preliminary stages of amphimixis is another matter. If we take a survey
of the observations that have been made up till now, we are confronted
at first by the undoubtedly striking fact that very little is known
about it as yet, for in fact the whole process is gone through even
in quite lowly forms of life in a manner very similar to that in the
higher forms. Amphimixis has been shown to be widespread even among
unicellular organisms, yet not in an _essentially_ simpler mode than
among multicellulars. We have seen that even in ciliated Infusorians
reducing division obtains, and that of the four nuclei which arise
from twofold division of the original nucleus three break up again,
and only the fourth, by a further division, separates into a male and
a female pronucleus, 'which then complete the amphimixis with the
corresponding pro-nuclei of another animal' (compare Fig. 85, 4-7, vol.
i. p. 321). This, and the existence of a dividing apparatus and of
chromosomes, make the process appear very little less complicated than
the fertilization of higher animals. The case is similar even in much
lower unicellular organisms, such as _Noctiluca_ (Fig. 83, vol. i. p.
317). In this form and in Rhizopods it is true that reducing divisions
have not yet been made out, but their occurrence in the lower Algæ
(_Basidiobolus_), and above all in those simple unicellular organisms
which give rise to malaria, and their allies, which live as 'Coccidia'
in the blood-cells and intestinal cells of animals, leads us to expect
that they may prove to be of general occurrence among unicellulars.
In the Coccidia, which are extremely simple unicellular organisms,
equipped, however, with a nucleus, the adaptations relating to
amphimixis are more extensive and more complex than in the Rhizopods.
For while in the latter the two conjugating cells are absolutely alike
in external appearance, in the former the male cell is distinct from
the female, and indeed the differences are as marked as those that
usually occur in multicellular animals.
[Illustration: FIG. 121. Life-cycle of _Coccidium lithobii_, a
cell-parasite of the centipede _Lithobius_; after Schaudinn. 1, a
'sporozoite'; 2, the same penetrating into an intestinal epithelial
cell; 3, the same growing into a 'schizont' capable of division; 4, the
same dividing, and 5, breaking up into numerous pieces which separate
from the 'residual body' in the centre, and either, as in 1, migrate
into epithelial cells and repeat the history, or pass on to the phase
of sexual reproduction. In the latter case, after eliminating a portion
of the nucleus (reduction) in 6 and 6 _a_, they form the 'macrogamete'
(the ovum); or within the mother-cell they produce microgametes (or
sperm-cells), 7 and 7 _a_. The penetration of a sperm-cell into an
egg-cell (amphimixis) is shown in 8, the fertilized egg-cell (9)
becomes the so-called oocyst or permanent spore, from which by repeated
division (10 and 11), new sporozoites, as in 1, arise, and begin the
cycle afresh.]
We owe our present knowledge of these processes especially to Schuberg,
Schaudinn, and Siedlecki, and, because of their theoretical importance,
I should like to summarize the essential points.
One of these Coccidia lives in the intestinal cells of a small
centipede, _Lithobius_; in Fig. 121 the parasite is shown as a
so-called 'Sporozoite,' that is, as a minute sickle-shaped cell,
which at first moves freely about the intestine of the host (1), but
then soon penetrates into an epithelial cell (2). There it grows to
a spherical shape (3), and then, after having devoured the cell,
it gives rise, by a peculiar process of division (Schizogony), to
a number of very minute nucleated pieces, again sickle-shaped, the
Schizonts, each of which bores its way into an epithelial cell as in
2, and follows the same path of development, so that a large number of
cells in the intestine of the same host are attacked in this manner.
But there is still another mode of reproduction, with which amphimixis
is associated, which leads directly to the formation of 'lasting'
germs which are enclosed in a capsule or cyst, reach the exterior with
the excrement of the host, and thus spread the infection to other
centipedes. The Schizonts which take this course develop into so-called
macro-gametes and microgametes, the former being the female, the latter
the male germ-cells. Then follows the penetration of a male gamete,
actively motile because of its two flagella, into the female gamete
(8). Amphimixis is accomplished, and the product of the fusion of the
two sex-cells (9) surrounds itself with a thinner cyst, within which
it multiplies by twofold division into four cells (10). These are the
'lasting' spores, which may dry up within the voided excrement of the
centipede (11), and if they be eaten by another animal of the species,
they infect it, for the sporozoites which have been formed by the
previous divisions creep out, and in form 1 begin the life-history anew.
