THE CELL AND PROTOPLASM.
==Vital Properties.==--We have seen that the general activities of the
body are intelligible according to chemical and mechanical laws,
provided we can assume as their foundation the simple vital properties
of living phenomena. We must now approach closer to the centre of the
problem, and ask whether we can trace these fundamental properties to
their source and find an explanation of them.
In the first place, what are these properties? The vital powers are
varied, and lie at the basis of every form of living activity. When we
free them from complications, however, they may all be reduced to four.
These are: (1) _Irritability_, or the property possessed by living
matter of reacting when stimulated. (2) _Movement_, or the power of
contracting when stimulated. (3) _Metabolism_, or the power of absorbing
extraneous food and producing in it certain chemical changes, which
either convert it into more living tissue or break it to pieces to
liberate the inclosed energy. (4) _Reproduction_, or the power of
producing new individuals. From these four simple vital activities all
other vital actions follow; and if we can find an explanation of these,
we have explained the living machine. If we grant that certain parts of
the body can assimilate food and multiply, having the power of
contraction when irritated, we can readily explain the other functions
of the living machine by the application of these properties to the
complicated machinery of the body. But these properties are fundamental,
and unless we can grasp them we have failed to reach the centre of the
problem.
As we pass from the more to the less complicated animals we find a
gradual simplification of the machinery until the machinery apparently
disappears. With this simplification of the machinery we find the
animals provided with less varied powers and with less delicate
adaptations to conditions. But withal we find the fundamental powers of
the living organisms the same. For the performance of these fundamental
activities there is apparently needed no machinery. The simple types of
living bodies are simple in number of parts, but they possess
essentially the same powers of assimilation and growth that characterize
the higher forms. It is evident that in our attempt to trace the vital
properties to their source we may proceed in two ways. We may either
direct our attention to the simplest organisms where all secondary
machinery is wanting, or to the smallest parts into which the tissues of
higher organisms can be resolved and yet retain their life properties.
In either way we may hope to find living phenomena in its simplest form
independent of secondary machinery.
But the fact is, when we turn our attention in these two directions, we
find the result is the same. If we look for the lowest organisms we find
them among forms that are made of a single _cell_, and if we analyze the
tissues of higher animals we find the ultimate parts to be _cells_.
Thus, in either direction, the study of the cell is forced upon us.
Before beginning the study of the cell it will be well for us to try to
get a clear notion of the exact nature of the problems we are trying to
solve. We wish to explain the activities of life phenomena in such a way
as to make them intelligible through the application of natural forces.
That these processes are fundamentally chemical ones is evident enough.
A chemical oxidation of food lies at the basis of all vital activity,
and it is thus through the action of chemical forces that the vital
powers are furnished with their energy. But the real problem is what it
is in the living machine that controls these chemical processes. Fat and
starch may be oxidized in a chemist's test tubes, and will there
liberate energy; but they do not, under these conditions, manifest vital
phenomena. Proteid may be brought in contact with oxygen without any
oxidation occurring, and even if it is oxidized no motion or
assimilation or reproduction occurs under ordinary conditions. These
phenomena occur only when the oxidation takes place _in the living
machine_. Our problem is then to determine, if possible, what it is in
the living machine that regulates the oxidations and other changes in
such a way as to produce from them vital activities. Why is it that the
oxidation of starch in the living machine gives rise to motion, growth,
and reproduction, while if the oxidation occurs in the chemist's
laboratory, or even in a bit of dead protoplasm, it simply gives rise to
heat?
One of the primary questions to demand attention in this search is
whether we are to find the explanation, at the bottom, a _chemical_ or a
_mechanical_ one. In the simplest form of life in which vital
manifestations are found are we to attribute these properties simply to
chemical forces of the living substance, or must we here too attribute
them to the action of a complicated machinery? This question is more
than a formal one. That it is one of most profound significance will
appear from the following considerations:
Chemical affinity is a well recognized force. Under the action of this
force chemical compounds are produced and different compounds formed
under different conditions. The properties of the different compounds
differ with their composition, and the more complex are the compounds
the more varied their properties. Now it might be assumed as an
hypothesis that there could be a chemical compound so complex as to
possess, among other properties, that of causing the oxidation of food
to occur in such a way as to produce assimilation and growth. Such a
compound would, of course, be alive, and it would be just as true that
its power of assimilating food would be one of its physical properties
as it is that freezing is a physical property of water. If such an
hypothesis should prove to be the true one, then the problem of
explaining life would be a chemical one, for all vital properties would
be reducible to the properties of a chemical compound. It would then
only be necessary to show how such a compound came into existence and we
should have explained life. Nor would this be a hopeless task. We are
well acquainted with forces adequate to the formation of chemical
compounds. If the force of chemical affinity is adequate under certain
conditions to form some compounds, it is easy to conceive it as a
possibility under other conditions to produce this chemical living
substance. Our search would need then to be for a set of conditions
under which our living compound could have been produced by the known
forces of chemical affinity.
