THOUGHTS TOWARDS A THEORY OF THE DEVELOPMENT OF ORGANISMS.[17]
Now that criticism of the germplasm theory has given us a bias in the right
direction, it is necessary to map out more clearly the path along which
solution of the problem may be sought. In general terms, our problem is the
necessary origin from an egg, always of the same organism, with its
manifold characters, and the explanation must avoid the attribution to the
egg of characters foreign to its nature as a cell. This is the more
necessary as Weismann objects to the supposition that cell-division is
doubling, holding that the supposition allows neither an explanation, nor
even the beginning of an explanation, of the differences that arise among
cells while the differentiation of the body occurs. 'Any explanation must
in the first place account for this differentiation,' says Weismann
(_Germplasm_, p. 224); 'that is to say, the diversity which always exists
amongst these cells and groups of cells arising from the ovum must be
referred to some definite principle. In fact, no one could even look at it
as giving a partial solution of the problem, if differentiation is supposed
to be due to that part alone of the germplasm becoming active which is
required for the production of the cell or organ under consideration. But
the higher we ascend in the organic world, the more limited does the power
of producing the whole from separate cells become, and the more do the
numerous and varied differentiations of the soma claim our attention and
require an explanation in the first instance. The presence of idioplasm in
all parts containing the primary constituents does not help us in this
respect.'
With this I cannot agree. Naturally, Naegeli, De Vries, Driesch and I
assume that, of the many rudiments present in every cell, only some come to
activity in each special case, and that the selection of those that become
active is due to causes arising in the course of development. Our
conception of the nature of these causes, and of their place of origin, is
diametrically opposed to Weismann's.
Weismann would make the causes of this orderly development of the rudiments
reside in the germplasm itself; for he considers that to be not only the
material but the motive force of the course of development. According to
him, every cell _must_ have become what it is, because it was provided only
with the definite rudiments assigned it beforehand, according to the plan
of the development of the germplasm.
On the other hand, we regard the development of the rudiments as depending
upon motive forces or causes that are external to the germplasm of the
ovum, but that none the less arise in orderly sequence throughout the
course of the development. The causes we recognise are first, the continual
changes in mutual relations that the cells undergo as they increase in
number by division, and second, the influence of surrounding things upon
the organism.
One may group together as _centrifugal causes_ of the process of
development the characters of the fertilised cells and the interrelations
between the products of their divisions, and distinguish them from the
_centripetal causes_, or motive forces that are provided by the action of
surrounding things. None the less, it must be borne in mind that there is
no sharp distinction between centrifugal and centripetal forces. On page 86
I showed how what is external in one stage of the process becomes internal
in the succeeding stage. The external constantly is becoming internal, and
the sum of the internal factors increases only at the expense of external
factors.
From the physiological point of view I regard the divergent differentiation
of cells as a reaction of the organic material to unlike impelling
forces--that is, to factors shown by experimental physiology to be actually
present and to rule the building up of the organism. 'It were superfluous
to detail,' as Naegeli says, 'how continually other forces external to the
idioplasm, but belonging to the individual, influence the idioplasm; every
cell, indeed, as it grows and divides, takes up a definite place in the
growing whole, and finds itself in a peculiar combination of conditions of
organisation.' 'Not only influences within the individual affect the
idioplasm, as that may be altered by external influences, and so may be
forced to grow in a new direction.' 'The influence of surroundings in
determining which of the rudiments contained in the idioplasm shall achieve
development is shown in the following example: it depends on their
nutrition whether certain trees shall bear foliage or flowers; while in an
unpropitious climate many plants refuse to bear flowers at all, but content
themselves with vegetative reproduction.'
This principle indicates the path along which explanation of the
differentiation of cells is to be sought. Although in no single case is it
yet possible to refer a known action to its appropriate cause--in other
words, to show a definite stimulus producing a definite reaction upon the
rudiment--this failure is not to be attributed to error in the principle.
It is the natural result of the enormous difficulties besetting an attempt
to understand the highly involved events of development. We can only ask
whether or no our general principle is harmonious with the facts displayed
in nature.
In the following pages I shall try to develop this view, taking, as
formerly, a few instances. I shall now proceed further with suggestions I
made in my treatise on _Old and New Theories of Development_. I start from
the conception that the ovum is an organism that multiplies by division
into numerous organisms like itself. I shall explain the gradual,
progressive organisation of the whole organism as due to the influences
upon each other of these numerous elementary organisms in each stage of the
development. I cannot regard the development of any creature as a mosaic
work. I hold that all the parts develop in connection with each other, the
development of each part always being dependent upon the development of
the whole.
The power of the egg to multiply by division is a chief and most important
factor in the production of complexity during the course of development. It
is only because the nuclear material, by a series of intricate, chemical
changes, assimilates reserve material from the egg and oxygen from the
atmosphere that it can give rise to continually increasing complexity
within itself. The increase in bulk results in a cleavage into two, four,
eight, and sixteen pieces, and so forth. The cleavage produces a constantly
changing distribution in space of the nuclear material. The two, four,
eight, and sixteen nuclei that arise by division diverge from each other
and take up new positions inside the egg, in definite relations to each
other. At first the particles of the egg were arranged around the
fertilised nucleus, which was a single centre of force; they become grouped
around as many centres of forces as there are nuclei, and so become
segregated into as many cells. Clearly enough, the egg, in its
single-celled condition, changes its quality in a marked degree when it
becomes multicellular, even although the change has occurred by doubling
division.
This, so clear in the early stages of development, continues to occur
throughout the later stages of growth. The continued cell-multiplication
causes not only changes of bulk, but also from time to time changes in
quality; for each shape is bound up with definite conditions. When the
conditions alter, the organic material, by its power of reaction, changes
its shape in a corresponding fashion.
As the nature of architectural plans depends upon the properties of the
wood, stone, or iron, as they must correspond with the material to be
employed (_i.e._, the span of a roof, the construction of a bridge depend
upon the material in shape and weight), so the nature of the organic
material determines to a large extent the shapes assumed in the course of
growth.
Shape in many respects appears to be a function of growth in an organic
material.
A few examples will make clear this important relation. A limit is set to
increase in the size of a blastosphere by the nature of the material of its
walls. Its wall is a membrane, composed of one or more layers of cells;
that this may preserve its curvature, a definite pressure from within must
be maintained, proportioned to the cohesive force of the cells; at the same
time the wall of the sphere must be able to withstand the strain and
pressure put upon it by external forces. All these, and many other factors
less easy to conceive, must be delicately adjusted to one another. If in
any direction a definite limit be exceeded, then either the structure will
be destroyed by disintegration of the component parts, or a new shape will
be assumed. The latter is the event in the case of a living substance
capable of reaction. The blastosphere, growing beyond its limits, folds
into a cup-shaped organism. Did we know all the influences affecting the
wall of the blastosphere, then we would understand the causes by which
growth beyond a definite limit must result in invagination. From the
occurrence of the gastrula in all the divisions of the animal kingdom, we
may conclude that it is a temporary phase, inevitable in the growth of
animals.
There may be noticed here a second connection between shape and organic
growth, exceedingly simple in its nature, but of fundamental importance in
its consequences. It may be stated in this saying: Growth always must be
such as to produce the greatest possible extension of surface. The reason
of this is simple, depending on the different natures of inorganic material
and living organic material.
A crystal in its mother liquor grows by attracting new particles and
depositing them upon its outer surface, according to the kind of
crystallisation peculiar to the material of which it is composed. These
particles, once crystallised, retain their position even when new layers
are deposited on their outer surfaces, and remain unchanged, perhaps, like
rock crystals, for thousands of years, until changed outer forces loosen
the bonds that bind them.
Organised material cannot grow in this fashion; it takes up material from
without, not, like the crystal, arranging it on the outer surface, but
ingesting it. Protoplasm cannot become fixed in any condition without being
destroyed; it exhibits perpetual interchanges with the outer world;
unceasing intake and output is a necessary accompaniment of its life. 'The
growth of idioplasm,' as Naegeli strikingly says, 'implies a constancy of
perpetual change.'
