changes and influences may determine whether the ripe ovum shall go on
to embryonic development or not, and that these influences, may be very
diverse in nature in different cases. I shall return later to these
important facts.
When we sum up the facts we have cited with reference to the reduction
of the number of chromosomes, it appears that nature is, as it were,
striving to keep the number constant for each species; that in
germ-cells which are destined for amphimixis they are reduced to half
the normal number, but that this halving of the number is suppressed
where fertilization is always absent, or that the reduction to half is
compensated for again in various ways, whether by subsequent fusion
of the two daughter-nuclei, which have arisen from the process of
reduction, or by an independent duplicating of the chromosomes in the
segmentation nucleus.
We might perhaps be inclined to conclude from all this that the
occurrence of development depended on the presence of the normal
number of chromosomes; and I used to regard this as possible. But
facts which have been more recently brought to light have excluded
this view. Above all, we now know that every nuclear division depends
on the presence of a dividing apparatus, a centrosphere, but that this
organ degenerates in the ova of most animals and is completely lost
after the second polar division has been effected. The mature ovum is
therefore in itself incapable of entering on its embryonic development,
no matter how many chromosomes its nucleus contains; it is only capable
of further division when the fertilizing sperm-cell brings with it
its dividing apparatus or centrosphere. In thread-like sperms this
lies in the median portion (Fig. 68_C_), and after the tail-piece has
been dissolved, which happens soon after the sperm enters the egg, the
central corpuscle, at first very small, can be recognized in front of
the sperm-nucleus, where it is soon transformed into an 'aster' and
divides into two. Then both spheres move apart (Fig. 75_D_, p. 296) and
form the nuclear spindle between them by the confluence of their rays.
From this the division of the ovum into the two first embryonic cells
proceeds. The two pronuclei in the ovum, the male and the female, are
thus exactly alike as to number of the chromosomes, and frequently
at least as to size and appearance (Fig. 75_C_). But they differ
in the possession or absence of a dividing apparatus, and in the
great majority of cases it is the male nucleus that brings with it
the central corpuscle which seems to be indispensable to embryonic
development (_B_, _cspt_). Hitherto, at least, only two exceptions to
this are known. In the little segmented worm, _Myzostoma_, which is
parasitic on sea-lilies or Crinoids, Wheeler observed that the ovum
retained its central corpuscle even after the polar divisions, while
the sperm-cell which penetrated into the egg had none. More recently
Conklin made the interesting discovery that in the egg of a marine
Gasteropod (_Crepidula_) both the egg-nucleus and the sperm-nucleus
retain their centrosphere and together form the segmentation spindle,
one lying at one pole and the other at the opposite.
All these observations confirm the view that the sperm and the
egg-cell are alike in this respect also. Each of them can, in certain
circumstances, bring with it the dividing apparatus indispensable to
development, though it is usually the sperm-cell that does so.
I should indeed assume that the sperm-cell and the egg-cell were
essentially alike, even although there were no exception to this
rule, that is, although the centrosome of the ovum perished in
all eggs which were fertilized. For this is obviously a secondary
arrangement, an adaptation to fertilization, that the ovum should be
incapable of development without fertilization, and it is made so by
the disappearance of its centrosome. In all other cells, as far as is
known, the central corpuscle persists after division, so that this
remarkable cell-organ is transmitted from cell to cell just like the
nucleus, and like it, never rises _de novo_. It is only in the egg-cell
that it disappears, though even there often very late, for it may be
present, as an aster, even after the sperm has penetrated into the
ovum and disclosed its own central body, or even brought it the length
of dividing into two (Fig. 80, _A_ and _B_). But the ovum-centrosome
disappears as soon as the second polar division is accomplished.
