a kind of self-fertilization, and these have been based on the
observations of Blochmann and Brauer, which showed that in the bee and
in the salt-water Crustacean, _Artemia salina_, the reducing second
maturation division of the ovum-nucleus is not suppressed, but is
regularly accomplished, and that the two daughter-nuclei which result
from this division unite with each other subsequently. I have already
noted that these statements do not hold true, at least with regard to
the bee. In this case the second maturing division takes place without
any subsequent fusion of the two daughter-nuclei. According to the
observations of Dr. Petrunkewitsch, which I have already mentioned, and
for the exactness of which I can vouch, the second maturation-spindle
is unusually long, so that the two daughter-nuclei are pushed very far
apart (Fig. 79, _Rsp 2_), and only the inner of the two nuclei (_K 4_)
becomes a segmentation nucleus, while the outer undergoes a remarkable
fate; it unites with the inner nucleus which results from the division
of the _first maturation cell_ (_K 2_), and from this union the
primitive _genital cells of the animal appear to arise_--an observation
the eventual theoretical importance of which can only be estimated
later.
Meantime all we can gain from it is a certain mistrust of the
interpretation of the processes of maturation in _Artemia_ which have
hitherto been given; at least we are tempted to suppose that the
copulation of two nuclei which Brauer observed in _Artemia_ may not
have led to the formation of the segmentation nucleus there either, but
may have had some other significance.
But, even if we leave this point entirely out of account, there
remain all the cases of regular parthenogenesis in which this mode of
reproduction occurs alone and not in alternation with the sexual mode.
In these only one maturing division is undergone, and only one polar
body is formed, and thus there can lie no possibility of supposing a
self-fertilization of the ovum.
[Illustration: FIG. 79. The two maturation divisions in the
unfertilized (drone-forming) egg of the bee, after Petrunkewitsch.
_Rsp 1_, first polar-body in division. _K 1_ and _K 2_, the two
daughter-nuclei thereof. _Rsp 2_, second directive spindle. _K 3_ and
_K 4_, the two daughter-nuclei thereof. In the subsequent stage _K
2_ and _K 3_ unite to form the primordial sex-cell nucleus. Highly
magnified.]
It is possible that we may yet discover species among unicellular
organisms which multiply without limit in the absence of any
amphimixis. R. Hertwig has recently observed phenomena in Infusorians
which he is inclined to refer to the suppression of an earlier habit of
conjugation, and so to a kind of parthenogenesis. But even if it should
be shown that amphimixis plays a part regularly and without exception
in the life of _all_ unicellular organisms, the facts in regard to
multicellular organisms are not affected; and, finally, the process of
amphimixis is one which we have not the slightest ground for assuming
to be either an awakener or a maintainer of life, and so I return
to the most essential part of the whole problem, the meaning of the
chromatin structures, the combination of which is the undoubted result
of amphimixis. Do they really represent, as we assumed earlier, _the
hereditary substance_, and what do we mean by this term?
As far as I know the literature and the development of biological
theories, the botanist Nägeli was the first to deduce, from the
considerable difference in size between the egg-cell and the
sperm-cell, the conclusion that the material basis on which the
hereditary tendencies depend must be a _minimal_ quantity of
substance. The difference is especially great in animals, even in
those species whose eggs may be called small, for instance, those of
sea-urchins or of mammals; even in these the mass of spermatozoon is
scarcely a thousandth part, often scarcely a hundred-thousandth part
of the mass of the ovum. And yet the inheritance from the father and
from the mother is equally great. Now as we know that vital powers
have always a material basis, a minute quantity, such as is contained,
for instance, in the spermatozoon of Man, must have implicitly in
it all the hereditary tendencies of the father; and the conclusion
is inevitable that in the ovum there can only be an equally minimal
quantity of substance which is the bearer of the hereditary powers, for
if there were a larger quantity of hereditary substance in the ovum its
power of transmission would also be greater[16].
[16] The improbable assumption that the hereditary substance of the
father may be in quality altogether different from that of the mother,
and so may have the same power of transmission, and yet take up much
less room, I leave out of the question altogether.
[Illustration: FIG. 69. Ovum of Sea-urchin (_Toxopneustes lividus_),
after E. B. Wilson, _zk_, cell-substance. _k_, nucleus (so-called
germinal vesicle). _n_, nucleolus (so-called germinal spot). Below
there is a spermatozoon of the same animal (_sp_), magnified in the
same proportion, about 750 times.]
[Illustration: FIG. 68. Diagram of a spermatozoon. After E. B. Wilson.
_sp_, apex. _n_, nucleus. _c_, centrosphere. _m_, middle portion. _ax_,
axial filament. _e_, terminal filament.]