We have thus an alternation of four generations which are all
unicellular, and of which one series (1-5) shows multiplication by
fission, while the other (6-11) includes, besides multiplication
by fission and as a condition of this, the process of amphimixis.
Amphimixis _must_ occur in order that the formation of 'lasting' spores
and new sporozoites may result. We have thus a regular alternation
of 'asexual' and 'sexual' reproduction, and the latter shows great
resemblance to that of multicellular organisms. The macrogamete
corresponds to the ovum, the microgametes to the spermatozoa, and they
resemble these also in their greater numbers and in their structure.
But the resemblance goes even further. The ovum is much larger than the
sperm-cell, and undergoes a kind of reduction of its nuclear substance;
shortly before fertilization the ovum-nucleus ('the germinal vesicle')
comes to the surface--just as in the case of animal ova--bursts, and
extrudes a part of its substance in the form of a sphere (Fig. 121, 6
and 7). A reduction of the nuclear substance in the male cell has not
been demonstrated in all cases, but in one of the _Lithobius_-Coccidia,
_Adelea ovata_, the relatively large microgamete (the sperm-cell, Fig.
122, _Mi_) places itself close to one pole of the female macrogamete
(the egg-cell) and then divides twice in succession, so that four small
cells arise (Fig. 122, _A-C_); of these only one penetrates into the
egg-cell (_D_, ♂_K_) and unites with it, the other three come to nought
(_D_, _Mi_). What a surprising resemblance this bears to the twofold
division of the mother sperm-cell in multicellular animals, through
which the number of chromosomes is reduced to half! In the conjugation
itself the thread-like chromosomes of the female nucleus are plainly
recognizable, while those of the male remain coiled up (Fig. 122, _D_).
That the nuclear substance can be separated into chromosomes (ids) even
in lowly unicellular organisms was probably first demonstrated by R.
Hertwig for _Actinosphærium_, a Heliozoon or freshwater sun-animalcule,
then by Lauterborn in regard to Diatoms, by Blochmann for an indigenous
Rhizopod, _Euglypha_, and by Ishikawa for the marine _Noctiluca_. Fresh
cases have been added in the last decade, so that we can now say that
a considerable number of unicellulars, from the ciliated Infusorians
and lower Algæ down to the Coccidia and Diatoms, exhibit a germ-plasm
composed of ids. These structures behave in the same way as those in
higher organisms, and Berger was able to demonstrate, in 1900, in the
case of a Radiolarian, their multiplication by spontaneous splitting.
[Illustration: 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.
[Illustration: 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.
Fusion of this kind was demonstrated in the Myxomycetes almost forty
years ago by De Bary, and it has been observed more recently in various
unicellular organisms, especially in Rhizopods and in Heliozoa.
These last often place themselves close together in pairs, threes,
or even more at a time, and then the delicate cell-bodies coalesce,
though no fusion of the nuclei takes place. With Hartog, we call this
process 'Plastogamy,' but we cannot agree with that observer when he
regards the importance of the process as consisting in the fact that
the nuclei thus come into contact with fresh cell-substance, after
having been surrounded for a very long time with the same cytoplasm.
If this were the import of amphimixis, then an _exchange of nuclei_
would take place, and this we find nowhere even among the lowest forms
of life, for everywhere there is a union of the nuclear substance of
two individuals. But this is by the way! Further cases of plastogamy
have been observed in many of the limy-shelled Rhizopods. A union of
this kind does not usually lead to any visible consequences, but in
some Foraminifera a group of young animals is developed within the
cell-bodies by the division of the nuclei and the cell-body; thus
multiplication follows the fusion just as in perfect amphimixis, and
we may therefore assume that there is a causal connexion between the
two. In the slime-fungi, too, the union of several amœba-like cells
into a multi-nucleated plasmodium is followed later by the development
of numerous encapsuled spores, but only after the plasmodium, which to
begin with is microscopically small, has grown to a macroscopically
visible, reticulated mass (_Æthalium_) sometimes a foot in extent. In
this case the fungus, creeping slowly over its foundation of decaying
substance, takes up nourishment from it, and it is not possible to
tell whether the union of the amœbæ yields any further advantage than
that of facilitating the spreading over large uneven surfaces, and
through this, later, the development of large fruit-bodies. But in the
case of the Foraminifera the plastogamy has obviously another effect,
unknown and mysterious, which as yet no one has ever been able to
define precisely. Words like 'stimulus to growth,' 'stimulating of
the metabolism,' and even 'rejuvenation,' give no insight into what
happens, but that something happens, that through the fusion of two or
more unicellulars a stimulus is exerted, which reveals itself later in
increased rapidity of growth, we may, and indeed must assume, because
this process has become a permanent arrangement in so many unicellular
organisms. Only what is useful survives, and the uniting individuals
must derive some advantage from the process of fusion, and it remains
to be seen whether we can find out with any clearness what this
advantage may be.