That this disappearance is really a secondary arrangement, which may be
again departed from, is proved by the case of those eggs which are able
to develop parthenogenetically, for in them the central body does not
disappear, but persists in the ovum after the first polar division, as
Brauer showed in _Artemia_. It then behaves exactly like the sphere of
the sperm-nucleus in the fertilized ovum, that is, it duplicates itself
and forms the segmentation spindle.
[Illustration: FIG. 80. Fertilization of the ovum of a Gasteropod
(_Physa_), after Kostanecki and Wierzejski. _A_, the whole spermatozoon
lies in the ovum. _sp_, its already divided centrosphere. _Rk 1_,
the first polar body. _Rsp 2_, the second directive spindle. _B_,
_spk_, the sperm-nucleus, the second directive spindle still has its
centrosphere, which afterwards disappears. The first polar body (_Rk
1_) has divided into two. Highly magnified.]
Thus the beginning of embryonic development in the ovum depends not on
a definite number of chromosomes, but on the presence of an apparatus
for division. Upon what the awakening of this to activity just at that
time depends cannot as yet be exactly stated; we can only indicate
that all parts of the cell have interrelations with each other, and
that, therefore, the division mechanism is dependent on the condition
of the rest of the cell-parts at the moment, and on the substances
which they contain or produce. From what we know experimentally in
regard to artificial parthenogenesis it is not difficult to imagine
that some sort of chemical substances are necessary to stimulate the
central corpuscle to activity. In any case, the whole nutrition of
the central corpuscle depends on the cell in which it lies, as is
shown by the fact that the sperm-nucleus, whose centrosome before
the entrance of the sperm into the ovum was inactive and scarcely
recognizable, grows rapidly after entrance and forms a large aster
round itself--is, in short, in the highest degree active (Fig. 80). As
the chromosomes certainly play an important part in the life of the
cell, and materially help to determine its various phases, it cannot
be disputed that they also may share in awakening the activity of the
central corpuscle. But this influence is only indirect; it is not the
mere number of chromosomes that decides whether the central corpuscle
is to become active or remain inactive. This cannot be assumed, because
we have in the maturation divisions a proof that division may take
place with a double number of chromosomes as well as with the undoubled
number; while in the divisions of the mother-egg-cells and the
mother-sperm-cells we have proof that a doubled number of chromosomes
does not in itself compel to division.
The exceptional and artificially produced cases of parthenogenesis
which we have discussed above are probably to be interpreted thus:
through slight differences in the constitution of the ovum, or through
certain mechanical or chemical stimuli, the metabolic processes in
the ovum are so altered that the centrosome of the ovum, instead of
breaking up, is stimulated to growth, and thus produces the active
dividing apparatus which is otherwise only brought into it by the
sperm. This is a more exact definition of the interpretation I gave
earlier (1891) of the 'chance' parthenogenesis of the silk-moth, which
was then the only case known, when I said 'the nucleoplasm of some ova
must possess the power of growth in a greater degree than the majority.'
But we are not yet in a position to go further, or to define more
exactly the nature of the processes of metabolism which are involved.
LECTURE XVI
FERTILIZATION IN PLANTS AND UNICELLULAR ORGANISMS, AND ITS IMMEDIATE
SIGNIFICANCE
Fertilization in a lichen, Basidiobolus--In Phanerogams--Here too
there is reduction of the number of chromosomes by a half--'Polar
cells' in lower and higher plants--Conjugation among unicellular
organisms--Noctiluca--The maternal and paternal chromosomes
remain apart--Actinophrys--Infusoria--Sexual differentiation
of the two conjugates in Vorticella--Importance of the process
of Amphimixis--Not a 'life-awakening' process--May occur
independently of multiplication--The Rejuvenescence hypothesis--Pure
parthenogenesis--The cycle idea--Does Amphimixis prevent natural
death?--Maupas' experiments with Infusorians--Bütschli's
view--Potential immortality of unicellular organisms--The
immortality of unicellular organisms and of the germ-cells depends
on the fact that there is no time-limit to the multiplication
of the smallest living particles--Parthenogenesis is not
self-fertilization--Petrunkewitsch's observations on the ova of
bees--Is the chromatin really the 'hereditary substance'?--Nägeli's
conclusion from the difference in size between ovum and
spermatozoon--Artificial division of Infusorians--Boveri's
experiments with the fertilization of pieces of ova not containing
a nucleus--Fertilization gives an impulse to development even to
non-nucleated pieces of ova--Merogony--The female and male nuclear
substances are essentially alike--Summary.
