the fact that machines cannot be directly made up of primary
physico-chemical forces or energies, but that, as Lotze said, forces of
a superior order are indispensable, which so dispose the fundamental
chemico-physical forces that they must act in the way aimed at by
the purpose of the machine. To produce a watch it is not enough to
bring together brass, steel, gold, and stones; to produce a piano it
is not enough to lay wood, iron, leather, ivory, steel, &c., side by
side, but these stuffs must be brought together in a definite form and
combination. In the same way, the mere juxtaposition of carbon and
water does not result in a carbohydrate like sugar or illuminating gas;
the component elements only yield what is desired when they are placed
in a particular and absolutely definite relation to each other, in
which they so act upon and with one another that sugar or illuminating
gas results, and the same is true of the component elements of a watch
or of a piano. In the watch and in the piano this relation is arranged
by human intelligence, by the workmen who form the different materials
and put them together in the proper manner. In this case, then, human
intelligence is, as Reinke says, the 'superior force' which compels the
energies to work together in a particular way.
But organisms also are machines which perform a particular and
purposeful kind of work, and they are only capable of doing so because
the energies which perform the work are forced into definite paths by
superior forces; these superior forces are thus 'the steersmen of the
energies.' There is undoubtedly a kernel of truth in this view, and I
shall return to it. Reinke, however, uses it in a way which I cannot
follow; that is, he infers from it a 'cosmic intelligence' which puts
these superior forces into the organisms, and thus controls these
machines to purposeful work, as the watchmaker puts 'superior forces'
into the watch by means of wheels, cylinders, and levers. In one case
it is human intelligence which controls the 'superior forces,' in
the other 'cosmic' intelligence. I cannot regard this reasoning from
analogy as convincing, because, in the first place, these 'superior
forces' are not 'forces' at all. They are constellations of energy,
co-ordinations of matter and the energies immanent therein under
complex and precisely defined conditions, and it is a matter of
indifference whether chance or human intelligence has brought them
together. If we take Reinke's own example of carbohydrates it is
certain that our coal-gas is due to the intelligence of man, which
brings together the carbon and the water in such a way that coal-gas
must arise. The 'superior forces' must here be looked for in the
arrangements of the coke-stove, and, in the second place, in the
intelligence of man. But when decaying plants in the marsh form another
carbon-compound, marsh-gas, where do the directing 'superior forces'
come in? Surely only in the fortuitous concomitance of the necessary
materials and the necessary conditions. Or may 'cosmic' intelligence
have established this laboratory in the marsh? If not, what can compel
us to refer the formation of dextrin or starch in the cells of the
green leaves of plants to 'superior forces' which are placed in them by
'cosmic' intelligence? I am far from believing that the great and deep
problem here touched upon can be put aside in any off-hand manner, but
I feel sure that it will never be solved by word-play about energies
and 'superior forces.'
Let us return to the kernel of truth in Reinke's thesis; it lies in
this, that, while the working of a machine does really depend on
the forces or energies which are bound up with the stuffs of which
it consists, it also depends on a particular combination of these
stuffs and forces, on a particular 'constellation' of them, as Fechner
expressed it. In the watch these 'constellations' are the springs,
the wheels, &c., and their position in relation to each other; but in
the organism they are the organs, down to the cells and cell-parts;
for the cell too is a machine, indeed a very complex one, as its
functions prove. There are thousands of kinds of 'constellations' of
elementary substances and forces which condition the activity of the
living machine, and only when all these constellations are present in
the proper manner and in the proper relations to each other can the
functions of the organism be properly discharged.
But the living machine differs essentially from other machines in
the fact that it constructs itself; it arises by development from a
cell, by going through numerous 'stages of development.' But none
of these stages is a dead thing, each is itself a living organism
whose chief function is to give rise to the next stage. Thus each
stage of the development may be compared to a machine whose function
consists in producing a similar but more complex machine. Each stage
is thus composed, just like the complete organism, of a number of such
'constellations' of elementary substances and elementary forces, whose
number in the beginning is relatively small, but increases rapidly with
each new stage.