Thus, growing protoplasm can assume only such shapes as allow it to remain
in constant touch with the outer world. A cubical or spherical mass of
cells could not grow by the formation of new layers of cells on the
outside, for these layers would deprive the centrally placed masses of
cells of their conditions of existence. Similarly, an extended membrane of
cells or an epithelial layer cannot add indefinitely to its thickness, else
would the cells furthest removed from the outside be injured in their
relations to surrounding things. To satisfy its essential conditions,
protoplasm can grow only with a proportionate extension of its external
surfaces. This is secured by the cells becoming arranged in threads and
membranes, and its result is that the threads by branching, and the
membranes by folding, produce structures whose complexity increases with
growth.
This conception that the shape of growing organisms is in many respects the
necessary consequence of the specific characters with which protoplasm is
endowed, explains the great contrast between animals and plants in their
general organisation. The contrast is the result of the difference between
animal and plant metabolism, and between the ways in which animals and
plants obtain their food. Plant cells elaborate protoplasm from the
carbonic acid of the air, water, and easily diffusible solutions of salts,
obtained from the sea or from the soil. For the chemical work of combining
these, they require the active energy of sunlight. We can now see the chief
requirements to which the constitution and arrangement of the cells in a
multicellular plant must be adapted. Plant cells may become clothed in a
thick membrane, as that would prove no hindrance to the passage of gases
and easily diffusible salts; but they must be arranged so as to present the
greatest possible surface to the surrounding media (_i.e._, to the soil and
the water, the air and the sunlight) whence is drawn their supply of matter
and force. The cells must turn a broad face to the outside; this they do by
becoming arranged in branching rows, or in leaf-shaped flattened organs.
That they may suck up water and salts from the soil, the cells are arranged
as a highly branched system of roots, covered with delicate hairs, and
penetrating the soil in every direction. To inhale the carbonic acid from
the air, and to be subjected to the influence of sunlight, the aerial part
of the plant stretches out its branches towards the light, and becomes
folded into the flat leaves, the structure of which reveals a suitability
for assimilation. Thus the whole architecture of a plant is superficial and
visible; internal differentiation into organs and tissues either is
wanting, or, compared with animals, is very scanty. It is only in the
higher plants that the internal fibro-vascular tissues appear; these serve
a double purpose: they act as channels along which the sap passes, so
bringing together the different materials absorbed by roots and leaves; and
they have the mechanical function of strengthening the stem and branches.
The different mode of nutrition of animals results in a totally different
structural plan. Animal cells absorb material that is already organised,
and that they may do so their cells are either quite naked, so affording an
easy passage for solid particles, or they are clothed only by a thin
membrane, through which solutions of slightly diffusible, organic colloids
may pass. Therefore, unlike plants, multicellular animals display a compact
structure with internal organs adapted to the different conditions which
result from the method of nutrition peculiar to animals. A unicellular
animal takes organic particles bodily into its protoplasm, and forming
around them temporary cavities known as food vacuoles, treats them
chemically. The multicellular animal has become shaped so as to enclose a
space within its body into which solid organic food-particles are carried
and digested, thereafter, in a state of solution, to be shared by the
single cells lining the cavity. In this way the animal body does not
require so close a relation with the medium surrounding it; its food, the
first requirement of an organism, is distributed to it from inside
outwards. In its further complication the animal organisation proceeds
along the same lines. The system of internal hollows becomes more
complicated by the specialisation of secreting surfaces, and by the
formation of an alimentary canal, and of a body cavity separate from the
alimentary canal.
In plants, it is the external surface that is increased as much as
possible. In animals, in obedience to their different requirements,
increase takes place in the internal surface. The specialisation of plants
displays itself in organs externally visible--in leaves, twigs, flowers,
and tendrils. The specialisation of animals is concealed within the body,
for the internal surface is the starting-point for the formation of the
organs and tissues.
Comparative embryology shows that, however varied the forms and functions
of the numerous animal organs may be, the method of their development is
remarkably similar. There are required only the slightest variations of a
few simple general laws. For these I may refer readers to a series of
special investigations (_Studies on the Germ-layer Theory_, Oscar and
Richard Hertwig), and to the fourth chapter of my Embryology, 'General
Discussion of the Principles of Development.'
In these works and in the foregoing pages I have tried to show that the
multiplication of the egg-cell by division is itself a source of increasing
complexity and an active principle in the determination of form, since the
products of the division unite to form a higher unity. But in another way
the multiplication of cells leads to differentiation among the cells
arising from the egg. Although each of these resembles the parent egg, from
which they arose by doubling division, yet they differ from it in one
point: they are no longer a whole, but have become the subordinate parts of
a higher unity, that is, of a higher organism. A cell that is no longer a
whole, but the part of a whole, has entered upon reciprocal relations with
other cells, and in the functions of its life is limited by these others
and by the whole. The further this is carried the more the cell falls short
of its independence as an elementary organism, and appears only as a part
with its functions subordinate and in dependence upon the whole.[18]
Although from the point of view of morphology it has become more and more
imperative to regard the cell as the unit of the higher organism, still,
from the physiological point of view the higher organisms must be regarded
as masses of material acting as wholes, and composed of several grades of
structural parts, subordinate in function to the whole, and displaying only
a limited division of capacities. And so the cell theory, according to
which the cell was exalted unduly as the unit of life, the centre of life,
the elementary organism, must take limitation and correction from these
wider views. This has already been insisted upon by many physiologists of
insight--for instance, by Naegeli (see p. 30), by Sachs, and by Voechting.
'Cell formation,' declares Sachs (_Physiology of Plants_, p. 73), 'is a
phenomenon very general, it is true, in organic life, but still only of
secondary significance; at all events, it is merely one of the numerous
expressions of the formative forces which reside in all matter, in the
highest degree, however, in organic substance.' 'Essentially, every plant,
however highly organized, is a continuous mass of protoplasm, surrounded
externally by a cell wall and penetrated internally by numerous transverse
and longitudinal partitions.'
My conception receives strong support from the way in which Voechting set
forth the relations of the cell to the whole:
'Is the circumstance that a cell, separated from the organism, is able to
survive and build up the whole again a proof of the independent life of the
cells while in the organism? I believe it to be only a proof that the life
of the organism is always dependent upon the cell, that the life is
inherent in the cell, and that the life of a compound organism is merely
the resultant of the vital phenomena of its single cells; but by no means
that the cell when isolated displays the same functions as while it is a
part of the organism. The cell while in the organism and the cell separated
from the organism and self-sufficing, are quite different. We must regard
the functions of a cell that is part of an organism, disregarding external
influences, as determined by the whole organism, and only by the cell
itself, in so far as that forms a greater or less part of the whole
organism. When not part of an organism, the cell is independent, and
entirely determines its own function. Nowhere is it easier than in this
case to confuse possibilities with facts, and nowhere is the confusion more
fatal. From a morphological point of view, one may confidently regard the
cell as an individual; but it must be borne in mind that an abstraction has
been made. Physiologically considered, the cell is an individual only when
it is isolated from a complex and is independent; of this no abstraction
can be made.'
According to the conception I have been explaining, cells merge their
independent individuality in that of the whole, and so the force that
directs their ultimate development, and that leads to their appropriate
elaboration, cannot be within them, cannot reside in special groups of
determinants, in the sense of Weismann. It is given by the relations in
which the cells come to stand to the whole organism and to the various
parts of the organism, and, on the other hand, to surrounding things.
Naturally, such relations differ with the place or position occupied by
cells in the whole organism, and in this way there come to be innumerable
conditions making for diverging directions of development, for division of
labour, and for dissimilar, histological differentiation. The part played
by a cell, as Voechting puts it, will depend upon the position it comes to
assume in the whole living unit. To use an expression of Driesch's,
dissimilar differentiation of cells is a 'function of position.' Such a
conception my brother and I, in our _Studies on the Germ-layer Theory_,
sought to establish clearly by many examples from the histology of the
coelenterates and of higher animals; such a conception for long has been
clearly expressed in physiological botany.
