insects in this respect, that there is no organ in the caterpillar
with a specific function corresponding to the wing of the butterfly.
Yet the two cases are exactly alike, and it would be a mistake to say
that the first primordium of the wing within the caterpillar is not
a part of the caterpillar at all. At first, certainly, it is only a
group of cells on the skin, occurring at a particular spot on the
dorsal surface of the second and third segments of the caterpillar,
and doubtless arising from a single cell of the embryo, the 'primitive
wing-cell,' which, however, has not as yet been demonstrated. But
it is nevertheless an integral part of the caterpillar, which could
neither be wanting, nor be larger or smaller, and so on; which, in
short, does mean something for the caterpillar, although perhaps not
more than any other of the skin-cells. For the butterfly, however, this
area on the skin means the rudiment of the wing; for from it alone
can there arise by multiplication the aggregate of cells which grows
out into a hollow protuberance, enlarges by degrees into a disk, the
imaginal disk, and eventually develops into the form of wing peculiar
to the species. This imaginal disk is connected very early with nerves
and with tracheæ, as may be beautifully seen especially in dipterous
larvæ (Fig. 89, _oi_), and these become later the nerves and tracheæ
of the wing, while thousands of peculiar scale-like hairs develop
on the upper surface; in short, the rudiment becomes a perfect wing
with its specific venation, and with the marking and colouring which
is often so complicated in Lepidoptera. Almost every little spot and
stripe of the latter is handed down with the most tenacious power
of transmission from generation to generation, and each can at the
same time be transmissibly varied; the same is true of the venation,
which is so important systematically just because it is so strictly
hereditary, yet it too can vary transmissibly, as can also the hooked
bristles, the odoriferous apparatus, and, in short, the whole complex
structure of the wing, with all its specific adaptations to the mode of
flight, to the manner of life, and to the colour of the environment.
How is it possible that all this can develop from a skin-cell? Is it
the influence of position that effects it, and could any other cell
of the caterpillar's skin do the same if it were placed in the same
position? Could any neighbour-cell of the primitive wing-cell replace
it if it were destroyed? It is hardly probable, and I think I can even
prove that this is not so. The experiment of killing such a cell in
the living animal has not yet been made; if it should succeed, we may
venture to say in advance that none of the neighbouring skin-cells
will be able to do its work and take its place in developing a wing;
the wing in question will simply remain undeveloped. In the summer of
1897 I hatched a specimen of _Vanessa antiope_ from the pupa, which,
though otherwise normal and well-developed, lacked the left posterior
wing altogether; no trace of it could be recognized. In this case, from
some cause which could no longer be discovered, the first formative
cell of the wing in the hypodermis, or its descendants, must have been
destroyed, and no substitution of another took place, as the defect
showed.
The young science of developmental mechanics attributes to the position
of a cell in the midst of a group of cells a determining value as
regards its further fate, and as far as the cells of the segmenting
ovum are concerned this seems to be true in certain cases, but the
assumption cannot be generally true except in a very subordinate sense.
The formative cell of the wing does not become what it is because of
its relative position in the organism. If this were so it could not
happen that a wing should develop instead of a leg, as was observed
in a _Zygæna_, nor could there be any of those deformities already
referred to, to which the name 'Heterotopia' is applied, and which
consist in the development of organs of definite normal structure, or
at any rate of apparently normal structure in quite unusual places, e.
g. an antenna on the coxa of a leg, or of a leg instead of an antenna
(in _Sirex_), or instead of a wing. It is therefore not some influence
from without that makes that particular skin-cell of the caterpillar
the rudiment of the wing, but the _reason lies within itself_, in its
own constitution. As the whole mass of determinants for the whole body
and for all the stages of its development must be contained within
the ovum and the sperm-cell, so the primitive cell of the butterfly's
wing must contain all the determinants for the building up of this
complicated part; and if the cell gets into a wrong position in the
course of development because of some disturbance or other, a wing
may develop from it in that position if the conditions are not too
utterly divergent. These heterotopic phenomena afford a further proof
of the existence of determinants, because they are quite unintelligible
without the assumption of 'primary constituents' or _Anlagen_.