But whence come these 'constellations' or, to keep to our metaphor,
the levers, wheels, and cranks of each successive stage in the making
of the organic machine? The epigenetic theory of a germ-plasm without
primary constituents answers by pointing to internal and external
influences which cause the germ-plasm, originally homogeneous, to
differentiate gradually more and more, bringing it into the most
diverse 'constellations.' But how can such influences introduce new
springs, levers, and wheels of a quite specific kind, as must be the
case if apparently similar germinal substances are to give rise to two
such different animals as a domestic duck and a teal? The cause must
lie in the invisible differences in the protoplasm, opponents will
answer, and we with them. But our studies up to this point have shown
us that the differences cannot be merely elementary differences, cannot
be merely of a physico-chemical nature depending on the composition of
the raw material and the implicated energies; they must depend on the
definite co-ordination of substances and energies, in other words, on
the occurrence of 'constellations' of these. Thus the germ-plasm must
be composed of definite and very diverse combinations of living units,
which are themselves bound up in a higher 'constellation,' so that they
act as a living machine at the first stage of development, and liberate
into activity the already existing constellations of the second
stage. The second stage in the series of living machines which arise
successively from each other liberates the sleeping 'constellations'
for the third, and so on.
These 'constellations' of matter and energy are the biophors, the
determinants, and the 'groups of determinants' which we may think
of as disposed in a manifold overlapping series. That they do not
enter into activity all at once, but successively take their part in
development, seems to me a necessary consequence of their successive
origin in the phylogeny; and the ontogeny, as we shall see later,
arises through a modified condensation of the phylogeny. Now since
every new determinant that arises in the course of phylogeny can only
develop by division and subsequent variation from the determinants
which were previously active at the same place in the organism, it is
quite intelligible that later on, when the phylogeny has been condensed
in the ontogeny, they should not enter upon their active stage at
the same time as their phyletic predecessors, but after them. The
theory of Oscar Hertwig, who starts from a germ-plasm without primary
constituents, that all parts of the germ-plasm become active at the
same time, seems to me quite untenable. How could the wheels, levers,
and springs of the complete vital machine, which arose so very slowly
in the course of phylogeny, arise to-day in the ontogeny in such rapid
succession unless they were already present in the germ-plasm and only
required to be incited to activity, that is, liberated by the stage
preceding them? Even Fechner supported this view when he supposed that
the interaction and mutual influences of the parts in the organism,
that is, of the 'constellations,' gave rise of themselves to the
succeeding stage, that is to say, to the new constellations peculiar
to the succeeding stage. To this Reinke reasonably objected that it
was like expecting the window frames of a house in process of building
to produce the panes of glass. The panes in the organism only develop
in the window frames if their determinants have been present in the
germ-plasm from the beginning, and are liberated by the development of
the frames, just as the activity of the glazier is liberated by the
sight of the completed frames. Neither new panes nor new determinants
could be produced rapidly; the former must be manufactured in the
glass factory, the latter in the developmental workshop of the form
of life in question, which workshop we call its phylogeny. But just
as it is unnecessary to erect a new glass factory for each new house
that is built, so the development of each individual does not require
the establishment each time of those numberless life-factories--the
constellations--whose business it is to produce anew the wheels,
levers, springs, and cylinders of the developmental machinery at each
stage, for they are all provided for in the germ-plasm, and it is only
on this account that they are capable of hereditary variation.
I have already directed attention to some embryological facts which
seem to be contradictory, if not to the germ-plasm theory itself, at
least to the assumption it makes that the germ-plasm is analysed out
during the ontogeny; and something more must be said on this head.
I refer to the numerous facts brought to light through the science
of developmental mechanics founded by Wilhelm Roux, and particularly
to the investigations as to the prospective significance of the
segmentation-cells of the animal ovum.