The simpler nature of plants in structure and function makes it easy to
conduct experimental observations upon this point.
I have already described how either side of the prothallus of a fern may be
made to produce male or female organs, according as it is kept in the light
or in the dark. Similarly, taking a willow slip, roots may be made to
appear at one end by moisture and darkness, while they will not appear on
the end kept in the light.
The experiments of botanists and of fruit-growers show that young buds and
the rudiments of roots are indifferent structures, the further growth of
which depends entirely upon the conditions in which they are placed. 'One
and the same bud may grow to a long or short vegetative shoot, to a floral
shoot, to a thorn, or may remain undeveloped. The same root rudiment may
grow to a main tap-root or may form a secondary lateral root. The
conditions that determine the mode in which these structures will develop
are quite within the power of the experimenter. We have shown already and
could show further, that he is able to determine the mode of growth by
cutting, bending, tying in a horizontal position, and so forth: For such
reasons, Voechting describes plants as masses of tissue, practically
plastic, and which may be moulded at the discretion of the investigator.
'For instance, in the case of _Prunus spinosa_, a branch may be produced in
place of a thorn by cutting a growing shoot at the proper height, in
spring. The buds below the point where the cut was made turn to shoots like
the rest of the plant and complete the interrupted growth, while on an
uncut stem they would have grown to thorns. Thus, the rudiment of a thorn
has been changed to that of a shoot' (Voechting).
Although it is more difficult to carry out experiments upon animals, some
good instances are known. If a piece cut from the stem of _Antennularia_ (a
hydroid polyp) be placed vertically, in a short time new branches and new
'roots' spring from it. In this case, again, the position of the new
growths is determined by the relation in which the stem is placed to
gravity. 'The tentacles arise only at the end turned towards the zenith;
the "roots" from the parts directed towards the ground' (Loeb).
A similar example may be taken from among vertebrates. The notochord arises
from a set of cells which are in close relation with the fused tips of the
blastopore. By exposing developing frog's eggs to abnormal conditions, I
was able, in some cases, to produce a hypertrophy of one of the lips of
the blastopore. When fusion of the lips took place the normal lip united
with the rim of the protruding hypertrophied lip. As a result of this the
notochord and the nerve plate came to arise, not from the usual set of
cells, but from those cells that, by the abnormal condition, had come to
lie in the place for the notochord. The protruding cells, which normally
would have developed into notochord and nerve plate, grew into a simple
fold of the external skin.
Moreover, it is well known in pathology that mucous membranes may lose
their proper character and assume the qualities and aspect of the external
skin, when, as in cases of prolapse, fistula, etc., they have been exposed
for some time to the air.
The relations of different parts to each other and to the whole are known
as correlations. Correlation exists in all the stages of the development of
an organism, sometimes in one way, sometimes in another. One must note very
carefully that Weismann's doctrine of determinants, according to which all
that happens in development follows a prearranged plan, is entirely in
opposition to this correlative character of the changes that occur during
development.
Here I shall give a few quotations from botanical and zoological writers:
'If the stem of a plant be cut so that it retains its roots, but is
deprived of leaves and shoots, then the adventitious buds will produce new
leaves and shoots. If, however, the stem be cut so as to deprive it of
roots, then the same cells that in the other case produced leaves and
shoots will now produce roots. Precisely the same occurs with a piece of
the root. In fact, it appears as if the idioplasm knew what parts of the
plant were wanting, and what it must do to restore the integrity and vital
capacity of the individual.' 'The idioplasm in the remaining part of a
plant must be affected when an important part has been removed, because the
idioplasm of the lost part is no longer capable of having influence.' 'It
is clear enough that necessity acts as a stimulus, and that each definite
need calls into existence the appropriate reaction.'
These are Naegeli's views, and they have been elaborated by Pflueger in his
important treatise on _The Teleological Mechanism of Living Nature_ (1877).
Voechting writes in similar fashion:
'In a tree that is growing under normal conditions, without being subjected
to injury, all the organs appear in definite relation to each other: so
many leaves correspond to a definite number of twigs and branches. These
spring from a stem of proportionate thickness, and the stem passes into a
definitely proportioned tap-root, from which arise a due array of lateral
roots. In normal conditions all these organs are in equilibrium. An
apple-tree, growing on the line where tilled garden ground meets a lawn,
grows more vigorously on the side towards the garden. If one of the roots
of an apple-tree with three main roots and three branches be amputated,
then the corresponding branch will lag behind in growth, although it may
not absolutely perish.' 'The equilibrium varies according to the specific
nature of the tree. It is shown in one way in the oak, in another in the
beech, and is different in the varieties of a species.'
Finally, consider this statement from Goebel's _Treatise on the Morphology
and Physiology of the Leaf_: 'The fact that lateral buds do not develop
while the axial bud is still growing vigorously depends upon the relation
between the two. That I denote as correlation of growth.'
The dependence of parts upon each other, and upon the whole, is specially
clear and instructive in cases where different plant individuals are united
by budding or grafting. To limit the growth of a tree, and to induce it to
become dwarfed, it is necessary only to graft it upon a nearly allied but
dwarf variety. When a pear-tree is grafted upon the quince, which is
characterized by its dwarf-like growth, the vegetative growth of the pear
is reduced exceedingly. It produces shorter and weaker shoots; all the
dwarf varieties of the pear employed as wall fruits, or growing into the
little pyramids spoken of in the trade as 'cordon'-trees and potting-trees,
could not have been produced unless the gardener had had the quince as a
natural dwarf stock (Voechting). With the dwarfing is associated a freer and
earlier production of fruit. Other kinds of fruit-trees, apples, apricots,
and so forth, show the same course.
'The capacity to withstand external influences and the duration of life may
be altered in the same way. The pistachio (_Pistazia vera_), cultivated in
Frankfort, which is destroyed by a temperature lower than 7.5 degrees of
frost, will survive 12.5 degrees if it has been grafted upon _P.
terebinthus_. Moreover, when it is grown from a seedling, it may reach the
age of 150 years; but when it has been grafted upon _P. terebinthus_ its
length of life is increased to 200 years; while, grafted on _P. lentiscus_,
it reaches only about 40 years' (Voechting).
Voechting's experiments upon beetroot are still more characteristic. 'The
stem of a beet plant that bore young buds gave rise to vegetative shoots
when it was united with a young, still growing root, but to a blossoming
stem when it had been grafted, in spring, upon an old root.'
Similarly, animal growth is correlative in all its stages. When a muscle
becomes unusually large it sets up corresponding correlations of growth in
many other parts of the body. The bloodvessels and nerves supplying it
become larger, and the increase in the nerves leads to corresponding
increase in the nerve centres. The tendons of origin and of insertion, and
the parts of the skeleton to which these are attached, must react to the
increased size of the muscle by growing larger; in fact for all the parts
of the animal body the conclusions which Naegeli and other physiologists
drew from plants are applicable. All the different elements of the body are
in definite and intimate touch with each other.
This is shown most beautifully and clearly in the extraordinarily
interesting phenomena called dimorphism and polymorphism. These seem to me
to show how very different final results may grow from identical rudiments,
if these, in early stages of development, be subjected to different
external influences.
Finally, I have a little to say about the sexual dimorphism that occurs so
generally in the animal kingdom.
Nearly all kinds of animals appear as male or as females. These differ from
each other not only in that they produce eggs or spermatozoa, but
frequently in a number of more or less striking characters affecting
different parts of the body, and known as secondary sexual characters. In
fact, the difference between the sexes may be so great that a systematic
naturalist, unacquainted with the mode of development of the creatures,
might place them in different species, genera, or even families, on account
of the striking differences in external characters.
As an instance, take _Bonellia_, a gephyrean, the strange case of which has
been remarked upon by Hensen and by Weismann. The male is about a hundred
times smaller than the female, in the respiratory chamber of which it lives
as a kind of parasite, and appears, so far as outward shape goes, more like
a turbellarian than a gephyrean. None the less, male and female are alike
not only while they are in the egg, but as larvae, and it is only towards
the period of sexual maturity that the great difference between them begins
to appear. So also is it with the dwarf males of the cirripedes.