Till within a few decades ago it was believed that in this process
one individual devoured the other, but this can now no longer be
maintained. If any one still seriously considers this possible,
Schaudinn's observations would convince him of his error, for in
_Trichosphærium_, a marine, many-nucleated Rhizopod, he observed, on
the one hand, the union of two or more animals, i. e. plastogamy, and
on the other hand, the swallowing and digesting of a smaller member
of the species by a larger one--two processes which are absolutely
different, for in the first case the cell-bodies of the two animals
remain intact, while an animal that is eaten becomes surrounded by a
food-vacuole, and is dissolved and digested within it. In the former
case the vital units (biophors) of each animal obviously remain intact
and capable of function; in the second, those of the over-mastered
animal are at once dissolved and chemically broken up; as biophors,
therefore, they cease to exist. Whether one or the other process takes
place may perhaps depend on whether the two animals differ greatly in
size, so that the smaller can be quite surrounded by the larger.
In a former lecture I have emphatically expressed my dissent from the
view which interprets amphimixis as a process of rejuvenation, meaning
thereby a necessary renewal of life, and I need not go into this again
in detail: for that the metabolism can continue through uncounted
generations without being artificially stimulated--that is, in any
other way than by nutrition--is proved by all those lowly organisms
which exhibit neither plastogamy nor complete amphimixis, and also by
the occurrence of purely parthenogenetic reproduction, &c. In what,
then, can the advantage lie which the conjugating unicellular organisms
derive from conjugation? Obviously not in that they impart to each
other what each already possessed, but only in the communication of
something special and individual, something that was peculiar to each,
and becomes common to both.
Haberlandt believed that the development of auxo-spores in Diatoms
pointed towards the processes which form the deepest roots of
amphimixis. As is well known, the hard and unyielding flinty shell
of these lowly Algæ involves a diminution of the organism at every
division, so that the Diatoms become smaller and smaller as they go
on multiplying, and if that went on without limit they would come
rapidly to extinction. But a corrective is supplied in the periodical
occurrence of conjugation of two organisms which have already
materially diminished in size, and this is followed by the growth of
the two fused individuals to the original normal size of the species.
It is, of course, obvious that in this case the union of two organisms
which have become too small may be of advantage in bringing them back
to the requisite normal size; but this is an isolated special case,
which certainly does not justify our regarding conjugation as a means
whereby diminished bodily size may be brought back to its normal
proportions. By far the greater number of unicellular organisms are not
permanently diminished in size by division, and even in the Diatoms the
mass of the two fused individuals does not amount to the normal size of
the species, so that even in this case there must be growth subsequent
to the conjugation before the normal is re-attained. It may be doubted,
therefore, whether the increase in mass is, even in the case cited, the
essential event in conjugation, and whether there are not other effects
which we cannot clearly recognize. Here, too, there must be differences
between the two conjugating individuals, as we have just seen, for if
they only communicated something similar to each other, the result
would be an increase only in their mass, not in their qualities.
Although we cannot demonstrate differences of this kind in the case
of the lowly organisms with which we are now dealing, we may assume
their existence from analogy with the higher organisms. We know,
especially through G. Jäger, that in Man every individual has a
specific exhalation, his particular odour, and that in the secretions
of his glands there are incalculably minute differences in chemical
composition, which justify the conclusion that the living substance
of the secreting cells themselves exhibits such differences, and that
all the various kinds of cells in an individual are not absolutely
identical with the corresponding cells of another individual, but
that they are distinguished from them by minute yet constant chemical
differences. The assumption that differences of this kind exist even
in unicellulars, and in all lowly organisms generally, is not a merely
fanciful one, but has much probability.