The hypothesis of determinants in the germ-plasm is so fundamental to
my theory of development that I should like to adduce another case
in its support and justification. The limbs of the jointed-footed
animals, or Arthropods, originally arose as a pair on each segment
of the body, and they were at first alike or very similar both in
their function and in their form. We find illustration of this in
the millipedes, and still more in the species of the interesting
genus _Peripatus_, which resembles them externally, as well as in the
swimming and creeping bristle-footed marine worms (Chætopods) belonging
to the Annelid phylum. We can quite well picture to ourselves that the
whole series of these appendages was represented in the germ-plasm by
a single determinant or group of determinants, which only required
to be multiplied in development. Without disputing whether this has
really been the case in the primitive Arthropods or not, it is certain
that it can no longer be the case in the germ-plasm of the Arthropods
of to-day. In these each pair of appendages must be represented by
a particular determinant. We must infer this from the fact that
the several pairs of these appendages have varied transmissibly,
independently of each other, for some are jaws, others swimming legs,
or merely bearers of the gills or of the eggs; others are walking legs,
digging legs, or jumping legs. In Crustaceans a forceps-like claw
is often borne by the first of the otherwise similarly constructed
appendages, or also by the second or the third, or there may be no
forceps, and so on; in short, we see that each individual pair has
adapted itself independently to the mode of life of its species. This
could only have been possible if each was represented in the germ-plasm
by an element, whose variations caused _a variation only in that one
pair of legs, and in no other_.
It may perhaps be objected that the differences in the appendages may
quite well have had their origin simply during the development of the
animal, while the primary constituents were the same for all, so that a
single determinant in the germ-plasm would suffice. But this could only
be the case if the differences depended not on internal but on external
causes, that is, if the same primary constituents gave rise to a set
of appendages which became different because they were subject in the
course of their development to different modifying influences. But
this is not the case, at least not to the extent that this supposition
would necessitate. Can it be supposed that, for instance, the jumping
legs of the water-flea (_Gammarus_) are a necessary consequence of the
somewhat divergent form of the segments from which they grow? A direct
proof to the contrary may be found in 'Heterotopia,' for in the place
where a posterior limb, modified for holding the eggs, normally occurs
in the crab an ordinary walking leg may exceptionally develop (Fig. 90,
Bethe), or an appendage resembling an antenna may take the place of an
extirpated eye (Herbst). But if there were really only one determinant
in the germ-plasm for all the appendages these would of necessity be
all alike, apart from the larger or smaller differences which might be
stamped upon them by growing from segments different in size and in
nutrition. Such differences, however, are far from being sufficient to
explain the great deviations seen among the appendages of most kinds
of Crustaceans, and still less to explain their adaptation to quite
different functions.
[Illustration: FIG. 90. The Common Shore-Crab (_Carcinus mænas_), seen
from below, with the abdomen forced back. In place of the swimmeret,
which ought to be borne by the fifth abdominal swimmeret, a walking leg
has grown on the left side, and one which properly should belong to the
right side (6). 1-5, thoracic limbs, _ps_1-4, swimmerets of the right
side. _s_6, _s_7, posterior segments of the abdomen. After Bethe.]
It need not be imagined that my argument can be controverted by
saying that _one_ appendage-determinant in the germ may split
itself in the course of development into a series of different
appendage-determinants. The question would then arise, How is it
able to do so? And the answer can be no other than that the single
first determinant had within it several different kinds of elements,
which subsequently separated to determine in different ways the
various appendages. But that is just another way of saying that this
single determinant actually includes within itself several different
determinants. For a determinant means nothing more than an element
of the germ-substance by whose presence in the germ the specific
development of a particular part of the body is conditioned. If we
could remove the determinants of a particular appendage from the
germ-plasm this appendage would not develop; if we could cause it to
vary the appendage also would turn out differently.