Males and females, whether they be more or less unlike, arise from the same
germinal material. The germinal material itself is sexless; that is to say,
there is not a male and a female germinal material. The phenomena of
inheritance in the sexual generation of hybrids show this clearly.
Characters appropriate both to males and to females are transmitted either
by eggs or by spermatozoa. In parthenogenetic animals both male and female
individuals appear at definite times from eggs produced without sexual
commerce. Whether the male or the female forms be produced depends, not
upon any difference in the germinal material, but on the external
influences, just as external influences determine whether the bud on a twig
shall give rise to a vegetative or to a flowering shoot, to a thorn or to a
stem. The influence of food, of temperature, or probably of other agencies,
determines in which direction the germinal material shall grow.
The experiments of a distinguished French investigator, M. Maupas, on the
determination of sex in _Hydatina senta_, a rotifer, have given striking
results.
In _Hydatina_, under normal conditions the eggs of certain individuals give
rise always to males, of others always to females. By raising or lowering
the temperature at the time when the eggs are being formed in the germaria
of the young females, the experimenter is able to determine whether these
eggs shall give rise to males or to females. After that early time the
character of the egg cannot be altered by food, light, or temperature.
In one experiment, in which five females not yet fully grown were kept in a
room at the temperature of 26 to 28 degrees centigrade, Maupas found that,
of 104 eggs only 3 per cent. gave rise to females, while in the case of
other five young females of the same brood, but kept in a cold chamber at a
temperature of 14 to 15 degrees centigrade, 95 per cent. of females were
produced. In another experiment, young animals were kept for a few days in
the cold, and then, until death, in a higher temperature. Of the eggs
produced while in the cold, 75 per cent. produced females, of those
deposited in the warmth, 81 per cent. became males.
With these results may be compared what happens with many plants. Melons
and cucumbers, which produce on the same stem both male and female flowers,
bear only male flowers in high temperatures, only female flowers when
subjected to cold and damp.
In the case of many insects in which parthenogenesis occurs, the
determination of sex depends upon fertilisation. Thus, among bees,
unfertilised eggs give rise to drones, fertilised eggs to females.
Sexual dimorphism in still another way reveals the intimate interactions
existing between all the parts of an organism in every stage of
development. It is well known, for instance, that among animals the early
removal or destruction of the sexual organs hinders the development of the
secondary sexual characters, or even may occasion the appearance of the
characters of the other sex. Old hens become cock-feathered; human eunuchs
have the high-pitched voice and the peculiarities of the larynx found in
women.
As much as sexual dimorphism, the phenomena of polymorphism show the
enormous influence exerted by external forces upon correlated variation of
the parts during development, and in this way upon the final structure.
In the question of polymorphism it is worth while to discuss at some length
the extreme polymorphism exhibited in the case of some of the colonial
animals--first, because the matter has recently occasioned an important
controversy between Herbert Spencer and Weismann; and, secondly, because
the discussion will serve to make still more clear the difference between
my views and those of Weismann upon the nature of the process of
development.
Among the colonial insects there arise, in addition to males and females,
sexless individuals known as neuters. These in certain cases are very
different from both males and females in structure and in social instincts.
Among bees there are the queens, sexually mature females; the workers,
females whose sexual organs are rudimentary, and parts of whose bodies--the
stings, the wings, the hind legs, with their pollen-collecting
apparatus--are peculiarly formed; and, lastly, the males, or drones.
In many of the ant and termite colonies still greater differences exist
between the different sets of individuals. In addition to males and
females, there are sexless workers, and these is many species are of two
kinds, known as workers and soldiers. The divergences of structure among
the three or four forms are shown, frequently by considerable differences
in size, by the presence and absence of wings, by differences in the
sense-organs, the brain, and the structure of the head. In the common
ant--_Solenopsis fugax_, for instance, as Weismann quotes from Forel--the
males have more than four hundred facets on their eyes, the females about
two hundred, and the workers from six to nine. Many soldiers possess
enormously large and heavy heads, with massive jaws, and naturally, with
the appropriate muscles much enlarged.
But as workers and soldiers, on account of the rudimentary state of their
sexual organs, cannot reproduce themselves, all the three or four kinds of
ants in the colony must be developed from eggs deposited by the females. In
this Weismann finds the most convincing proof of the omnipotence of natural
selection, and, I venture to add, for the omnipotence of his doctrine of
determinants.
He says (_Contemporary Review_, vol. lxiv., p. 313): 'It fortunately
happens that there are animal forms which do not reproduce themselves, but
are always propagated anew by parents which are unlike them. These animals,
which thus cannot transmit anything, have nevertheless varied in the past,
have suffered the loss of parts that were useless, and have increased and
altered others; and the metamorphoses have at times been very important,
demanding the variation of many parts of the body, inasmuch as many parts
must adjust themselves so as to be in harmony with them.' 'None of these
changes' (p. 318) 'can rest on the transmission of functional variations,
as the workers do not at all, or only exceptionally, reproduce. They can
thus only have arisen by a selection of the parent ants, dependent on the
fact that those parents which produced the best workers had always the best
prospect of the persistence of their colony. No other explanation is
conceivable, and it is just because no other explanation is conceivable
that it is necessary for us to accept the principle of natural selection.'
According to Weismann's conception, 'every part of the body of the ant'
(_loc. cit._, p. 326) 'that is differently formed in the males, females,
and workers is represented in the germplasm by three (sometimes four)
corresponding determinants; but on the development of an egg never more
than one of these attains to value--_i.e._, gives rise to the part of the
body that is represented--and the others remain inactive.' This structure
of the germplasm Weismann attributes to the operation of selection. 'For in
the ant state' (_loc. cit._, p. 326) 'the barren individuals or organs are
metamorphosed only by the selection of the germplasm, from which the whole
state proceeds. In respect of selection, the whole state behaves as a
single animal. The state is selected, not the single individuals, and the
various forms behave exactly like the parts of one individual in the course
of ordinary selection.'
Naturally, from the views on the germplasm theory and on the doctrine of
determinants that I have expressed in this book, I cannot accept the
explanation Weismann thus gives of the facts. It is true that Weismann
holds his own explanation to be the only conceivable explanation. 'For
there are only two possible _a priori_ explanations of adaptations for the
naturalist, namely, the transmission of functional variations and natural
selection' (_loc. cit._, p. 336); 'but as the first of these can be
excluded' (on account of the infertility of workers and soldiers), 'only
the second remains.'
But are the alternatives really only as Weismann suggests? Is there no
choice left for the naturalist?
When I was reading his _All-sufficiency of Natural Selection_, kindly sent
me by the author, it came into my mind that I could not accept his dilemma.
For the different individuals in the insect states may be explained in a
third way--in a way overlooked by Weismann. This third explanation is
nothing more than the subject of all this treatise of mine. It is that, in
obedience to different external influences, the same rudiments may give
rise to different adult structures.
I am glad that the same answer has been made to Weismann's _All-sufficiency
of Natural Selection_ by two biologists, Herbert Spencer and Emery,
simultaneously with mine. Emery, a specialist upon the structure of ants,
and Herbert Spencer, relying upon the investigations of several Englishmen,
have sought to prove that the differences between the individuals in the
colonies of ants, bees, and termites, have been slowly called into
existence by the operation of external influences affecting the egg in its
situation and food during development.
It has been shown fully by experiment and by observation that the
fertilised eggs of the queen bee may become either workers or queens. This
depends merely on the cell in the hive in which the egg is placed, and on
what food the embryo is reared. In the specially large cells, known as
queens' chambers, and with specially nutritious diet, they become queens.
With poor food, and in smaller cells, they become workers. Even if worker
larvae be supplied in time with a richer diet, they may be turned into
queens.
Similarly, the differences that exist among termites and ants, as Emery
shows, may be described as polymorphism due to food. The Italian zoologist,
Grassi, has shown that termites have it in their power to alter the
relative numbers of workers and soldiers, and to produce as many of the
latter as may be required, and they are able to accelerate the sexual
maturity of other individuals by supplying nourishment suitable for
stimulating the maturation of the genital organs.