I NOW turn to the consideration of the process of fertilization in
plants and unicellular organisms.
With regard to plants, it can now be definitely asserted that in them,
too, fertilization is essentially a conjugation of nuclei; it depends
on the union of the nuclei of the two 'sex-cells.' These sex-cells are
usually very small among lower plants, indeed up to the phanerogams;
this is especially true of the zoosperm-like male germ-cells, but it
usually holds also true of the ovum, which is but seldom burdened
with an abundant supply of yolk. In spite of the many difficulties
which this smallness of size puts in the way of observation, the
untiring exertions of a host of excellent investigators have succeeded
in following the process of fertilization in all the larger groups
of plants--in algæ, fungi, mosses, ferns, and horse-tails among
cryptogams, and in phanerogams.
I shall first give an example from among the lower plants (Fig. 81).
In one of the lichens, _Basidiobolus ranarum_, each of two adjacent
cells in the fungus-thread gives off a bill-like process, and the two
processes become closely apposed (Fig. 81, _a_). The nucleus of each
cell moves into the bill-shaped process, is there transformed into a
nuclear spindle (_B_, _ksp_) and divides, so that one daughter-nucleus
comes to lie in the apex point of the bill, the other at the base.
The cell-body also divides, though very unequally, and the final
outcome of the process is two cells in each, of which one is small
and occupies the apex of the bill, while the other is large and fills
all the rest of the cell-space. The former do not play any further
part of importance, but break up, the latter are the sex-cells, the
cytoplasm of which now coalesces through a gap in the cell-walls, while
their nuclei become closely apposed and ultimately unite (_C_, ♂ and
♀ _k_). From this union arises the fertilized spore, the so-called
'zygote' (_D_). The two small abortive cells so greatly resemble in
their origin the polar cells of the animal ovum that it is difficult to
resist the supposition that they bring about a reduction in the number
of chromosomes. But the number of the chromosomes has not yet been
determined either in them or in the sex-nuclei.
[Illustration: FIG. 81. Formation of polar bodies in a lichen,
_Basidiobolus ranarum_. _A_, the two conjugating cells with the
bill-like processes in which the nuclei lie. _B_, the nuclei dividing.
_ksp_, the nuclear spindle. _C_, after the division into a polar body
(_rk_) and a sex-nucleus (♂ _k_ and ♀ _k_). _D_, after the union of
the nuclei to form a conjugation nucleus (_copk_); the fertilized ovum
is surrounded by envelopes and modified into a lasting spore. After
Fairchild.]
We have come to know the processes of fertilization among phanerogams
chiefly through Strasburger, Guignard, and more recently through the
Japanese botanist Hirase. The agreement with the animal process is
surprisingly great, notwithstanding the notable differences in the
external conditions of fertilization.
As is well known, the male cells in the highest flowering plants are
not zoosperms but roundish cells, each of which, enclosed, together
with a sister-cell--the so-called 'vegetative' cell--in a thick
cellulose capsule constitutes a pollen-grain. The pollen-grains reach
the stigma, under which, buried deep within the 'ovule,' the female
sex-cell rests, enclosed in a long, sac-like structure called the
'embryo-sac' (Fig. 82, _A_). Beside it (_eiz_) there lie several other
cells, usually seven in number, two of which, the so-called 'synergidæ'
(_sy_), have their place at one end of the embryo-sac, just in front of
the ovum (_eiz_). Probably these give off a secretion which exercises
an attractive (chemotactic) influence on the male fertilizing body
('the pollen-tube'), and thus, so to speak, show it the way to the ovum.