But suppose, on the other hand, that we find this simplest bit of living
matter is not a chemical compound, but is in itself a complicated
machine. Suppose that, after reducing this vital substance to its
simplest type, we find that the substance with which we are dealing not
only has complex chemical structure, but that it also possesses a large
number of structural parts adapted to each other in such a way as to
work together in the form of an intricate mechanism. The whole problem
would then be changed. To explain such a machine we could no longer call
upon chemical forces. Chemical affinity is adequate to the explanation
of chemical compounds however complicated, but it cannot offer any
explanation for the adaptation of parts which make a machine. The
problem of the origin of the simplest form of life would then be no
longer one of chemical but one of mechanical evolution. It is plain then
that the question of whether we can attribute the properties of the
simplest type of life to chemical composition or to mechanical structure
is more than a formal one.
==The Discovery of Cells.==--It is difficult for us to-day to have any
adequate idea of the wonderful flood of light that was thrown upon
scientific and philosophical study by the discoveries which are grouped
around the terms cells and protoplasm. Cells and protoplasm have become
so thoroughly a part of modern biology that we can hardly picture to
ourselves the vagueness of knowledge before these facts were recognized.
Perhaps a somewhat crude comparison will illustrate the relation which
the discovery of cells had to the study of life.
Imagine for a moment, some intelligent being located on the moon and
trying to study the phenomena on the earth's surface. Suppose that he is
provided with a telescope sufficiently powerful to disclose moderately
large objects on the earth, but not smaller ones. He would see cities in
various parts of the world with wide differences in appearance, size,
and shape. He would see railroad trains on the earth rushing to and fro.
He would see new cities arising and old ones increasing in size, and we
may imagine him speculating as to their method of origin and the reasons
why they adopt this or that shape. But in spite of his most acute
observations and his most ingenious speculation, he could never
understand the real significance of the cities, since he is not
acquainted with the actual living unit. Imagine now, if you will, that
this supramundane observer invents a telescope which enables him to
perceive more minute objects and thus discovers human beings. What a
complete revolution this would make in his knowledge of mundane affairs!
We can imagine how rapidly discovery would follow discovery; how it
would be found that it was the human beings that build the houses,
construct and run the railroads, and control the growth of the cities
according to their fancy; and, lastly, how it would be learned that it
is the human being alone that grows and multiplies and that all else is
the result of his activities. Such a supramundane observer would find
himself entering into a new era, in which all his previous knowledge
would sink into oblivion.
Something of this same sort of revolution was inaugurated in the study
of living things by the discovery of cells and protoplasms. Animals and
plants had been studied for centuries and many accurate and painstaking
observations had been made upon them. Monumental masses of evidence had
been collected bearing upon their shapes, sizes, distribution, and
relations. Anatomy had long occupied the attention of naturalists, and
the general structure of animals and plants was already well known. But
the discoveries starting in the fourth decade of the century by
disclosing the unity of activity changed the aspect of biological
science.
