Part 36

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.

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.

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, _mi_^4). 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).

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.

As these differences are only adaptations to the necessity that the animals or cells shall find each other and unite, so also are all the other differences of a sexual kind, the thousand-fold differences between the sperm-cell and the egg-cell, and the not less numerous differences between male and female animals, both in 'primary' and especially in the diverse 'secondary' sexual characters which we have previously discussed; all these are only means for bringing about the process of the union of two germ-cells to form a fertilized 'ovum' which is capable of development. The essential part of this so-called 'sexual reproduction' does not, however, depend on these differences, neither on the sexual differences of the germ-cells nor on those of the whole organism; it lies solely in the actual union of the two germ-cells. Let us remember the idea we have already emphasized, that the _essential part_ of the so-called 'sexual reproduction' does not depend on these differences, and let us hold fast to the idea already indicated, that the chromosomes of the nucleus are the real bearers of the hereditary tendencies; then we see that the mingling, or, better, the union of the hereditary substances of two different individuals, whether single-celled or many-celled, is the result of the process which we have hitherto called fertilization or conjugation, but which we shall henceforward designate by the more general term 'Amphimixis' which means the mingling of substances contributed from two distinct sources.

Having made ourselves acquainted with the phenomena of amphimixis in animals, plants, and unicellular organisms, we have to face the problem of the significance of this remarkable and complicated process. What is it that happens, and what meaning can we attach to it?

The first thing to be done is to show that the old and long-prevailing conception of fertilization as _a life-awakening process_ must be entirely abandoned. That a new individual can arise even among highly organized animals, quite independently of fertilization, is proved by the parthenogenetic eggs of insects and crustaceans; fertilization is not the spark 'which falls into the powder-cask' and causes the explosion; it is only an indispensable condition of development. As we have seen, there are germ-cells which are not sexually differentiated, such as the spores of the lower plants, which are all capable of development without amphimixis; and parthenogenetic ova prove that even differentiated female germ-cells, that is, germ-cells originally adapted for amphimixis, may in certain circumstances develop without it; amphimixis is thus not the fundamental cause of development, but is only, for many germ-cells, one of the conditions which must be fulfilled before development can set in. It is a condition which, under certain circumstances, may be dispensed with.

If, then, the multiplication of individuals by single-celled germs can take place independently of amphimixis, we may conclude that the establishment of amphimixis has nothing to do with the capacity for multiplication, that it is not a life-awakening process, but is a process of a unique kind, which means something quite different. The whole conception of the awakening of life in the germ is antiquated and out of harmony with the present state of our knowledge. _Life never begins anew_, as far as we can see, and apart from the possibility that, unknown to us, a spontaneous generation (_Urzeugung_) of the lowest forms of life is still taking place, life is continuous and consists of an infinite series of living forms between which there is no real interruption. Life, in fact, is like a continuous stream, the larger and smaller waves of which are particular species and individuals. Only a few decennia ago a morphologist, who was rightly held in high esteem, could champion the idea that the mature ovum of animals was lifeless material, which had to be quickened in order to develop, but now such a theory is untenable, since we have become aware of the phenomena of maturation in the ovum, and know that most important vital processes, the reducing divisions, take place at the time of maturation, quite independently of fertilization.

Thus we do not even require to take into account the conjugation of unicellular organisms to make it clear that amphimixis is not the cause of the origin of new individuals, but a process, _sui generis_, which may indeed be associated with the beginning of embryonic development, but which may also occur independently of it, as we see in the case of unicellular organisms. If, on the one hand, we see development taking place in spores and parthenogenetic ova independently of amphimixis, and on the other hand amphimixis occurring without reproduction in unicellular organisms, we must regard the two phenomena, amphimixis and reproduction, as processes of a distinct kind, which may, however, occur in association with and interdependence upon each other.