[Illustration: FIG. 82. Fertilization in the Lily, _Lilium martagon_,
after Guignard. _A_, the embryo-sac before fertilization; _sy_,
synergidæ; _eiz_, ovum; _op_ and _up_, upper and lower 'polar nuclei';
_ap_, antipodal cells. _B_, the upper part of the embryo-sac,
into which the pollen-tube (_pschl_) has penetrated with the male
sex-nucleus (♂_k_) and its centrosphere; below that is the ovum
with its (also doubled) centrosphere (_csph_). _C_, remains of the
pollen-tube (_pschl_); the two sex-nuclei are closely apposed. Highly
magnified.]
When a pollen-grain has reached the stigma it sends out a tube, usually
after a few hours, which penetrates into the soft tissue of the
style, and grows deep down into the interior of the ovule, ultimately
penetrating as far as the embryo-sac through a special little opening
in the covering of the ovule, the so-called 'micropyle' (Fig. 82_B_,
_pschl_). Its blunt end is now closely apposed to this, so that the
true sperm-nucleus (_B_, ♂_k_), surrounded by some protoplasm, can
leave the pollen-tube and wander in among the cells of the embryo-sac.
Later on we shall see that two generative nuclei migrate from the
pollen-tube, but in the meantime we shall devote our attention only to
one of them, the fertilizing nucleus, which immediately moves towards
the ovum-nucleus and apposes itself closely to it. Then follows the
fusion or conjugation of the two nuclei, which are alike in size and
appearance, just as in the fertilization of the animal ovum (_C_, ♂ _k_
and ♀ _k_). Whether in this case, too, the sperm-nucleus brings with it
a central corpuscle, or whether, as Guignard believed he observed, the
ovum retains its central corpuscle (_C_, _csph_), or finally, whether
both modes occur, is not yet known with certainty. The fact that, as
a rule, seeds capable of reproduction only form in an ovule when the
stigma has been previously dusted with pollen, leads us to suppose
that, in this case, as among animals, the ovum lacks something that is
necessary to induce embryonic development, only retaining this power
in very exceptional cases, namely, when adapted for parthenogenesis.
And this something may very well be the dividing apparatus of the cell,
the centrosome with the centrosphere. But whether this supposition
prove correct or not, a nuclear spindle always forms simultaneously
with the fusion of the two sex-nuclei into a segmentation nucleus, and
this spindle is the starting-point of the young plant, thus exactly
corresponding to the first segmentation of the animal ovum. It agrees
with it also in the important respect that it again contains the full
number of chromosomes--twenty-four in the lily--while the two nuclei,
male and female, only exhibit half the number each, that is, twelve.
Thus a reduction in the number of chromosomes to half takes place in
plants also, but it is not yet known with certainty whether this is
brought about in the same way as among animals, namely, by reducing
divisions. Without entering more fully into this still unsolved and
very complex problem, I should like to state that I consider this very
probable; indeed, I agree with the view of V. Häcker[13], that the
reducing divisions of plants are only more difficult to recognize as
such, and, furthermore, are often disguised by the fact that they often
occur alongside of, or between divisions which are not reducing. If it
were possible to reduce the number of chromosomes in a cell to half
without the aid of cell-division, if, for instance, only half were to
integrate again from the chromatin-network, this must have been quite
as possible in the case of animal cells, and then, moreover, the single
chromosome would not have had the significance of an individuality,
and no special form of nuclear division would have been introduced to
reduce their number. That it has been introduced seems to me to prove
that it was necessary, and since it was so among animals, it could not
have been dispensed with among plants either.
[13] See V. Häcker, _Praxis und Theorie der Zellen- und
Befruchtungslehre_, Jena, 1899, pp. 144-5.
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.
[Illustration: 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.