==The Cell Doctrine==.--The cell doctrine is, in brief, the theory that
the bodies of animals and plants are built up entirely of minute
elementary units, more or less independent of each other, and all
capable of growth and multiplication. This doctrine is commonly regarded
as being inaugurated in 1839 by Schwann. Long before this, however, many
microscopists had seen that the bodies of plants are made up of
elementary units. In describing the bark of a tree in 1665, Robert Hooke
had stated that it was composed of little boxes or cells, and regarded
it as a sort of honeycomb structure with its cells filled with air. The
term cell quite aptly describes the compartments of such a structure, as
can be seen by a glance at Fig. 7, and this term has been retained even
till to-day in spite of the fact that its original significance has
entirely disappeared. During the last century not a few naturalists
observed and described these little vesicles, always regarding them as
little spaces and never looking upon them as having any significance in
the activities of plants. In one or two instances similar bodies were
noticed in animals, although no connection was drawn between them and
the cells of plants. In the early part of the century observations upon
various kinds of animals and plant tissues multiplied, and many
microscopists independently announced the discovery of similar small
corpuscular bodies. Finally, in 1839, these observations were combined
together by Schwann into one general theory. According to the cell
doctrine then formulated, the parts of all animals and plants are either
composed of cells or of material derived from cells. The bark, the wood,
the roots, the leaves of plants are all composed of little vesicles
similar to those already described under the name of cells. In animals
the cellular structure is not so easy to make out; but here too the
muscle, the bone, the nerve, the gland are all made up of similar
vesicles or of material made from them. The cells are of wonderfully
different shapes and widely different sizes, but in general structure
they are alike. These cells, thus found in animals and plants alike,
formed the first connecting link between animals and plants. This
discovery was like that of our supposed supramundane observer when he
first found the human being that brought into connection the widely
different cities in the various parts of the world.
[Illustration: FIG. 7.--A bit of bark showing cellular structure.]
Schwann and his immediate followers, while recognizing that the bodies
of animals and plants were composed of cells, were at a loss to explain
how these cells arose. The belief held at first was that there existed
in the bodies of animals and plants a structureless substance which
formed the basis out of which the cells develop, in somewhat the same
way that crystals arise from a mother liquid. This supposed substance
Schwann called the _cytoblastema_, and he thought it existed between the
cells or sometimes within them. For example, the fluid part of the blood
is the cytoblastema, the blood corpuscles being the cells. From this
structureless fluid the cells were supposed to arise by a process akin
to crystallization. To be sure, the cells grow in a manner very
different from that of a crystal. A crystal always grows by layers being
added upon its outside, while the cells grow by additions within its
body. But this was a minor detail, the essential point being that from a
structureless liquid containing proper materials the organized cell
separated itself.
This idea of the cytoblastema was early thrown into suspicion, and
almost at the time of the announcement of the cell doctrine certain
microscopists made the claim that these cells did not come from any
structureless medium, but by division from other cells like themselves.
This claim, and its demonstration, was of even greater importance than
the discovery of the cells. For a number of years, however, the matter
was in dispute, evidence being collected which about equally attested
each view. It was a Scotchman, Dr. Barry, who finally produced evidence
which settled the question from the study of the developing egg.
The essence of his discovery was as follows: The ovum of an animal is a
single cell, and when it begins to develop into an embryo it first
simply divides into two halves, producing two cells (Fig, 8, _a_ and
_b_). Each of these in turn divides, giving four, and by repeated
divisions of this kind there arises a solid mass of smaller cells (Fig.
8, _b_ to _f_,) called the mulberry stage, from its resemblance to a
berry. This is, of course, simply a mass of cells, each derived by
division from the original. As the cells increase in number, the mass
also increases in size by the absorption of nutriment, and the cells
continue dividing until the mass contains thousands of cells. Meantime
the body of the animal is formed out of these cells, and when it is
adult it consists of millions of cells, all of which have been derived
by division from the original cell. In such a history each cell comes
from pre-existing cells and a cytoblastema plays no part.
[Illustration: FIG. 8.--Successive stages in the division of the
developing egg.]
It was impossible, however, for Barry or any other person to follow the
successive divisions of the egg cell through all the stages to the
adult. The divisions can be followed for a short time under the
microscope, but the rest must be a matter of simple inference. It was
argued that since cell origin begins in this way by simple division, and
since the same process can be observed in the adult, it is reasonable to
assume that the same process has continued uninterruptedly, and that
this is the only method of cell origin. But a final demonstration of
this conclusion was not forthcoming for a long time. For many years some
biologists continued to believe that cells can have other origin than
from pre-existing cells. Year by year has the evidence for such "free
cell" origin become less, until the view has been entirely abandoned,
and to-day it is everywhere admitted that new cells always arise from
old ones by direct descent, and thus every cell in the body of an
animal or plant is a direct descendant by division from the original
egg cell.
