleus (n mi).
Although the course of this remarkable process exhibits all manner of differences in detail in different species, it is everywhere the same in its essential feature, and this essential feature is undoubtedly the union of an equal quantity of the nuclear substance of two animals to form a new nucleus. It is thus essentially the same process which we have already recognized among higher animals as 'fertilization.' The differences are of minor importance, and they arise partly from the fact that the sex-cells of multicellular animals are not independent self-supporting units, and partly from their differentiation into 'male' and 'female' cells. The minuteness of the sperm-cell, for instance, conditions its penetration of the ovum, which is always much larger and passive, and also the thorough fusion of its cell-body with the cell-body of the ovum. That this difference has very little deep significance is best seen from the fact that, even among Infusorians, there are forms in which the two conjugating individuals are quite different, especially in size, and in which the much smaller 'male' animal fuses completely with the much larger 'female,' and indeed bores its way into it after the manner of a sperm-cell. This is the case among the bell-animalcules (Vorticellinæ) ([Fig. 86]), the conjugating pairs of which had been observed long before our present insight into these processes had been attained. Indeed, the facts had been interpreted as a kind of 'budding process,' the minute and differently shaped 'male' animal (mi), which at the time of conjugation is attached to the larger 'female' (ma), was regarded as its bud. This supposed bud, however, does not grow out from the animal, but into it!
Thus we see here again that a differentiation of individuals as males and females may occur among unicellular organisms, just as in the sex-cells of higher animals and plants, and this proves to us once more that all these differences of sex, whether in reproductive cells of multicellular organisms, or in the entire multicellular animal or plant, or finally, in unicellular organisms, are not of essential, but only of secondary significance, however important they may be for securing fertilization or conjugation in each special case. They are always only adaptations to the special conditions, and only occur where they are necessary to ensure the union, and always in such a manner that the union of the two cells is facilitated. In most Infusorians such a differentiation into male and female animals was not necessary, because these organisms are very motile, and are thus readily able to meet and unite; it was therefore sufficient for them to remain hermaphrodite. The bell-animalcules, however, are sedentary, and for them it was obviously an advantage that, at the time of conjugation, smaller, free-swimming, and also more simply organized individuals should arise, which were able to seek out the larger sedentary forms. Here, then, as in many other unicellular animals, these little male individuals only occur when they are necessary, that is, at the time of conjugation. Similarly, in the green alga, Volvox, male and female cells arise only at the time of conjugation, reproduction being at other times effected by means of parthenogonidia, that is, by elements which require no fertilization.
Fig. 86. Conjugation of an Infusorian. Vorticella nebulifera, showing sexual differentiation of the whole organism. After Greef. I, the 'microgonidium' or male individual (mi) attaches itself to the 'macrogonidium' or female individual (ma); cv, contractile vacuole; st, contractile stalk. II, the ciliated circle on the male individual has disappeared. The male has become firmly embedded in the female by means of a sucker-like retraction of its lower end. III, the fusion of the two individuals has been completed; the bristly residue of the male (ct) is about to be thrown off; the stalk (st) is contracted into a spiral. Magnified about 300 times.