If we inquire as to the part of the spermatozoon which bears this
hereditary substance, we may exclude both the tail-thread and the
middle piece (Fig. 68), the former because it obviously fulfils
quite a specialized physiological function and is histologically
adapted to this function, the latter because, from observation on the
spermatozoon which has made its way into the ovum, we know that it
contains the centrosome, the dividing apparatus of the nucleus. Thus
there only remains the 'head' of the spermatozoon, which includes the
nucleus, as the possible vehicle of the heritable substance. Therefore
we are led to seek for the hereditary substance in the nucleus. But
the hereditary substance cannot be a perishable substance which may
at need be dissolved, in the literal sense of the word, and be formed
anew; therefore we cannot look for it in the nuclear membrane, and just
as little in the 'nuclear sap' which fills the meshes of the nuclear
network, since the material on which heredity depends must necessarily
be solid. Nägeli has clearly shown that we must assume a stable,
that is, a solid molecular architecture. There thus remains only the
nuclear reticulum with its chromatin granules, and when we remember
what we have learnt of the behaviour of this chromatin substance during
division and amphimixis we can entertain no doubt that the sought-for
bearer of the inheritance is contained in the substance of the
chromosomes.
The great care with which the chromosomes are halved by means of the
complicated division apparatus led us earlier to regard them as a
substance of complex and manifold qualities and of great physiological
importance; their constant number in any one species, and the reduction
of that number to half by means of the reducing divisions, justify us
in concluding that they are permanent structures, physiological and
morphological units, which undergo no more than an apparent irregular
dispersion during the resting state of the nucleus. Finally, the fact
that these supposed vehicles of inheritance occur in equal numbers in
each of the conjugating germ-cells, and that this number is _always_,
both in animals and in plants, half of the normal number occurring in
somatic cells, is decisive. The logical necessity that the hereditary
substance of both parents should be transmitted to the offspring in
equal quantity could not be more precisely met than it is by the
fact that half the normal number of chromosomes occurs in each of
the sex-nuclei in the ovum. Personally, I have long been certain, on
these grounds, that the chromosomes of the nucleus are the hereditary
substance, and I expressed my conviction on this point almost
simultaneously with Strasburger and O. Hertwig[17].
[17] More precisely, my conclusions were published several months
later than those of the investigators named (1885). I think, however,
that no one who is familiar with my writings for the years immediately
preceding, which are collected in _Aufsätzen über Vererbung und
verwandte biologische Fragen_ (Jena, 1892), will dispute that the idea
was reached by me independently. I attach importance to this because
all my later work is based upon this idea.
But there is also a physiological proof of the meaning of the nuclear
substance; and this we owe, again, to the simultaneous and independent
researches of two investigators, M. Nussbaum and A. Gruber, the
latter working in the Zoological Institute here (in Freiburg), and
at my request. They made experiments on regeneration in unicellular
organisms, and found that Infusorians which were artificially divided
into two, three, or four pieces were able to build up a whole animal
out of each piece, provided that it contained a portion of the
nucleus (macronucleus). The large blue trumpet-animalcule, _Stentor
cœruleus_, is well suited for such experiments, not only on account of
its size, but because it possesses a very long rosary-like nucleus,
which can be easily cut two or three times. When a piece is cut off
which does not contain a portion of the nucleus, it may indeed live
for some days and swim about and contract, but it is incapable of
reconstructing the lost parts, and thus of forming a whole animal, and
it perishes. It is in the nucleus, therefore, that we have to look
for the substance which stamps the material of the cell-body with a
particular form and organization, namely, the form and organization
of its ancestors. But that is exactly the conception of a hereditary
substance or idioplasm (Nägeli). Some modern biologists deny that there
is any hereditary substance _per se_, and believe that the whole of
the germ-cell, cell-body and nucleus together effects transmission.
But though it must be admitted that the nucleus without the cell-body
cannot express inheritance any more than the cell-body without the
nucleus, this is dependent on the fact that the nucleus cannot live
without the cell-body; if it be removed from the cell and put, say,
into water, it bursts and is dissolved. But the cell-body without the
nucleus lives on, though of course only for a few hours or days, and
its metabolism ceases only when it is brought to a standstill by the
failure to replace by nutrition the used-up material. Thus the argument
used by those who deny the existence of a hereditary substance would be
paralleled if we denied that Man possesses a thinking substance, and
maintained that he thinks with his whole body, and even that the brain
cannot think by itself without the body.