In this general sense the determinants of the germ-plasm are not
hypothetical, but actual; just as surely as if we had seen them with
our eyes, and followed their development. Hypothesis begins when we
attempt to make creatures of flesh and blood out of these mere symbols,
and to say how they are constituted. But even here there are some
things which may be maintained with certainty; for instance, that they
are _not_ miniature models, in Bonnet's sense, of the parts which they
determine; and, further, that they are not lifeless material, mere
substances, but living parts, vital units. If this were not so they
would not remain as they are throughout the course of development,
but would be displaced and destroyed by the metabolism, instead of
dominating it as living matter alone can do--doubtless undergoing
oxidation, but at the same time assimilating material from without,
and thereby growing. There cannot be lifeless determinants; they must
be living units capable of nutrition, growth, and multiplication by
division.
And now we have arrived at the point at which a discussion of
the organization of the living substance in general can best be
interpolated.
The Viennese physiologist, Ernst Brücke, forty years ago promulgated
the theory that living matter could not be a mere mixture of chemical
molecules of any kind whatever; it must be 'organized,' that is, it
must be composed of small, invisible, vital units. If, as we must
certainly assume, the mechanical theory of life is correct, if there
is no vital force in the sense of the 'Natur-Philosophie,' Brücke's
pronouncement is undoubtedly true; for a fortuitous mixture of
molecules could no more produce the phenomena of life than a _single_
molecule could, because, as far as our experience goes, molecules
do not live; they neither assimilate, nor grow, nor multiply. Life
can therefore arise only through a particular combination of diverse
molecules, and all living substance must consist of such definite
groups of molecules. Shortly after Brücke, Herbert Spencer likewise
assumed the reality of such vital 'units,' and the same assumption
has been made in more recent times by Wiesner, De Vries, and myself.
In the meantime we can say nothing more definite about the composition
of these bearers of life, or 'biophors,' as I call them, than that
albumen-molecules, water, salts, and some other substances play the
chief part in their composition. This has been found out by analysis
of dead protoplasm; but in what form these substances are contained in
the biophors, and how they affect each other in order to produce the
phenomena of life by going through a ceaseless cycle of disruptions and
reconstructions, is still entirely hidden from us.
We have, however, nothing to do with that here; we content ourselves
with recognizing in the biophors the characteristics of life, and
picturing to ourselves that all living substance, cell-substance, and
nuclear substance, muscle-, nerve-, and gland-substance, in all their
diverse forms, consist of biophors, though, of course, of the most
varied composition. There must be innumerable kinds of biophors in all
the diverse parts of the millions of forms of life which now live upon
the earth; but all must be constructed on a certain fundamental plan,
which conditions their marvellous capacity for life; all possess the
fundamental characters of life--dissimilation, assimilation, growth,
and multiplication by division. We must also ascribe to them in some
degree the power of movement and sensibility.
As to their size, we can only say that they are far below the limits
of visibility, and that even the minutest granules which we can barely
perceive by means of our most powerful microscopes cannot be small
individual biophors, but must be aggregates of these. On the other
hand, the biophors must be larger than any chemical molecule, because
they themselves consist of a group of molecules, among which are some
of complex composition, and therefore of relatively considerable size.
It may now be asked whether the determinants, whose existence we have
already inferred, are not identical with these 'biophors' or smallest
living particles; but that is not the case, at least not generally. We
called determinants those parts of the germ-substance which determine a
'hereditary character' of the body; that is, whose presence in the germ
determines that a particular part of the body, whether it consists of
a group of cells, a single cell, or a part of a cell, shall develop in
a specific manner, and whose variations cause the variations of these
particular parts alone.
Again, it may be asked how large and how numerous such 'hereditary
parts' may be, whether they correspond to every distinct part of a
cell, or to every cell of the body, or only to the larger cell groups.