Emery explains this polymorphism by attributing it to the general laws of
growth in the insect organism under the influence of different external
stimuli. He thinks that 'the production of workers depends upon a special
capacity of the germplasm to respond to the abundance or scantiness of
certain nutritive materials by a greater growth of certain parts of the
body, and a lesser growth of other parts. Workers' food stimulates growth
in the jaws and brain, retards growth in the wings and sexual cells.
Queens' food has the opposite action.' There is a correlation between
retardation of the sexual glands and acceleration of the development of the
head, just as in vertebrates there is a correlation between the sexual
glands and the secondary sexual characters. 'The characters by which the
workers differ from the queens, therefore, are not innate, but are produced
secondarily.'
Quite independently, but simultaneously, Herbert Spencer has suggested the
same explanation as Emery. Moreover, he has used the conditions that exist
among the state-forming insects as a strong argument against Weismann's
doctrine of determinants. The observations of many careful persons, such as
Charles Darwin, Emery, and others, show that in many species of ants the
extreme types of individuals are connected by many intermediate forms.
(_Apud_ Emery, this is the case in many _Myrmicidae_, in most _Camponotidae_,
and in _Azteca_.) These forms are transitional, not only in general size,
but in the degree to which the genital organs have been arrested, and in
the peculiarities of the jaws.
Spencer explains these transitional forms, and I agree with him, by
supposing that the stoppage in food supply has taken place at different
times after development has begun. ('It must happen that the stoppage of
feeding will be indefinite.') Thus, the existence of transitional forms
presents no difficulty on the theory of the agency of food. But how can the
doctrine of determinants be applied to it? 'If he is consistent' (says
Spencer, _Contemporary Review_, lxiv., p. 901), 'he must say that each of
these intermediate forms of workers must have its special set of
"determinants," causing its special set of modifications of organs; for he
cannot assume that while perfect females and the extreme types of workers
have their different sets of determinants, the intermediate types of
workers have not. Hence we are introduced to the strange conclusion that,
besides the markedly distinguished sets of determinants, there must be, to
produce these intermediate forms, many other sets slightly distinguished
from one another--a score or more kinds of germplasm, in addition to the
four chief kinds. Next comes an introduction to the still stranger
conclusion, that these numerous kinds of germplasm producing these numerous
intermediate forms are not simply needless, but injurious--produce forms
not well fitted for either of the functions discharged by the extreme
forms, the implication being that natural selection has originated these
disadvantageous forms. If, to escape from this necessity for suicide,
Professor Weismann accepts the inference that the differences among these
numerous intermediate forms are caused by arrested feeding of the larvae at
different stages, then he is bound to admit that the differences between
the extreme forms, and between these and perfect females, are similarly
caused. But if he does this, what becomes of his hypothesis that the
several castes are constitutionally distinct, and result from the operation
of natural selection?'
My course of thought leaves me with little to add to this criticism by
Spencer. In this case, as in many others that I have pointed out, Weismann
makes his usual mistake. He incorporates in the rudiment what really are
stimuli coming from external conditions during the process of development;
he makes a grave confusion between the rudiment and the conditions of its
development.
In my view, in these cases of polymorphism in the colonies of insects
Nature exhibits a series of most important experiments, and their plain
meaning is that the same germinal material, when subjected to different
external influences, may produce very different final products. When from
the neutral germinal material of an insect egg there is produced a male or
female creature, or a worker or soldier (as this or that influence acts),
the process is no other, and presents no greater difficulties, than when an
experimenter, taking the young bud of a plant, according to the conditions
to which he subjects it, can turn it into a vegetative or into a
reproductive shoot, a thorn or a root; no different to what occurs when the
investigator, cutting into a _Cerianthus_, produces a second or third
mouth, surrounded by tentacles, or in the case of _Cione_ surrounded by
eye-spots.
It has been shown, I think, in these pages that much of what Weismann would
explain by determinants within the egg must have a cause outside the egg.
The chief factors in the process of development we have found to be: (1)
The multiplication of cells by division (growth as a moulding factor); (2)
the relations of cells to their external environment (position in its
widest sense as a factor); (3) the interrelations of the parts of a whole
(cells, tissues, and organs) to one another and to the whole (correlative
development). There remains to be considered the extent to which the
germinal material in the egg determines the course of development of the
organism. Here, before all things, it must be insisted that the individual
nature of the cell determines the specific fashion in which the cell will
react to the varying stimuli coming from varying conditions. The same
agency produces very different results upon different organisms. These
differences must be attributed to the differences in the nature (different
intimate structure) of the active material.
Sachs speaks strikingly on this point (_Physiology of Plants_, p. 602): 'If
the same external cause induces exactly opposite effects in the organs, the
explanation of this must simply be sought in the different structure of the
organs. If one organ, when illuminated from one side, becomes curved so as
to be concave on the side turned towards the centre of light, while another
becomes convex on that side, the cause can only lie in the internal
structure of the organ. But it is just on such differences of structure
that the great variety of reactions which the most different plant organs
exhibit towards the same external influences depends; and, fundamentally,
all that we term biology--the mode of life of organisms--depends upon the
fact that different organisms react differently towards the same external
influences, and these reactions differ not only qualitatively, but also
quantitatively, the finest gradations existing in both cases.'
For instance, in a plant-embryo roots are produced at the lower end under
the influence of the soil and of gravity. But it is upon the specific
nature of the protoplasm of different kinds of plants that the special
shape of the whole root system depends: whether, for instance, the root
system ramifies superficially or strikes deep into the soil; whether the
rootlets grow quickly or slowly; in what fashion they fork, and whether or
no they form special structures like bulbs.
Thus, even from my point of view, explanation of the process of development
requires the assumption of the existence of different kinds of germinal
material in different kinds of organisms. These germinal substances must be
possessed of an extraordinarily complex organisation, and must be able to
react in specific fashion--that is to say, in a fashion different in each
species--to all the slightest internal and external stimuli encountered
from time to time as the organisation becomes formed by cell division.
In this sense I agree with what Naegeli says:
'The egg-cells contain all actual specific characters as truly as the adult
organisms; when they exist in the condition of eggs, organisms are as
distinct from each other as in the adult condition. The species is present
as truly in the fowl's egg as in the fowl, and the egg of a fowl differs as
much from the egg of a frog as the fowl differs from the frog. Men,
rodents, ruminants, invertebrates display more or less important and
outwardly visible differences in constitution; so also the sexual cells to
which they give rise, since they represent the rudiments of the future
adults, must be different from each other in the constitution of the
rudiments, although we are not yet able to prove these differences by
observation.'
In this assumption of a specific and highly-organized germinal substance
with which a development begins, I agree with evolutionists; but in its
details my conception is quite different from their conception. For I can
ascribe to the germinal substance only such characters as are appropriate
to the true nature of a cell, but I cannot ascribe to it the numerous
characters that can come into existence only by the interrelations of many
cells and the action of the environment.
Haacke, in his recently-published book (_Gestaltung und Vererbung)_, has
expressed a doubt that my conception of development is, after all, a
preformational theory. 'For preformation,' he says, 'it is not necessary to
imagine that the egg contains a miniature of the adult. If only, like
Hertwig, one assumes to be present in the germinal material a
prearrangement of qualitatively different idioblasts, one has steered into
the harbour of preformation with all sails set.'
In reply, I plead that, like Naegeli, De Vries, Driesch, and others, I have
tried to blend all that is good in both theories. My theory may be called
_evolutionary_, because it assumes the existence of a specific and
highly-organised initial plasm as the basis of the process of development.
It may be called _epigenetic_, because the rudiments grow and become
elaborated, from stage to stage, only in the presence of numerous external
conditions and stimuli, beginning with the metabolic processes preceding
the first cleavage of the egg-cell, until the final product of the
development is as different from the first rudiment as adult animals and
plants differ from their constituent cells.