I am convinced that it is just as mistaken to maintain that every
part of an organism must contain the hereditary tendencies in the
same degree, or that in unicellular organisms the cell-body is as
important in inheritance as the nucleus (Conklin). If one feels any
doubt on this point, one has only to call to mind Nägeli's inference,
from the minuteness of the spermatozoon, that the hereditary substance
must be minimal in quantity. But even theoretically there is not the
smallest ground for the assumption that the cell-body as well as the
nucleus contains the hereditary qualities, since we find in general
that functions are distributed among definite substances and parts of
the whole organism, and it is just on this division of labour that
the whole differentiation of the body depends. And why should this
principle not have been employed just here where the most important
of all functions is concerned? Why should all living substance be
hereditary substance? Although Nägeli thought of his 'idioplasm'
otherwise than we now think of hereditary substance, although he
wrongly imagined it in the form of strands running a parallel course
through the cell-substance and forming a connected reticulum throughout
the whole body, he recognized at least so much quite correctly,
that there are two great categories of living substance--hereditary
substance or idioplasm, and 'nutritive substance' or trophoplasm, and
that the former is much smaller in mass than the latter. We now add
to this, that the idioplasm must be sought for in the cell-nucleus,
and indeed in the chromatin granules of the nuclear network and of the
chromosomes.
But incontrovertible proof of the fact that the nuclear substance
_alone_ is the hereditary substance was furnished when it was found
possible to introduce into a non-nucleated piece of a mature ovum of
one species the nucleus of another related species, and when it was
seen that the larva that developed from the ovum so treated belonged
to the _second_ species. Boveri made this experiment with the ovum and
spermatozoon of two species of sea-urchin, and believed that he had
succeeded in getting from non-nucleated pieces of the ovum of the first
species, fertilized with the sperm of the second, larvæ of this second
species; but, unfortunately, later control-experiments made by several
investigators, especially by Seeliger, have shown that this result
cannot be regarded as quite certain and indubitable.
I must emphasize again that I am far from regarding the cell-protoplasm
of the ovum as an indifferent substance. It is certainly not only
important but indispensable for the development of the embryo, and it
has assuredly its own specific character, as in every other kind of
cell. It represents, so to speak, the matrix and nutritive environment
in which alone the hereditary substance can unfold its wonderful
powers; it has developed historically, like every other kind of cell,
but it contains nothing more than the inherited qualities of this one
kind of cell-protoplasm, not those of the other cells of the body.
But although the essence of fertilization lies, as we have seen, in
the union of the hereditary substance of two individuals, and not in
a 'quickening' of the ovum, we may quite well speak of a quickening by
fertilization in another sense, if we mean the impulse to embryonic
development, for this is really supplied by the entrance of the
sperm-nucleus with its centrosphere into the ovum. But even this
impulse can, under certain circumstances, be given in another way,
and certainly the awakening of it is not the _end_ of fertilization,
but only the condition without which the end, the union of two kinds
of nuclear substance, could not be attained. There is no indication
whatever that this 'quickening' of the ovum would be necessary for any
other reason except that _the ovum was previously made incapable of
development_. There would be no 'fertilization' were not the mingling
of hereditary substances of fundamental importance for the organic
world.
Moreover, an ovum, or a fragment of an ovum, may also develop of
itself, having only _one_ of the sex-nuclei, and the union of the
hereditary substance of two cells is therefore not indispensable for
the mere production of a new individual.
What has been observed in regard to fragments of ova is particularly
interesting in this connexion. Ernst Ziegler first succeeded in halving
a newly fertilized sea-urchin ovum, so that one half contained the
female and the other the male pronucleus. The latter alone contained
a centrosphere, and developed a blastula larva. Delage carried these
experiments further, and cut an unfertilized but mature sea-urchin
ovum into pieces, and then 'fertilized' the non-nucleated pieces with
spermatozoa. These pieces developed and yielded young larvæ of the
relevant species; so it is clearly seen that even a piece of mature
ovum-protoplasm may undergo embryonic development, provided that
a nucleus furnished with a dividing apparatus penetrates into it.
Unfortunately it is technically impossible to cut such a non-nucleated
and then fertilized fragment of ovum so that one half shall contain
the male nucleus the other its centrosphere. Even without this
_experimentum crucis_ we may say that the half with the male nucleus
would not multiply by division, and that the other probably would,
though it would not go through the regular course of segmentation
processes, because the hereditary substance absolutely necessary for
these was wanting.
But these and similar experiments prove something more, namely, that
the nuclei of the sperm-cell and egg-cell do not, as was formerly
believed, stand in a primary and essential contrast to each other,
which may be described as male and female, but that both are alike in
their deeper essence, and may replace each other. They only differ from
each other as far as the cells to which they belong differ, in this,
namely, that they are mutually attractive; they find each other and
unite, and then go on to develop, which each was previously unable to
do by itself. Widely as the sperm-cell and egg-cell differ in size,
constitution, and behaviour, in regard to essential character they are
alike; they bear the relation--as I expressed it twenty years ago--of
1:1; that is, _they both contain an equal quantity of essentially
similar hereditary substance_, and the quality of this substance is
only individually variable. We should, therefore, speak not of a 'male'
and 'female,' but of a 'paternal' and a 'maternal' nucleus.