Obviously the areas which are individually determined from the germ
must differ in size, according as we have to do with an organism which
is small or large, simple or more complex. Unicellular organisms,
such as Infusorians, probably possess special determinants for a
number of cell-organs and cell-parts, although we cannot directly
observe the independent and transmissible variation of these organs;
lowly multicellular animals, such as the calcareous sponges, will
require a relatively small number of determinants, but in the higher
multicellular organisms, as, for instance, in most Arthropods, the
number must be very high, reaching many thousands if not hundreds of
thousands, for in them almost everything in the body is specialized,
and must have varied through independent variation from the germ. Thus
in many Crustaceans the smelling-hairs occur singly on special joints
of the antennæ, and the number of joints furnished with a smelling-hair
is different in different species; the size, too, of the smelling-hairs
themselves varies greatly, being, for instance, much smaller in our
common Asellus than in the blind form from the depths of our lakes, in
which the absence of sight is compensated for by an increased acuteness
of the sense of smell. Thus the smelling-hairs may vary transmissibly
in themselves, while any joint of the antennæ may also produce one
independently through variation. In this case accordingly we must
assume that there are special determinants for the smelling-hairs, and
for the joints of the antennæ. But we cannot always and everywhere
refer identical or approximately similar organs, when there are many
of them, to a corresponding number of determinants. Certainly the
hairs of mammals or the scales of butterflies' wings do not all vary
individually and independently, but those of a certain region vary
together, and are therefore probably represented in the germ-plasm by
a single determinant. These regions often appear to be very small,
as is best seen by the fine lines, spots, and bands which compose
the marking of a butterfly's wing, and still more in the odoriferous
scales occurring in some butterflies, as, for instance, in the blue
butterflies (_Lycæna_). These little lute-shaped scales do not occur
in all species, and they occur in very unequal numbers even in those
which possess them; there are certain species which exhibit only about
a dozen, and these are all on one little spot of the wing. Since these
odoriferous scales must have arisen as modifications of the ordinary
hair-like scales, as one of my pupils, Dr. Köhler, has demonstrated by
comparative studies, these ordinary hair-like scales must have varied
transmissibly at certain spots, that is, their determinants have varied
while those of the surrounding scales have not.
The case is the same in respect to the sound-producing apparatus of
many insects. Many grasshoppers produce sounds by fiddling with the
thigh of the hind leg on the wing, others by rubbing one anterior wing
upon the other, and, indeed, always with one particular vein in one
upon a particular vein in the other. One of these serves as the bow,
the other as the string, of the violin, and the bow is furnished with
teeth (Fig. 91), ranged beside each other in a long row, which have
the same function as the colophonium of the violin, that is, to grasp
and release the strings alternately, and thus to produce resounding
vibrations. My pupils, Dr. Petrunkewitsch and Dr. Georg von Guaita,
have recently proved that these teeth have arisen as modifications of
the hairs which are scattered everywhere over the wing and leg. But
only in this one place, on the so-called 'stridulating-vein,' have
they been modified to form stridulating teeth (_schr_). Thus this
vein must be capable of transmissible variation by itself alone, that
is, there must be parts contained in the germ-plasm, the variation of
which causes a variation solely of this individual vein and its hairs,
possibly even a variation only on certain hairs on this vein.
[Illustration: FIG. 91. Hind leg of a Locustid (_Stenobothrus
protorma_), after Graber. _fe_, femur. _ti_, tibia. _ta_, tarsal
joints. _schr_, the stridulating ridge.]
On the other hand, there are also large regions, whole cell-masses
of the body, which in all probability vary only _en bloc_, as, for
instance, the milliards of blood-cells in Man, the hundreds of
thousands or millions of cells in the liver and other glandular organs,
the thousands of fibres of a muscle, or of the sinews or fascia, the
cells of a cartilage or a bone, and so on. In all these cases a single
determinant, or at least a few in the germ-plasm, may be enough. But in
numerous cases it is impossible to say how large the region is which
is controlled by a single determinant, and it is, of course, of no
importance to the theory. In unicellular organisms the determinants
will control parts of cells, in multicellular organisms often whole
cells and groups of cells.
