that is, a perfect individual with all its parts, develops from the
ovum, and not a defective one. For suppose that each mature germ-cell
contained only half the primary constituents of the body, it would be
impossible that these halves should always exactly complete themselves
to form a whole embryo when they are brought together in fertilization,
after having been halved by mere chance during the preceding reducing
division; it would be much more likely to happen that they did not
complete themselves, and that their union would therefore result
in an individual with certain parts wanting. If, for instance, in
the sperm-cell only the anterior half of the body was potentially
present, and this united with an ovum which likewise contained only
the primary constituents of the anterior half, the embryo resulting
from their union would lack the posterior half of the body, and so
on. Of course so rough a division of the primary constituents is not
to be thought of, but however fine we can imagine the halving of the
mass of primary constituents to be, there would never be any guarantee
that the two cells uniting in amphimixis would complete the mass of
primary constituents again; indeed, the chance that the two exactly
complementary halves of the mass would meet would rather become less
the finer and more complex one imagines the halving by reducing
divisions to be. A perfect embryo with all its parts would rarely
arise, but now one group of parts, now another would be wanting, while
another group might be developed double, or at least would be doubly
present in the primary constituents.
But in addition to this the facts of inheritance show us that the
resemblance to mother and father may express itself simultaneously
in all the parts, or at least in the same parts of the child, as may
be seen with especial clearness among plant-hybrids, and thus the
conclusion is inevitable that even in the half number of chromosomes
all the primary constituents of the whole body are present.
Let us go a generation further. If the species possess four
chromosomes the child will have in its cells two maternal chromosomes
(_A_) and two paternal chromosomes (_B_); what form will this
proportion take in the germ-cells produced by the child? The maturation
division can effect the reduction to two chromosomes in different ways;
there may, for instance, be two paternal chromosomes (_B_) left in the
one, and two maternal chromosomes (_A_) in the other daughter-cell, or
one paternal (_B_) and one maternal (_A_) in the one, and a similar
combination in the other cell. Let us follow the latter case further.
A sperm-cell which contained the combination _A_ and _B_ might meet
in amphimixis with an egg-cell of different origin also containing a
similar combination of chromosomes, let us say a chromosome _C_ from
the mother, and a chromosome _D_ from the father. We should then have
in the segmentation nucleus of the fertilized ovum four different
chromosomes, each of which contained the hereditary substance of one
grandparent; we should have the four chromosomes, _A_, _B_, _C_, _D_,
as the hereditary substance of the grandchild.
_But since, as we have seen, the halved hereditary substance still
contains the whole mass of primary constituents, each one of these
chromosomes must contain the collective primary constituents of the
whole body of the relevant grandparent_[18]. _The hereditary substance
in the fertilized ovum thus consists of several complexes of primary
constituents (chromosomes) each of which (an 'id') comprises within
itself all the primary constituents of a complete individual._
[18] When I say the 'collective' primary constituents of the whole body
of the grandparent this is not expressing it quite precisely, for, as
we shall see later, each individual must arise from the co-operation
of different chromosomes of different origin, not merely from one of
the chromosomes contained in its germ-plasm. In the example given
above, the body of each grandparent cannot have arisen only from
a single chromosome, which was transmitted to his grandchild, but
from the co-operation of this chromosome with three others, which
have distributed themselves along other genealogical paths. But this
does not affect the above chain of reasoning, for here it is not a
question of whether all the primary constituents of the grandparent
are present in the child--that can never be the case--but whether the
primary constituents transmitted by him represent the whole body of an
individual.
It can be made clear in yet another way that, as a consequence of
sexual reproduction, the germ-plasm of each species must be composed
of several 'ids,' _individually different_. Let us assume that there
was as yet no amphimixis, and that we could look on at its introduction
into the organic world; the hereditary substance of the beings which
had previously lived and multiplied by division would consist of
more or less numerous chromosomes similar to each other, so that,
for instance, each individual would contain sixteen identical 'ids.'
But if amphimixis were now to take place for the first time, in the
same manner as it does to-day--that is, after the reduction of the
number of the ids to half--in the first amphimixis eight paternal
ids would unite with eight maternal ids to form the germ-plasm of the
new individual, as is indicated in Fig. 87 by a circle of spheres,
of which ten are white and ten black as a sign of their difference.