To explain more clearly my conception of the nature of the process of
development, especially in the relations that I conceive to exist between
the rudiment and the adult, I shall conclude by reverting to my comparison
between a human community and an organism.
As a man arises from an egg-cell by cell multiplication and cell
differentiation, so the human community, a composite organism of a still
higher nature, has arisen from separate human beings as its starting-point.
Culture and civilization are the wonderfully complicated results of the
co-operation of many individuals united in society. By the manifolding of
their relations and their combinations, men in society have brought about a
higher complexity than man, left by himself, ever would have been able to
develop from his own individual properties--a complexity that has arisen
by the interaction of the same characters of many men in co-operation.
Similarly the activity of the egg in growth and cell-formation is an
inexhaustible source of new complexity; for the self-multiplying systems of
units, always binding themselves into higher complexes, continually enter
into new interrelations, and afford the opportunity for new combinations of
forces--in fact, of new characters.
Both cases--the course of the development of the egg-cell into a man, and
of men into a state--depend upon epigenesis, not upon evolution.
The comparison may be carried into details.
The more complex and higher organisation of human society occurs in this
fashion: of the numerous single individuals, all of which are endowed with
the various incipient human characters, some individuals elaborate some
incipient characters, others other characters, and these come to play
correspondingly different parts. The special differentiation undergone by
any individual depends upon the special place he comes to occupy in the
whole of which he is a part, not upon really different organisation
residing in him from his birth. Beside those characters which have
developed specially in his case, there lie dormant the rudiments of all the
characters possessed by men, and, under different conditions, these might
have come to development.
Differentiation in multicellular organisms takes a similar course. Every
cell, by doubling division of the egg, receives all the rudiments of its
kind; of these rudiments, some in one set of cells, others in another,
come to develop, according to the part of the whole in which the cells come
to lie during the progress of the development, and according to the
relations to the whole they come to assume. Thus, here they assume the
characters of the external skin; there, they become gland-cells of the
intestine; here, muscle-fibres; there, sense-cells or nerve-cells; in one
place they serve the whole organism, in the form of blood-corpuscles, as
agents for nutrition and respiration; there, becoming connective tissue or
bone, they form skeletal elements of the body.
Thus, during the course of development, they are forces external to the
cells that bid them assume the individual characters appropriate to their
individual relations to the whole; the determining forces are not within
the cells, as the doctrine of determinants supposes. The cells develop
those characters that are suggested by their relation to the external world
and their places in the whole organism.
But I must insist here that the subordination of the cells to the whole
organism, in both multicellular animals and in plants, is much more
complicated than that of the units to the human state. In the latter case,
the individuals are separate from one another; they are independent
organisms and are bound together only in social relations. None the less,
consider how in a civilized state the apparently sovereign individual is
conditioned in all his circumstances; how each change in the general state
exercises an influence on the individual's disposition freedom of will, and
method of life (dwelling, food, institutions, health); then reflect how
much greater in the animal and the plant is the domination of the whole,
and the subordination of the units, as in them cell is directly joined to
cell--indeed, in most cases united materially by threads of protoplasm. In
such cases the self-sufficiency of the cell as an elementary, living
organism is so far prevented, that it becomes a subordinate part, with its
function in dependence on the whole.
One other point our comparison will make clearer: I refer to the relation
of the specific nature of the rudiment to the specific nature of the
product of the rudiment.
The different organisations and qualities of the communities formed by
different animals may be explained by the special characters of the animals
forming them. Those of the bee colonies depend on the nature of bees; of
ant colonies on the nature of ants; of the societies of men on the nature
of men; indeed, in the latter case we see how they differ as they are
formed by Italians, Germans, Slavs, Turks, Chinese, or Negroes. Similarly,
the specific organisation of the cell determines the kind of animal which
may be built up by it.
In my theory two assumptions of totally contrasting nature are made: I
assume a germplasm of high and specific organisation, and I assume that
this is transformed into the adult product by epigenetic agencies. To a
certain extent, therefore, I reconcile the opposition between evolution and
epigenesis, these opponents so prominent last century.
But my theory does not pretend to explain all the many problems involved
in the course of organic development. In this respect it differs from
Weismann's doctrine of determinants, as that is a closed system, finding
within itself a formal explanation of all development. So far it seems to
me an abandonment of explanation rather than an explanation; for it
explains by signs and tokens that elude verification and experiment, and
that cannot encounter concrete investigation. His explanation is no more
than a description, in other words, of the visible events of development.
To be more than this, it would be necessary to explain how in each case the
biophores and determinants and ancestral plasms are constituted, and how
they are arranged in the architecture of the germplasm so as to produce the
development of the egg-cell in this or that fashion. It must, at the least,
offer such possibilities as the structural formulae of chemists offer. But
in the present stage of our knowledge Weismann's method is unpromising; it
merely transfers to an invisible region the solution of a problem that we
are trying to solve, at least partially, by investigation of visible
characters; and in the invisible region it is impossible to apply the
methods of science. So, by its very nature, it is barren to investigation,
as there is no means by which investigation may put it to the proof. In
this respect it is like its predecessor, the theory of preformation of the
eighteenth century.
FOOTNOTES:
[17] The second section contains references to the following treatises:
C. V. NAEGELI: _Mechanisch-physiologische Theorie der Abstammungslehre_
(1884).
HERTWIG, OSCAR: _Lehrbuch der Entwicklungsgeschichte des Menschen und der
Wirbelthiere_; 4th edit.
SACHS: _Lectures on Plant Physiology_; English edition, Clarendon Press.
VOECHTING: _Ueber die Theilbarkeit im Pflanzenreich und die Wirkung innerer
und aeusserer Kraefte auf Organbildung an Pflanzentheilen._ _Pflueger's
Archiv._, vol. xv., 1877.
Ibid.: _Ueber Organbildung im Pflanzenreich_, 1, 2; Bonn, 1878, 1884.
GOEBEL: _Beitraege zur Morphologie und Physiologie des Blattes._ _Bot.
Zeit._, 1880.
PFLUeGER: _Die teleologische Mechanik der lebendigen Natur_; Bonn, 1877.
MAUPAS: _Sur le determinisme de la sexualite chez l'Hydatina senta._
_Comptes rendus des seances de l'Academie des Sciences_; Paris, 1891.
WEISMANN: _Die Allmacht der Naturzuechtung. Eine Erwiderung an Herbert
Spencer_; Jena, 1893.
HERBERT SPENCER: _A Rejoinder to Professor Weismann._ _Contemporary
Review_, 1893.
Ibid.: _Die Unzulaenglichkeit der 'Natuerlichen Zuchtwahl.'_ _Biol.
Centralblatt_, vol. xiv., No. 6.
EMERY: _Die Entstehung und Ausbildung des Arbeiterstandes bei den Ameisen._
_Biol. Centralb._, vol. xiv., No. 2, 1894.
HAACKE; _Gestaltung and Vererbung_ (1894).
[18] The assumption of doubling division does not involve the assumption
that the germinal mass is unalterable. Although I do not regard the process
of division as a mechanism for breaking up the idioplasm into dissimilar
groups of determinants, I regard the idioplasm--and here I agree with
Naegeli--as only relatively stable. In course of time external and internal
forces may slowly alter it. On the one hand, the idioplasm of the
reproductive cells in the course of generations may slowly alter, while, on
the other hand, the idioplasm of cell groups in an organism may acquire a
local character in correspondence with their different topographical and
functional positions in the whole creature, and in relation to their place
in the organic division of labour, just as in human communities individuals
become altered by the lifelong exercise of some calling.
Nor does the doctrine of doubling divisions conflict with those conclusions
of pathology according to which, in the process of regeneration, cells and
tissues give rise only to cells and tissues of their own order. For further
details see my treatise, _Ei und Samen-Bildung bei Nematoden_, pp. 97-99.
These slight suggestions are only to prevent misconceptions.
INDEX AND GLOSSARY
A
ACINETA, a group of protozoa, development of, 41.
Acquired characters, question of their inheritance, x.
Amphioxus, a marine animal, representative of the primitive vertebrate
stock, experiments on eggs of, 61.