Perhaps an inference as to the nature of the determinants may be drawn
from this with some probability, in as far as mere parts of cells may
be supposed to have simpler determinants than whole cells and groups of
cells. The determinants in the chromosomes of unicellular organisms
may therefore often consist of single biophors, so that in this case
the conception of biophors would coincide with that of determinants.
In multicellular organisms, on the other hand, I should be inclined
on the whole to picture the determinant as a group of biophors, which
are bound together by internal forces to form a higher vital unity.
This determinant must live as a whole, that is, assimilate, grow, and
multiply by division, like every vital unit, and its biophors must be
individually variable, so that the separate parts of a cell controlled
by them may also be capable of transmissible variation. That they are
so, every highly differentiated cell of a higher animal teaches us;
even the smelling-hairs of a crab exhibit a stalk, a terminal knob, and
an internal filament, and many muscle-, nerve-, and gland-cells are
much more complex in structure.
LECTURE XVIII
THE GERM-PLASM THEORY (_continued_)
Structure of the germ-plasm--Vital affinities--Division--O.
Hertwig's chief objections to this theory--Male and female eggs
in the Phylloxera show differential division--Dispersal of the
germ-plasm in the course of Ontogeny--Active and passive state
of the determinants--Predetermination of cells--There are no
determinants of characters--Liberation of the determinants--Accessory
idioplasm--Herbst's lithium larvæ--Plant galls--Cells with several
facultative determinants--Connective tissue in vertebrates--Mesoderm
cells of Echinoderms--Sexual dimorphism--Female and male
ids--Polymorphism (_Papilio merope_)--Ants.
I HAVE endeavoured to prove that the germ-substance proper must be
looked for in the chromatin of the nucleus of the germ-cell, and more
precisely still in those ids or chromosomes which we conceive of as
containing the primary constituents (_Anlagen_) of a complete organism.
Such ids in larger or smaller numbers make up the whole germ-plasm of
a germ-cell, and each id in its turn consists of primary constituents
or determinants, i.e. of vital units, each of which determines the
origin and development of a particular part of the organism. We have
now to make an attempt to picture to ourselves how these determinants
predetermine those cells or cell-groups to which they correspond. In
doing so we have to fall back upon mere hypotheses, and in stating any
such hypothesis I wish expressly to emphasize that I am only following
up one of the possibilities which our imaginative faculty suggests.
Nevertheless, to endeavour to form such a conception is certainly not
without use, for it is only by elaborating a theory to the utmost
that we are able to use it in application to concrete cases, thus
stimulating the search for corroboratory or contradictory facts, and
leading gradually to a recognition of the gaps or mistakes in the
theory.
The first condition that must be fulfilled in order that a determinant
may be able to control a cell or cell-group is that it should
succeed in getting into it. It must be guided through the numerous
cell-divisions of ontogeny so that it shall ultimately come to lie
in the cells which it is to control. This presupposes that each
determinant has from the very beginning its definite position in
relation to the rest, and that the germ-plasm, therefore, is not a mere
loose aggregate of determinants, but that it possesses a structure, an
architecture, in which the individual determinants have each their
definite place. The position of the determinants in relation to one
another cannot be due to chance, but depends partly on their historical
development from earlier ancestral determinants, partly on internal
forces, such as we have already assumed for keeping the determinants
together. We may best designate these hypothetical forces 'affinities,'
and in order to distinguish them from mere chemical affinities we
may call them 'vital.' There must be forces interacting among the
different determinants which bind them together into a living whole,
the id, which can assimilate, grow, and multiply by division, in the
same manner as we were forced to assume for the smaller units, the
biophors and single determinants. In the ids, however, we can observe
the working of these forces quite directly, since each chromosome
splits into two halves of equal size at every nuclear division, and not
through the agency of external forces, e.g. the attraction which we may
assume to be exerted by the fibrils of the nuclear spindle, but through
purely internal forces, often long before the nuclear spindle has been
formed at all.