We may think of the figure as representing the 'equatorial plate'
of a nuclear spindle with its ids arranged in a circle. Now, if
two organisms of this generation, with two kinds of ids, unite in
amphimixis after previous reduction of the ids, we have figure _B_,
in which the paternal ids (_pJ_) are seen to the left of the line and
the maternal ids (_mJ_) to the right, while each semicircle is in its
turn made up of two kinds of ids, those of the grandparents (_p_^2_J_
and _m_^2_J_, _p_^2_J_^1 and _m_^2_J_^1). The figures _C_ and _D_ show
the two following generations, in which the number of identical ids is
each time reduced to half, because eight strange ids are again mingled
with them; in _C_ only two ids are still identical, and in _D_ all the
ids are individually different, because they have come from different
ancestors of the same species. Of course this would only be the case
if inbreeding were excluded, because through it the ids of the same
forefathers from two or more sides would meet; but prolonged inbreeding
is a rare exception in free nature.
[Illustration: FIG. 87. Diagram to illustrate the operation of
amphimixis on the composition of the germ-plasm out of diverse
ancestral plasms or 'ids.' _A_-_D_, the ids of the germ-plasm of
four successive generations: _A_, consisting of only two kinds of
ids; _B_, of four; _C_, of eight; _D_, of sixteen kinds. _pJ_ and
_mJ_, paternal and maternal ids. _p_^2_J_, grandpaternal; _p_^3_J_,
great-grandpaternal; _p_^4_J_, great-great-grandpaternal ids. The marks
in the ids themselves indicate their individually distinct characters.]
I shall now call the hereditary substance of a cell its 'idioplasm,'
after Nägeli's example, although he sought it in the cell-substance,
not in the nucleus, and had a different theoretical conception of its
mode of action. It was he, however, who conceived and established
the idea of the idioplasm as the bearer of the primary constituents,
an _Anlagensubstanz_, determining the whole structure of the
organism in contrast to the general nutritive protoplasm. Every cell
contains idioplasm, since every cell-nucleus contains chromatin,
but I call the idioplasm of the germ-cells _germ-plasm_, or the
primary-constituent-substance of the whole organism, and the complexes
of primary constituents necessary to the production of a complete
individual--whose presence we have just shown to be theoretically
necessary--I call _ids_. In many cases these 'ids' might be synonymous
with chromosomes, at least in all the cases in which the chromosomes
are simple, that is, are not composed of several similarly formed
structures. Thus in the salt-water Crustacean, _Artemia salina_,
which possesses 168 minute granular chromosomes, each of these
chromosomes must be regarded as an id, for each can in certain
circumstances be thrown out from the ovum by the reducing division,
or it can be brought into the most various combinations with other
chromosomes by fertilization. Each of them must therefore consist of
perfect germ-plasm in the sense that all the parts of an individual
are virtually contained in it; _each is a biological unity, an id_.
But when we see in many animals larger band-shaped or rod-shaped
'chromosomes,' and when these are composed of a series of granules, as
they are, for instance, in the often mentioned _Ascaris megalocephala_,
each of these granules is to be regarded as an id. In point of fact,
we find, instead of the two or four large rod-shaped chromosomes
of _Ascaris megalocephala_, a larger number of smaller spherical
chromosomes in other species of _Ascaris_.
Compound chromosomes consisting of several ids, such as all rod or
band-like elements of the nuclear substance probably are, I designate
'idants.' That they are composed of several individual ids is not
always clearly apparent because of the smallness of the object, and
even in larger ones this may only be seen in certain stages. Thus
we have in Fig. 88, _A_ and _B_, two 'mother-sperm-cells' of the
salamander; _A_ at an earlier stage, in which the individual ids are
not visible; _B_ at a later stage, in which the band has split, and
the rosary-like structure has become at once apparent. It is not
possible, then, to see at once whether each chromosome corresponds to
one or to several ids. A more exact investigation of the processes
of reducing division has shown that there are chromosomes of simple
spherical form, that is, composed of several ids whose 'plurivalence'
cannot be directly recognized, but can only be inferred from their
further development; there are bivalent chromosomes of double value
and quadrivalent chromosomes of fourfold value, which we have to think
of as made up of two or four ids. It would lead us too far to go into
this more precisely, nor does it fall within the scope and intention
of these lectures to inquire into these intimate and still disputed
details.