Among these investigations we find experiments in compressing
certain eggs (sea-urchin's) in the early stages of segmentation.
The blastomeres are prevented by artificial pressure from grouping
themselves in the normal manner; they are compelled to spread out side
by side in the _same plane_. If the pressure is removed, they change
their grouping, and yield a normal embryo. I will not here discuss
whether these results can only be interpreted as showing that each
segmentation-cell has the same prospective significance, and that it
is only its relative position which decides what part of the embryo is
to be formed from it; this could not be done without going into great
detail; I therefore assume it to be true, and confine my survey to the
second group of experiments, those on isolated segmentation-cells.
It has been shown that in the eggs of the most diverse animals,
for instance in the sea-urchin once more, each of the two first
blastomeres, if separated from one another, can develop into a complete
larva. Indeed, in the eggs of sea-urchin and some other animals each
of the first four, or any of the first eight, blastomeres, and indeed
any segmentation-cell during the earlier stages, possesses the power
of developing to a certain point, namely, as far as the so-called
'blastula-larva.' This seems to contradict a theory which assumes that
the primary constituents become separated in the successive stages of
ontogeny. But in the first place the blastomeres of all animals do
not behave in this way, and, moreover, the facts can be quite well
explained without entirely renouncing the assumption of the segregation
of the determinant-complexes. It is only necessary to assume that the
segmentation-cells, which develop in the isolated condition as if they
were intact eggs, still contain the complete germ-plasm, and that the
differential segregation into groups of determinants with dissimilar
hereditary tendencies takes place later. This would certainly load
the theory with further complications, and I shall not enter into the
question here, since the facts which we should have to consider are as
yet by no means undisputed.
But in any case the facts of developmental mechanics referred to, which
we owe to numerous excellent observers of the last decade,--I need only
name W. Roux, O. Hertwig, Chun, Driesch, Barfurth, Morgan, Conklin,
Wilson, Crampton, and Fischel--not only leave the essential part of the
germ-plasm theory untouched, but rather strengthen than endanger its
more subordinate points, such as the assumption of a segregation of the
components of the germ-plasm in the course of ontogenesis.
As to the fundamental ideas expressed in the theory, I have already
shown that these remain unaltered, even if we do not assume a
disintegration or segregation of the germ-plasm, but think of all the
developing cells as equipped with the complete germ-plasm. In that case
the determinants would be liberated to activity solely by specific
stimuli. But in regard to the assumption of disintegration, it must be
noted that the facts cited relative to the sea-urchin's ova do not by
any means hold true of the eggs of all animals.
In various animal types each of the first two segmentation-cells,
when separated from its neighbour, produces only a half-embryo, and
any one of the first four cells a quarter-embryo. This 'fractional
embryo' is, however, in some cases able later to develop into a whole
embryo (to 'postgenerate' itself, as W. Roux says). The isolated
blastomere shows, to begin with, an activity of only a half of the
primary constituents of the animal, as was first established by W. Roux
and maintained conclusively, in spite of many attacks, until it was
established beyond doubt by the detailed corroboratory investigations
of Endres. The secondary completion of the embryo, which, however, is
still disputed, must be regarded as a regeneration, and, to explain it,
a co-operation of the complete but not yet wholly active germ-plasm in
both segmentation-cells must therefore be assumed.
It would carry us too far if I were to deal in detail even with the
most important of the numerous facts that the last decade has brought
to light; I shall restrict myself to the most essential.