Anabolism, the formation of more complex chemical bodies by the agency of
protoplasm, 86.
Animal cells, characteristic mode of growth, 111.
Antennularia, Loeb's experiment, 117.
Ants, polymorphism in, 125.
Ascidians, tunicate animals, 46.
Atavism, the occurrence in an organism of a character abnormal in it, but
normal in an ancestor, 24.
B
Bees, polymorphism in, 125.
Beetroot, grafting experiments, 121.
Begonia, reproduction from leaves, 46.
BEET, experiments on rats, 73.
BERESOWSKY, skin-grafting, 75.
BEYERINCK, upon galls, 51.
Biophores. Each determinant, according to Weismann, is composed of a number
of ultimate living pieces, the biophores, which are the active agents that
direct the functions of a mature cell, ix, 22.
Blastosphere,
an early stage in embryonic development; the embryo consists of a hollow
sphere, the walls of which consist of a single layer of cells, and the
cavity of which is called the segmentation cavity, xvii;
explanation of formation, 97, 98.
Blood, transfusion of, 75.
BLUMENBACH, _nisus formativus_, 5;
upon galls, 50.
Bone-grafting, 73, 74.
Bonellia, sexual dimorphism in, 122.
Bryozoa, a group of minute animals which form encrustations on seaweeds and
stones, 46.
Buds, origin of, 28;
reproduction and regeneration by, 46.
C
Cell, description of, 31;
characters possible in, 88;
differentiation of, in development, 112;
as units in morphology and physiology, 113;
Sachs on, 114;
Voechting on, 114, 116.
Cell theory, relation of, to heredity, 31.
Centrosome, an organ of cells most obvious during nuclear division, 93.
Cerianthus, experimental heteromorphoses, 51.
CHABRY, destruction of segmentation sphere, 62.
Chromatin, a material found in the nucleus of cells, so called because it
absorbs stains with avidity: germplasm and, viii, xiv;
relation of, to specific character of cells, 36, 37.
Chromosomes, definite, visible bodies, as which the chromatin of a dividing
nucleus appears, xiv, 93.
Crystal, growth of, compared with organic growth, 108.
Cione, experimental heteromorphoses, 52.
Clavellina, reproduction from buds, 46.
Cleavage-planes,
the planes separating the daughter-nuclei, or daughter-cells, in the
early division of a fertilised egg-cell, xvii;
relation between appearance of, and structure of eggs, 95.
Coelenterata, a major division of multicellular animals, including such
creatures as sea-anemones, corals, and jelly-fish, 46.
Continuity of the germplasm, 26.
Continuity of life, the doctrine opposed to spontaneous generation, 2.
Correlations, 118, 121.
D
DARWIN, pangenesis, 21.
Determinants. Each _id_ of germplasm is supposed by Weismann to be composed
of minor pieces, arranged in a complicated fashion that is the result of
the past history of the species. For every part of the body, large or
small, that may be different in different individuals or species, there is,
at least, one determinant in the _id_. The determinants are so grouped in
the _id_ that they are liberated and become active when the time comes for
the development of that part of the body they control, viii, 22;
arguments against, 82;
relation to cells, 87.
Determinates, the smallest parts of an organism which vary independently,
and which are supposed by Weismann to be represented in the germplasm by
special pieces, 23, 25.
Differentiating division, such a division of the nucleus as would result in
daughter-nuclei unlike each other, and unlike the parent nucleus. The
qualities of the parent nucleus are supposed to have been distributed
between the daughter-nuclei, xi;
absence of visible evidence for, xv, 25;
objections to occurrence of, 34, 78.
Dimorphism, the appearance of the same species in two different forms,
sexual dimorphism, 122, 124.
Disharmonic union in grafting, 70.
Double monsters, as examples of heteromorphosis, 63.
Doubling division. When an amoeba reproduces by simple division, the
daughter-amoebae are identical, and each is identical with the parent except
in size; from one amoeba two have been formed. A doubling division of the
nucleus is such as would result in the formation of two nuclei alike in
every respect, ix;
visible evidence for, xv, 24;
in unicellular organisms, 40;
occurrence of, with differentiating division, 78.
DRIESCH, experiments on eggs, 54;
separation of segmentation spheres, 60.
E
Echinoderms, a group of marine animals, of which the star-fish is the most
familiar type, eggs of, 54.
Echinoidea, a group of echinoderms, 61.
Ectoderm, the tissue in an adult derived from the epiblast (which see), 19.
Egg, relation between structure and division of, 94;
specific character of, 135.
EMERY, on polymorphism in ants, 128.
Endoderm, the tissue in an adult, derived from the hypoblast (which see), 19.
Enfoldment. See Evolution.
Epiblast. In the development of all multicellular animals, the young embryo
soon becomes divided into two sets of cells, the epiblast and hypoblast;
where a gastrula is formed, the outer layer of cells is the epiblast, the
inner layer the hypoblast, xviii.
Epigenesis, the doctrine that the formation of a new individual is not the
mere out-growing of particles hidden in the egg-cell, but the result of
moulding external forces, xiii;
Roux's definition of, 7;
Weismann's denial of, 9;
epigenetic explanation of stages in development, 98;
summary of Hertwig's acceptance of, 136.
Evolution. Originally the term was applied, not to the origin of existing
forms of life from common ancestors, but to the doctrine that every living
creature contained within it the whole series of its future descendants,
and that the growth of a living creature was evolving of one of these
enfolded miniatures, xiii, 1, 2, 3;
Roux's contrast of, with epigenesis, 6;
the new evolution, 10;
Hertwig's partial agreement with, 135, 136.
Experiment, Weismann's caution against, 10.
F
Fertilisation, the union of the nuclear matter of a male cell with the
nuclear matter of a female cell, xii, xiv.
Foraminifera, a group of protozoa provided with shells, 44.
FOREL, on eyes of ants, 126.
Frogs' eggs, Hertwig's experiments upon; development of, under compression,
57-60.
Funaria, reproduction from chopped pieces, 46.
G
Galls, 50.
Gastrula, an early embryonic stage, most simply formed from the
blastosphere by the invagination of one side of the wall, and consisting of
a hollow sac, the walls of which are formed by two layers of cells,
xviii, 60;
formation of, 99.
Gemmules. See Pangenesis.
Germ, the youngest embryonic stage of an individual or organ, 10.
Germplasm, the substance supposed to be the material bearer of inherited
qualities: Weismann's conception of, viii, 20;
identification of, with nuclear matter, 21;
account of Weissmann's theory, 21-28.
Germ-tracks, the hypothetical paths along which germplasm passes in an
unaltered condition during development, 27;
objections to, 81.
GOEBEL, on plasticity of plants, 120.
Grafting, 68, 70;
of Hydra, 72;
bone-grafting, 73, 74;
skin-grafting, 74, 120, 121.
GRASSI, polymorphism due to food, 129.
Gregarines, a group of parasitic protozoa, development of, 41.
H
HAACKE, declaration that Hertwig is evolutionary, 135.
Haemoglobin, the red colouring matter of blood, 75.
Harmonic union in grafting, 70.
Heteromorphosis, explanation of, 49;
cases of, 51, 52;
embryonic cases, 54.
His, presence of foci in the germ, 13.
Histogenous, producing microscopical characters, 20.
Histology, study of the microscopical characters of cells and tissues,
differentiation, 115.
Hydatina, determination of sex, 5;
temperature, 123.
Hydra, regeneration in, 47;
grafting of, 72.
Hydromedusae, a group of invertebrate animals, the typical members of which
are branched colonies of polyps: Weismann's investigations on, viii, xii.
Hypoblast. See Epiblast, xvi.
Hypotrichous infusoria, a group of protozoa, 41.
I
_Ids_, hypothetical individual pieces, a number of which are supposed by
Weismann to be present in the germplasm of every sexual cell, and each of
which is supposed to contain the inherited material necessary for a
complete new organism. It has been suggested that tiny beads seen within
the chromosomes of a sexual cell are the _ids_, viii, 23, 33.