The germ-plasm of every species of plant or animal is thus composed
of a larger or smaller number of ids or primary constituents of an
individual, and it is through the co-operation of these that the
individual which develops from the ovum is determined.
[Illustration: FIG. 88. Sperm-mother-cells (spermatocytes) of the
salamander. _A_, cross-section of the cell in the aster-stage; the
chromosomes (_chr_) or idants do not reveal that they are compounded
out of many ids, which are, however, quite distinctly seen in
_B_ (_Jd_), where the chromosomes or idants (_chr_) are already
longitudinally split. _zk_, cell-substance. _csp_, centrosome. _c_,
centrosome in division. After Hermann and Drüner.]
We have further to inquire what conception we can form of the
constitution of an id and of its mode of operation. I have already
spoken of 'primary constituents' (_Anlagen_) of which the germ-plasm
consists, but what right have we to think of the parts of an animal as
already contained in the germ in any form whatever? Is it not equally
possible that the germ consists of parts, none of which bear any
definite relation in advance to the parts of the finished animal? Might
not the germ-cell, along with its nucleus, undergo transformations and
regular changes which would successively give rise to new conditions,
namely, the different stages of development, until finally the complete
animal was attained?
We stand here before an old problem, before the two opposed
interpretations--the theory of 'Evolution' and the theory of
'Epigenesis,' which were first ranged against each other long ago, and
which are a cause of strife even now, although in somewhat different
guise.
The theory of 'Evolution' is especially associated with the name of
Bonnet, who elaborated it in detail in the eighteenth century. It
maintains that the development of the ovum to the perfect animal is not
really a new creation, but only an unfolding of invisible small parts,
which were already present in the ovum. It assumes that the parts of
the perfect organism are already preformed in the ovum, and on this
account it is called the 'Preformation Theory.' Bonnet often speaks
of the preformation of the perfect animal in the germ as a 'miniature
model,' although his conception of 'evolution' was not really so crude
as has been often alleged. He expressly emphasized that this miniature
model was not exactly like the perfect animal, but consisted of
'elementary parts' only, which he thought of as a net whose meshes were
filled up during development and by means of nutrition with an infinite
number of other parts. But after all, his conceptions, and those of his
time generally, were very far removed from the biological thinking of
our own day, as may perhaps be most readily understood when I mention
that he regarded death and decay as an 'involution,' as a folding back,
so to speak, by means of which all the parts gained though nutrition
were removed again, so that the net of the miniature model shrank
together to the invisible minuteness that it had in the ovum. So it
remained, he fancied, till it was reawakened at the resurrection,
using the term in the religious sense! He afterwards dropped this
fancy, because the objection was made to it that human beings who had
lost a leg or an arm in this life would necessarily be maimed at the
resurrection!
In Bonnet's time the facts of development were quite unknown, and
not even the stages of the development of the chick from the egg had
been observed. When this was afterwards done the prevalent theory
of 'evolution' necessarily collapsed, for men saw with their own
eyes that a miniature model of the chick did not gradually grow into
visibility and ultimately into the young chick, but that first of all
parts showed themselves in the egg which bore no resemblance at all
to the chick, that these first rudiments were then altered, and that
through continual new formations and transformations the chick finally
appeared. Upon this K. von Wolff based his theory of 'Epigenesis,' or
development through new formations and transformations. He maintained
that the doctrine of 'Evolutio' was false; that there is no miniature
model invisibly contained within the egg; but that from the simple
egg-substance there arises, through the agency of the formative powers
inherent in it, a long series of stages of development, of which each
succeeding one is more complex than the one before, until ultimately
the perfect animal is reached.