That isolated segmentation-cells have the capacity of developing into
embryos which are complete but correspondingly smaller in size has been
demonstrated in animals of various groups, though it does not seem to
go to the same length in all. In the Medusæ we find that not only one
of the first two, but one of the first four, eight, and even sixteen
segmentation-cells may develop a whole larva when isolated (Zoja). In
the sea-urchin at least any one of the first eight blastomeres may do
so. And Driesch's experiments in cutting up the young larvæ at the
blastula-stage (a single-layered ball of cells) leads us to assume that
each of these cells still possesses the complete germ-plasm. Beyond
that stage, however, the primary constituents obviously divide into
those of the ectoderm and those of the endoderm, for the subsequent
two-layered stage in the sea-urchin's development, the gastrula, does
not complete itself if it be artificially divided into fragments which
consist only of cells from the outer, or only of cells from the inner
layer. In corroboration of this experiment made by Barfurth, Samassa
was able to demonstrate in regard to the egg of the frog that, even
after the third division of the ovum, the segmentation-cells are so
different from each other in respect of their primary constituents
that they were not able to replace each other mutually. When this
investigator killed the ectoderm-cells alone by means of an induction
current, or the endoderm-cells alone, the dead half could not be
replaced by the half which remained alive, and the whole ovum perished.
If these facts may be adduced in favour of a separation of the primary
constituents at an earlier or later stage, we find even stronger proofs
among the Ctenophores, Gastropods, Bivalves, and Annelids. In the
last-named group Wilson has shown it to be probable that development
is really a 'mosaic work,' as Roux and I had assumed. The older
observations made by Chun at an earlier date on the Ctenophora, and
the more recent experiments of Fischel on the same animals, prove the
same thing for this group. In this case complete larvæ are easily
distinguished from mere 'partial developments' by the number of the
characteristic 'ciliated meridional rows' or ribs, which extend from
one pole of the larva to another. In the complete larva there are
eight of these, but in larvæ from one of the first two blastomeres
(isolated) there are only four, and in those which have arisen from one
of the first four blastomeres there are only two. If an ovum at the
eight-cell stage can be successfully divided into separate blastomeres,
each of these will form an 'eighth larva,' always with only one
ciliated rib. Even in the succeeding sixteen-cell stage it could still
be demonstrated that the substance responsible for the formation of
the ribs only lies in particular places and always suffices only for
eight ribs. The sixteen-cell stage consists of eight large cells and
eight small ones, the 'macromeres' and the 'micromeres'; if an ovum
at this stage be cut so that one piece contains five macromeres and
five micromeres, a partial larva will develop which possesses only
five ribs, while the larva from the other portion will have only
three. But the localizing of the rib-determinants can be followed
still further, for in larvæ in which individual micromeres have
been displaced from their normal position there is a correlated
displacement of the corresponding ribs, and a dislocation of their
ciliated comb-plates. The determinants of the ribs must therefore
lie in the micromeres, and we must conclude that at the antecedent
division they were only imparted to one daughter-nucleus, while the
other, that of the macromere, did not receive this kind of determinant.
Here then we have an example of dissimilar or differential division.
Those who oppose this theory of qualitative division will hardly be
likely to admit this, but will rather seek to maintain that 'external
influences,' such as relative position, determine which cells are to
give rise to the ciliated ribs and which are not. But the fact that
artificial displacement of the micromeres leads to a disarrangement of
the ciliated comb-plates, of which the ribs are made up, invalidates
this suggestion, and at the same time overthrows the interpretation
that it may be the cells which lie on particular meridians that are
determined by this position to the production of ciliated plates.
Obviously, the converse of this is true; those cells which contain
the rib-determinants come to lie in the regular course of development
in these eight meridians, and the cells lying between them, though of
the same descent (from micromeres), contain no such determinants and
therefore form no ribs. But if those cells which are equipped with
rib-determinants be artificially displaced, then they give rise to
swimming-plates elsewhere than on the aforesaid meridians.
The experiments made by Crampton on a marine Gastropod, _Ilyanassa_,
likewise go to prove that a disintegration or segregation of the
primary constituents does occur in the course of development. In
this case, when the first two or first four segmentation-cells were
artificially separated from each other, they developed exactly as
if they still belonged to the complete ovum, that is, each isolated
segmentation-cell yielded, respectively, a half or a quarter-embryo.
And these 'partial embryos' were not able in this case to give rise
subsequently to the missing parts or to form complete embryos.