Idioblasts, Hertwig's name for hypothetical ultimate units of living
matter, 22, 82;
the ultimate units of living matter, according to De Vries, 22.
Idioplasm, as opposed to germplasm, which is the nuclear material of
germ-cells; idioplasm is the nuclear material of tissue-cells, xi, 38.
Immortality, definition of, 82;
of germ-cells, ix;
of unicellular organisms, 17;
of germ-cells, 80.
Individuality of cells, 115.
Invagination, the infolding of a layer of cells, as, for instance, in the
transformation of a blastosphere into a gastrula, xvii.
Isotropism, explained in footnote, 33.
K
Karyokinesis, a complicated process of nuclear division, xiv.
Katabolism, the formation of less complex chemical bodies by the agency of
protoplasm, 86.
L
Labile, unstable, constantly changing, 38.
LANDOIS, experiments on transfusion of blood, 75.
LEIBNITZ, on immortality, 82.
LOEB, on heteromorphoses, 49;
on plasticity of animals, 117.
M
MAUPAS, experiments on sex of rotifers, 123.
Melons, determination of sex by temperature, 124.
Mesoblast, in the development of the coelomata, or three-layered
multicellular animals; a third set of cells, the mesoblast, arises between
the epiblast and hypoblast, xviii.
Monsters, relation of, to division of egg-cell, 63.
Mosaic theory of Roux, 56.
Morphoplasm, the general protoplasm of a cell, 35.
Multicellular organisms, those in which the body is composed of many cells,
specialized in different directions; cell-division in, 43.
Mus, experiments on grafting among mice and rats, 74.
Myxomycetes, sometimes called 'slime fungi,' a group of low organisms,
consisting of creeping masses of protoplasm with many nuclei, 33.
N
NAEGELI, biological units, 30;
cross-fertilization and grafting compared, 69;
heredity, 92;
environment in development, 104;
on plasticity of plants, 119;
on specific characters of eggs, 134.
Nais, regeneration in, 47.
Notochord, formation of, from unusual cells, 117.
Nucleus, a specialized portion of the protoplasm of cells, different in
chemical and physical properties (see Chromatin, Chromosomes), as the
bearer of heredity, 19.
NUSSBAUM, views on origin of germ-cells, 17.
Nutrition, influence of, on development, 2.
O
OLLIER, bone-grafting, 73.
Ontogeny, the development of an individual from the egg upwards, 9.
Osteoblasts, cells which are the active agents in bone-formation, 73.
Ovogenesis, the formation of egg-cells in the ovary, 13.
P
Pangenesis, Darwin's provisional hypothesis, that the sexual cells were
composed of minute particles (gemmules), given off by all the cells of the
body, 21.
Periosteum, a cellular sheath of bones, 73.
Physiological units, Herbert Spencer's name for hypothetical ultimate units
of living matter, 22.
Pistachio, influence of temperature on, 121.
Plant-cells, mode of growth, 110.
Plasomes, Hertwig's name for theoretical units of protoplasm, 32.
Plasticity of plant tissues, 117, 119, 120.
Pluteus, a free-swimming larval stage in the development of echinoderms, 54.
Podophrya, reproduction of, 41.
Polymorphism, the appearance of the same species in several different forms
in ants and social insects, 125.
PONFICK, on transfusion of blood, 75.
Preformation, identical with the original meaning of evolution, which see.
Prothallus, the leaf-shaped green organism that grows from the spore of a
fern and produces sexual organs, 49.
Pseudopodia, extensions of protoplasm beyond the general contour of the
cell, 41.
R
Radiolaria, a group of protozoa, 44.
Regeneration in plants and animals, 45, 47.
Rhipsalis grafted on Opuntia, 71.
ROUX, contrast between epigenesis and evolution, 6;
mosaic theory of, 56.
Rudiment, used here as a translation for the word _anlage_, which means the
first plotting-out or beginning of a living structure. Darwin showed that
rudimentary organs in adult creatures were for the most part vestiges of
organs that had lost their use. In this treatise 'rudiment' is applied to
an organ or structure in its incipient condition, whether that incipient
state be visible in a young embryo, or a hypothetical structure in the
germplasm, 6;
latent rudiments, 37.
S
SACHS, on cells, 114;
on reaction and protoplasm, 133.
Salix purpurea, reproduction from galls, 51.
SCHMITT, bone-grafting, 74.
Segmentation, the early division of a developing egg, xvii.
Segmentation spheres, the cells resulting from the early divisions of a
developing egg, separation of, by Wilson and Driesch, 60.
Segmentation cavity. See Blastosphere.
Sex, determination of, by temperature, 123, 124.
Sexual cells (spermatozoa in male, ova or egg-cells in female), the
nucleated pieces of protoplasm which are the starting-point of the new
generation in sexual reproduction, origin of, 18.
Soma, the body of a plant or animal as contrasted with the reproductive
cells contained within it, 45.
Somatic cells, the cells of the soma; mortality of, 17.
SPENCER, HERBERT, controversy with Weismann on polymorphism in
insects, 125.
Spermatogenesis, the formation of spermatozoa in the testis, 13.
Spontaneous generation, 2.
Stolon, a strand of tissue connecting the individuals of colonial
animals, 46.
STRASBURGER, the value of the nucleus in heredity, 13, 18.
T
Termites, polymorphism in, 125.
Transfusion of blood, 75.
Transplantation of bone, 73, 74.
TREMBLEY, grafting of Hydra, 72.
Triton, an amphibian, experiments on the egg by constriction, 64.
Tubularia, experimental heteromorphoses, 51.
Tunicata, a group of marine animals clad with a leathery tunic, 14.
U
Unicellular organisms, animals (protozoa) and plants (protophyta) with the
simplest structure, each being a single cell: immortality of, 17;
division doubling in, 40.
Unit, definition of a biological, 30.
V
Vegetative affinity, 66 _et seq._
Vertebrates, regeneration of lost parts, 47.
VOECHTING, experiments on grafting, 70;
harmonic and disharmonic union, 70;
on cells, 114, 116;
on plasticity of plants, 117, 119;
on grafting, 120.
W
WEISMANN and preformation, 8-10;
caution against experiment, 12;
sources of his theory, 20, 21;
Hertwig's description of his theory, 22;
absence of proof for differentiating division, 34;
symmetry of egg and adult, 55;
immortality of germ-cells, 17, 80, 82;
germ-tracks, 83;
doubling division, 102;
controversy with Spencer, 125.
Willow, reproduction from slips, 46.
Wilson, separation of segmentation spheres of amphioxus egg, 60.
WOLFF, _Theoria Generationis_, 4.
Wounds, healing of, in relation to idioplasm, xii.
Y
Yolk, nutritive material stored in an egg-cell, xvi.
THE END.
BILLING AND SONS, PRINTERS, GUILDFORD.
Heinemann's Scientific Handbooks.
MANUAL OF BACTERIOLOGY.
By A. B. GRIFFITHS, Ph.D., F.R.S. (Edin.),
F.C.S. Crown 8vo., Illustrated, 7_s._ 6_d._
MANUAL OF ASSAYING GOLD,
SILVER, COPPER, AND LEAD ORES. By
WALTER LEE BROWN, B.Sc. Revised, Corrected,
and considerably Enlarged, with a
chapter on the Assaying of Fuel, etc. By
A. B. GRIFFITHS, Ph.D., F.R.S. (Edin.),
F.C.S. Crown 8vo., Illustrated, 7_s._ 6_d._
GEODESY. By J. HOWARD GORE.
Crown 8vo., Illustrated, 5_s._
THE PHYSICAL PROPERTIES
OF GASES. By ARTHUR L. KIMBALL, of the
Johns Hopkins University. Crown 8vo.,
Illustrated, 5_s._
HEAT AS A FORM OF ENERGY.
By Professor R. H. THURSTON, of Cornell
University. Crown 8vo., Illustrated, 5_s._
LONDON:
WILLIAM HEINEMANN,
21, BEDFORD STREET, W.C.
End of Project Gutenberg's The Biological Problem of To-day, by Oscar Hertwig