This certainly marked considerable progress, for it meant the
beginning of a science of embryology, that is, the science of the
form-development of the animal or plant from the ovum. The result was
not so important in its theoretical aspect, for though the knowledge
had been gained that the young animal goes through a long series of
different stages, it had not been discovered how nature works this
wonder and causes an animal of complex structure to arise from the
apparently simple substance of the ovum. A solution of the difficulty
was found by attributing to the ovum a formative power, afterwards
called by Blumenbach the _nisus formativus_, which possessed the
capacity of developing a complex animal from the simple 'slime,' or, as
we should say, the simple protoplasm.
If we contrast the strictly theoretical part of the two theories, we
find that Bonnet regarded the ovum as something only apparently simple,
but in reality almost as complex as the animal which developed from it,
and that he thought of the latter, not as being formed anew, but as
being unfolded or evolved. That is to say, he thought that rudiments
present from the outset in the ovum gradually revealed themselves and
became visible. Wolff, on the other hand, regarded the ovum as being
what it seemed, something quite simple, out of which only the _nisus
formativus_ could, by a series of transformations and new formations,
build up a new organism of the relevant species.
Wolff's Epigenesis routed Bonnet's theory so completely from the
field that, until quite recently, epigenesis was regarded as the only
scientifically justifiable theory, and a return to the 'evolutionist'
position would have been looked upon as a retrograde step, as a
reversion to a period of fancy which had been happily passed. I myself
have been repeatedly told, with regard to my own 'evolutionistic'
theory, that the correctness of epigenesis was indisputably
established, that is, was a fact, verifiable at any time by actual
observation!
But what are the facts? Surely only that there is a succession of
numerous developmental stages, which we know very precisely in the
case of a great many animals, and that the miniature model which
Bonnet assumed to be in the egg does not exist. Both these facts
are now no longer called in question. But that does not furnish us
with a theory of development, for theory is not the observation of
phenomena or of a series of phenomena, _it is the interpretation of
them_. Epigenesis, as formulated first by Aristotle and again by
Harvey, Wolff, and Blumenbach, certainly offered an interpretation of
development, not, however, by referring only to what was observable,
but by going far beyond it; on the one hand taking the _appearance_ of
a homogeneous germ-substance for reality, and, on the other, assuming
a special power, which caused a heterogeneous organism to arise from a
homogeneous germ.
We cannot now accept either of these assumptions, for we know that the
germ-substance is not homogeneous, and indeed is not merely a substance
but a living cell of complex structure; and we no longer believe
in a special vital force, and therefore not in a special 'power of
development,' which could only be a modification of the former. We are
thus as little able to accept the old epigenesis as the old evolution,
and we must establish a theory of Development and Heredity on a new
basis.
What this basis must be is in a general way beyond doubt. Since it
is the endeavour of the whole of modern biology to interpret life
more and more through the interactions of the physical and chemical
forces bound up with matter, development, too, comes within this aim,
for development is an expression of life. We seek to understand the
mechanism of life, and, as a part of that, the mechanism of development
and of heredity which is closely associated with it.
If we wished to attack the problem of heredity at its roots we
should first of all have to try to understand the process of life
itself as a series of physico-chemical sequences. Perhaps this will
be achieved up to a certain point in the future, but if we were to
wait for this we should in the meantime have to abandon all attempts
at a theoretical interpretation of the phenomena of development and
heredity, and might indeed have to postpone them to the Greek Kalends.
That would be as though, in the practice and theory of medicine, all
investigation into and speculation regarding disease had to wait until
the normal, healthy processes of life were thoroughly understood. In
that case we should now know nothing of bacteria diseases and the
hundred other acquisitions of pathological science: physiology too
would have remained far behind its present level if it had lacked the
fruitful influence of experience in cases of disease, and the ideas
and theories, true and false, which have been based thereon. In the
same way we require a theory of development and heredity if we are to
penetrate deeper into these phenomena, and must have it in spite of the
fact that we are still very far from having a complete causal knowledge
of the processes of life. For the raw material of observation, which
is to some extent fortuitous, will never bring us any further on;
observation must be guided by an idea, and thus directed towards a
particular goal.
It is, however, quite possible to leave aside for the present all
attempts at an explanation of life, and simply to take the elements of
life for granted, and on this basis to build up a theory of heredity.