There are thus two contrasted groups of animals, in one of which a
segregation of the mass of primary constituents apparently takes place
at the very beginning, while in the other it does not take place in the
first stages of development, but apparently occurs later on. We may
distinguish these two groups, with Heider, as those having 'regulation
ova' and those having 'mosaic ova.' But I do not see that this affords
any reason why we should give up our conception of the successive
segregation of the germ-plasm into its determinants, even although, as
I said before, I may modify it so far as to say that the segregation
does not necessarily take place in all groups and species of animals at
the same time, but occurs earlier in some and later in others.
Now that I have shown how the germ-plasm theory may be brought into
harmony with the phenomena of ontogeny, I wish to go on to show what
the theory can accomplish in clarifying our understanding of the
phenomena of reproduction and heredity. I shall at the same time give a
brief exposition of some of the most important of these phenomena.
First, a few words in regard to the development of the reproductive
cells. We may leave aside in the meantime the question whether they
are sexually differentiated or not; we are only concerned just now
with the main problem: How is it possible for the organism to produce
germ-cells, that is, cells which contain the complete germ-plasm with
all its determinants, when the building up of the body in ontogeny,
according to our theory, involves a disintegration or segregation of
the determinant-architecture into smaller and smaller groups? It is
impossible that specific determinants should arise _de novo_, just
as an animal cannot arise otherwise than from its germ, nor a cell
otherwise than from a cell, nor a nucleus otherwise than from an
already existing nucleus. If vital units ever originate _de novo_
at all, it is only conceivable in the case of the very simplest
biophors, as we shall see later when we come to speak of 'Spontaneous
Generation.' Specific biophors and the determinants composed of them
have behind them a phylogeny, a history, which conditions that they
shall arise only from their like.
Thus we see that germ-cells can only arise where all the determinants
of the relevant species arranged as ids are already present. If we
could assume that the ovum, just beginning to develop, divides at
its first cleavage into two cells, one of which gives rise to the
whole body (soma) and the other only to the germ-cells lying in this
body, the matter would be theoretically simple. We should say, the
germ-plasm of the ovum first doubles itself by growth, as the nuclear
substance does at every nuclear division, and then divides into two
similar halves, one of which, lying in the primordial somatic cell,
becomes at once active and breaks up into smaller and smaller groups
of determinants corresponding to the building up of the body, while
the germ-plasm in the other remains in a more or less 'bound' or 'set'
condition, and is only active to the extent of gradually stamping as
germ-cells the cells which arise from the primordial germ-cell.
As yet, however, only one group of animals is known to behave
demonstrably in this manner, the Diptera among insects; in all others
the cell from which the germ-cells exclusively arise, the 'primordial
germ-cell,' makes its appearance later in development, usually
during embryogenesis and often very early in it, after the first few
divisions of the ovum, but sometimes not till long after the end of
embryogenesis, and not even in the individual which arises from the
ovum, but in descendants which arise from it by budding. This last
case occurs especially in the colonial hydroid polyps, which multiply
by budding. Here the primordial germ-cell is separated from the ovum
by a long series of cell-generations, and the sole possibility of
explaining the presence of germ-plasm in this primordial cell is to
be found in the assumption that in the divisions of the ovum the
whole of the germ-plasm originally contained in it was not broken up
into determinant groups, but that a part, perhaps the greater part,
was handed on in a latent state from cell to cell, till sooner or
later it reached a cell which it stamped as the primordial germ-cell.
Theoretically it makes no difference whether these 'germ-tracks,' that
is, the cell-generations which lead from the ovum to the primordial
germ-cell, are short or very long, whether they consist of three or
six or sixteen cells, or of hundreds and thousands of cells. That
all the cells of the germ-track do not take on the character of
germ-cells must, in accordance with our conception of the 'maturing'
of determinants, be referred to the internal conditions of the cells
and of the germ-plasm, perhaps in part also to an associated quantum
of somatic idioplasm which is only overpowered in the course of the
cell-divisions.