All the more recent experiments on 'merogony,' that is, on the
development of fragments of the ovum, confirm this view. Thus Boveri
had already observed that even small pieces of sea-urchin ova which did
not contain the nucleus of the ovum developed, after the spermatozoon
had entered them, into small but otherwise normal larvæ of the species.
More recently Hans Winkler proved the same thing for the ova of plants,
by dividing the ovum of a marine alga (_Cystosira_) into two pieces,
then fertilizing these with water containing sperms, with the result
that he got from both pieces, the nucleated and the non-nucleated, an
embryo of normal appearance. In the latter it could only have been a
'paternal' nucleus which directed the development.
To sum up. Our investigation into the meaning of amphimixis has led
us to the conclusion that it consists in the union of two equal
complements of hereditary substance, contributed by two different
individuals, into one unified nucleus, and that the sole immediate
result of this is _the combination of the hereditary tendencies of two
individuals in one_. Among multicellular organisms this one individual
of dual origin always implies the beginning of a new life, since
amphimixis is indissolubly associated with reproduction, and even
among unicellular organisms it can hardly be disputed that the two
Infusorians which separate after conjugation are no longer the same
as they were before. After amphimixis they must contain a different
combination of hereditary substance from what they had before, and
this must reproduce the parts of the animal in a somewhat modified
form. This is theoretically beyond doubt, although it can scarcely be
established by observation.
We thus know now what 'fertilization' is. Through the labours of the
last decade the veil has been torn from a mystery of nature which for
thousands of years confronted humanity as unapproachable; a riddle has
been solved for the solution of which a few centuries ago men did not
even dare to hope. Not a few have taken part in these labours; some I
have already named, but it is impossible that I should here mention
all who have shared in the achievement by observation or reflection.
Whoever has helped it on even a single step may say to himself that
he has taken an active part in bringing about what must be called
essential progress in human knowledge.
But in the science of nature every new solution implies the cropping up
of a new riddle, and we are immediately confronted with the problem,
Why should nature, in the course of evolution, have interpolated this
process of the mingling of different hereditary substances almost
everywhere in the organic world? This, however, is a problem which we
cannot attack until we have first made ourselves more fully acquainted
with the phenomena of inheritance, and have attempted to reason back
from these to the nature of the hereditary substance. We must, in
short, think out a theory of heredity.
LECTURE XVII
THE GERM-PLASM THEORY
Conception of the 'id' deduced from the process of
fertilization--Hereditary substance, 'idioplasm' and
'germ-plasm'--'Idants'--Evolution or Epigenesis--Herbert Spencer's
uniform germinal substance--Determinants--Illustrations: _Lycæna
agestis_--The leaf-butterflies--Insect metamorphosis, limbs of
segmented animals--Heterotopia--The ultimate living units or
biophors--Number of determinants--Stridulating organ of the
grasshopper.
IN proceeding to expound the theory of heredity which has shaped itself
in my mind in the course of my own scientific development, I should
like to begin by pointing out that the hereditary substance of the
germ-cell of an animal or of a plant contains not only the primary
constituents (_Anlagen_) of a single individual of the species, but
rather those of several, often even of many individuals. That this is
so can be proved in several ways.
I start from what I hold to be the proved proposition, that the
chromatin substance of the nucleus is the hereditary substance. We
have seen that this is present in the germ-cells of every species in
the form of a definite number of chromosomes, and that in germ-cells
destined for fertilization, that is, in sex-cells, this number is first
reduced to half, the reduction being effected, as is now proved in
regard to a whole series of animals, by the two last cell-divisions,
the so-called maturation divisions.
We know that the full number is only reached again through amphimixis,
by which process the half number of chromosomes in the male and female
germ-cells are united in a single cell, the 'fertilized ovum,' and in a
single nucleus, the so-called segmentation nucleus. Thus the hereditary
substance of the child is formed half from the paternal, half from the
maternal hereditary substance, and we have seen that this remains so
during the whole development of the child, since, at every succeeding
cell-division each of the paternal and each of the maternal chromosomes
doubles by dividing, and the resulting halves are distributed between
the two daughter-nuclei.
Now if the complete hereditary substance of a germ-cell before the
reducing divisions contains potentially all the primary constituents
of the body, which it does as a matter of course, then it follows that
after the reduction each germ-cell must either contain only half the
primary constituents of the parents or all the primary constituents
must be contained in the half number of chromosomes. The latter seems
to me the only possible assumption, as I shall immediately proceed to
show, and this is as much as to say that the primary constituents of at
least two complete individuals must be contained in the chromosomes of
the segmentation nucleus.