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.
[Illustration: 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.]
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).
[Illustration: 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;
_mi_^1, the micronucleus has already increased considerably in
size and is beginning to divide. 2. each micronucleus has divided
into two daughter-nuclei (_mi_^2), which are connected only by the
division-strand (_ts_). 3, to the left each of the daughter-micronuclei
(_mi_^2) 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 (_mi_^3). 5,
three of these are in process of dissolution, the fourth is dividing
into two great-grand-daughter-nuclei (_mi_^4), 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.
[Illustration: 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.]
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.
It was by chance that human observation brought the latter fact to
light first, and therefore led us for so long to accept the idea that
_fertilization_, that is, amphimixis, and _development_, that is,
reproduction, are one and the same; and thus it happens that even now
there are many naturalists who cannot rid themselves of the idea that
amphimixis, if not a life-awakening, is at least a _life-renewing_
process, a so-called 'process of rejuvenescence.'
More than ten years ago[14] I disputed this view, and since then
the facts which make it untenable have become more and more clear.
Notwithstanding this I see that it is still adhered to, at least in a
modified form, by many esteemed naturalists, and so it does not seem
superfluous to discuss it in more detail.
[14] _Die Bedeutung der sexuellen Fortpflanzung für die
Selektionstheorie_, Jena, 1886.
I have already noted that we see in conjugation an amphimixis without
reproduction, and in spores and parthenogenetic ova reproduction
without amphimixis, and I do not doubt that every unprejudiced critic
will admit this; many among us, however, are not unprejudiced, but are
under the spell of earlier ideas, so that they cannot forget that it
was long believed that fertilization was an indispensable condition
of development; they therefore regard the divisions which recommence
sooner or later after conjugation, and which may be repeated hundreds
of times, _as conditioned by the conjugation which preceded them_,
and compare them to the series of cells which, in the Metazoa, lead
from the fertilized ovum to the fully-formed animal. They regard both
series of cell-generations as a developmental cycle, which leads from
fertilization to fertilization again, from conjugation to conjugation,
and which would be impossible without either fertilization or
conjugation.
This play with the idea of a 'cycle' reminds me vividly of similar
fantastic play from the time of the much-despised 'Naturphilosophie' of
a hundred years ago. As men sought to find the analogues of 'solar' and
'planetary' systems in animal and plant, and believed they had stated
something when they compared the motile animals to planets and the
sedentary plants to the sun (!), so it is now imagined that a deeper
insight has been gained by the recognition of cycles of development.
By all means let us regard the development of a multicellular organism
as cyclic; it returns again to its starting-point, but this no more
explains the forces which produce the cycle, and thus the meaning of
fertilization, than a comparison with the circling planets explains
the causes of locomotion in animals. With quite as much reason the
cycle of development might be made to start from the parthenogenetic
ovum, and then the whole conclusion of the fanciful cycle idea in
regard to the meaning of fertilization falls to the ground, for in
this case the cycle begins without fertilization. Attempts are made to
get over this difficulty by showing that in many cases parthenogenesis
alternates regularly or irregularly with sexual reproduction, as in
the water-fleas (Daphnids), the Aphides, and so on. The mysterious
rejuvenating power of amphimixis is supposed to suffice for several
generations, a purely gratuitous assumption, which is also in open
contradiction to the facts. For there are species which now reproduce
exclusively by parthenogenesis, among plants for instance, a number
of fungi, among animals a few species of Crustaceans. Of the latter
it can be demonstrated that ages ago they reproduced sexually, for
they still possess the sac which serves for receiving spermatozoa, but
this sac remains empty, for there are now no males, at least in any
habitat of the species known to us. To this set belongs an inhabitant
of stagnant water, _Limnadia hermanni_, a species of Crustacean which
was found thirty years ago in hundreds, all of the female sex, near
Strassburg, and also many of the little Ostracods (_Cypris_) which
inhabit especially the muddy bottom of our pools and marshes. I bred
one of these (_Cypris reptans_) in numerous aquaria for sixteen years,
during which there were about eighty generations, and throughout this
time no male ever appeared, nor did the sperm-sac of the female ever
contain spermatozoa. The after-effects of the 'rejuvenating' power of
an amphimixis supposed to have taken place earlier must in this case
have been enduring indeed!
For these reasons it seems to me useless to make comparisons between
the developmental cycle of unicellular organisms and the ontogeny of
multicellular organisms. Both processes have indeed many points of
resemblance--long series of cells, then interruption of the divisions
and the occurrence of amphimixis--so that we may quite well speak of
cyclic development in the physiological sense, in as far as certain
internal conditions periodically recur and compel the organism to
conjugation, but we must not suppose that there is more in this than,
for instance, in the 'cyclic development' of Man, which consists in
the fact that he finds himself periodically impelled to take food. The
feeling of hunger which forces him to do so is the signal which warns
the organism that it is time to supply fresh combustible material to
the metabolism. In the same way, after a long series of generations
of Infusorians the necessity for conjugation arises; the whole colony
suffers an 'epidemic of conjugation,' and the animals unite in pairs;
in the meantime we know not why, and must content ourselves with
formulating what is observable, that _the nuclear substances of two
individuals are thereby mingled in each conjugate_.
Obviously the impulse to conjugation is a signal in the same sense as
the feeling of hunger is, and we know well from the higher animals what
a mighty influence it exerts, an influence hardly less potent than that
of hunger. In Schiller's words, 'Durch Hunger und durch Liebe, erhält
sich dies Weltgetriebe.'
We can see clearly enough why Nature should have given animals the
feeling of hunger, but the reason for the need of conjugation is not so
plain; we can only say in the meantime that it must be of some value in
maintaining the forms of life, for only that which fulfils a purpose
can be permanently established.