We have already taken a step in that direction by establishing that
the whole substance of the fertilized ovum does not take part in
heredity in the same degree, but that only a small part, the chromatin
of the nucleus, is to be looked upon as the bearer of the hereditary
qualities, and by deducing, further, that this chromatin is made
up of a varying number of small but still visible units, the ids,
each of which virtually represents the whole organism, or, as I have
already expressed it, each of which contains within itself, as primary
constituents, all the parts of a perfect animal.
It was these 'primary constituents' which led us to the digression in
regard to Bonnet's theory of 'Evolutio' and Wolff's 'Epigenesis.'
Let us now inquire what must be the constitution of such a chromatin
globule, an id, so that, shut up within the nucleus of a living
reproductive cell, it can direct the development of a new organism
which resembles its parent. Two fundamental assumptions present
themselves, and these can be related to every conception of a
'germ-plasm,' even independently of the assumption of ids. Either we
may think of the id as made up of similar or of different kinds of
parts, none of which has any constant relation to the parts of the
perfect animal, or we think of it as composed of a mass _of different
kinds of parts, each of which bears a relation to a particular part
of the perfect animal_, and so to some extent represents its 'primary
constituents' (_Anlagen_), although there may be no resemblance
between these 'primary constituents' and the finished parts. The
assumption of a germ-plasm composed of similar parts, which has
been made, for instance, by Herbert Spencer, may be called the
modern form of epigenesis, while the other assumption is the modern
form of the 'evolution' theory. As the former theory can no longer
call to its aid a 'formative power' as a _Deus ex machina_, it can
only explain development as induced by the influence of external
conditions--temperature, air, water, gravity, position of parts--upon
the chemical components of the germ-plasm, which are everywhere
uniformly mingled; and it makes no difference whether this uniform
germ-plasm is thought of as composed of many different kinds of parts,
as long as those parts are mingled uniformly to make the germ-plasm
and bear no relation to definite parts of the developing animal.
Oscar Hertwig has recently outlined such a theory. Although I cannot
expound it here I must say at least so much with regard to it, and to
all other theories of development founded on a similar basis, that
they could not be accepted even if they were able to offer a workable
explanation of the development of the individual, and for this reason,
that ontogeny is not an isolated phenomenon which can be interpreted
without reference to the whole evolution of the living world, for it
is most intimately associated with this, being indeed a piece of it,
having, as we shall see, arisen from it, and, furthermore, preparing
for its continued progress. _Ontogeny must be explained in harmony
with phylogeny and on the same principles._ The assumption of a
germ-plasm without primary constituents, or of a completely homogeneous
germ-plasm, as Herbert Spencer maintained, is irreconcilable with this,
for, as will be seen, it contradicts certain facts of inheritance and
variation. Therefore all theories founded on this assumption must be
rejected.
There is another and, I believe, weighty consideration which forbids us
to assume a germ-substance without primary constituents. I shall return
to this later, but in the meantime I wish to build up more completely
my own 'germ-plasm' theory.
I assume that the germ-plasm consists of a large number of different
living parts, each of which stands in a definite relation to particular
cells or kinds of cells in the organism to be developed, that is, they
are 'primary constituents' in the sense that their co-operation in the
production of a particular part of the organism is indispensable, the
part being _determined_ both as to its existence and its nature by the
predestined particles of the germ-plasm. I therefore call these last
_Determinants_ (_Bestimmungsstücke_), and the parts of the complete
organism which they determine _Determinates_, or hereditary parts.
It is easy to show on what basis this assumption rests; the phenomena
of inheritance taken in conjunction with those of variation seem to
me to compel us to it. We know that all the parts of an organism are
variable, and that in one individual the same part may be larger, in
another smaller. Not all variations are transmissible, but many of
them, and some very minute ones, are. Thus, for instance, in many human
families there occurs a small pit, hardly as large as the head of a
pin, in the skin of the ear, whose transmission I have observed from
the grandmother to the son and to several grandchildren. In such a case
there must be a minute something in the germ-plasm, not present in
that of other human beings, which causes the origin, in the course of
development, of this little abnormality in the skin.