==The Cell==.--But what is this cell which forms the unit of life, and to
which all the fundamental vital properties can be traced? We will first
glance at the structure of the cell as it was understood by the earlier
microscopists. A typical cell is shown in Fig. 9. It will be seen that
it consists of three quite distinct parts. There is first the _cell wall
(cw)_ which is a limiting membrane of varying thickness and shape. This
is in reality lifeless material, and is secreted by the rest of the
cell. Being thus produced by the other active parts of the cell, we will
speak of it as _formed_ material in distinction from the rest, which is
_active_ material. Inside this vesicle is contained a somewhat
transparent semifluid material which has received various names, but
which for the present we will call _cell substance_ (Fig. 9, _pr_). It
may be abundant or scanty, and has a widely varying consistency from a
very liquid mass to a decidedly thick jellylike substance. Lying within
the cell substance is a small body, usually more or less spherical in
shape, which is called the _nucleus_ (Fig. 9, _n_). It appears to the
microscope similar to the cell substance in character, and has
frequently been described as a bit of the cell substance more dense than
the remainder. Lying within the nucleus there are usually to be seen one
or more smaller rounded bodies which have been called _nucleoli_. From
the very earliest period that cells have been studied, these three
parts, cell wall, cell substance, and nucleus have been recognized, but
as to their relations to each other and to the general activities of the
cell there has been the widest variety of opinion.
[Illustration: FIG. 9.--A cell; _cw_ is the cell wall; _pr_, the cell
substance; _n_, the nucleus.]
==Cellular Structure of Organisms==.--It will be well to notice next just
what is meant by saying that all living bodies are composed of cells.
This can best be understood by referring to the accompanying figures.
Figs. 10-14, for instance, show the microscopic appearance of several
plant tissues.
[Illustration: FIG. 10.--Cells at a root tip.]
[Illustration: FIG. 11.--Section of a leaf showing cells of different
shapes.]
At Fig. 10 will be seen the tip of a root, plainly made of cells quite
similar to the typical cell described. At Fig. 11 will be seen a bit of
a leaf showing the same general structure. At Fig. 12 is a bit of plant
tissue of which the cell walls are very thick, so that a very dense
structure is formed. At Fig. 13 is a bit of a potato showing its cells
filled with small granules of starch which the cells have produced by
their activities and deposited within their own bodies. At Fig. 14 are
several wood cells showing cell walls of different shape which, having
become dead, have lost their contents and simply remain as dead cell
walls. Each was in its earlier history filled with cell substance and
contained a nucleus. In a similar way any bit of vegetable tissue would
readily show itself to be made of similar cells.
In animal tissues the cellular structure is not so easily seen, largely
because the products made by the cells, the formed products, become
relatively more abundant and the cells themselves not so prominent. But
the cellular structure is none the less demonstrable. In Fig. 15, for
instance, will be seen a bit of cartilage where the cells themselves are
rather small, while the material deposited between them is abundant.
This material between the cells is really to be regarded as an
excessively thickened cell wall and has been secreted by the cell
substance lying within the cells, so that a bit of cartilage is really a
mass of cells with an exceptionally thick cell wall. At Fig. 16 is shown
a little blood. Here the cells are to be seen floating in a liquid. The
liquid is colourless and it is the red colour in the blood cells which
gives the blood its red colour. The liquid may here again be regarded
as material produced by cells. At Fig. 17 is a bit of bone showing small
irregular cells imbedded within a large mass of material which has been
deposited by the cell. In this case the formed material has been
hardened by calcium phosphate, which gives the rigid consistency to the
bone. In some animal tissues the formed material is still greater in
amount. At Fig. 18, for example, is a bit of connective tissue, made up
of a mass of fine fibres which have no resemblance to cells, and indeed
are not cells. These fibres have, however, been made by cells, and a
careful study of such tissue at proper places will show the cells within
it. The cells shown in Fig. 18 (_c_) have secreted the fibrous material.
Fig. 19 shows a cell composing a bit of nerve. At Fig. 20 is a bit of
muscle; the only trace of cellular structure that it shows is in the
nuclei (_n_), but if the muscle be studied in a young condition its
cellular structure is more evident. Thus it happens in adult animals
that the cells which are large and clear at first, become less and less
evident, until the adult tissue seems sometimes to be composed mostly of
what we have called formed material.
[Illustration: FIG. 12.--Plant cells with thick walls, from a fern.]
[Illustration: FIG. 13.--Section of a potato showing different shaped
cells, the inner and larger ones being filled with grains of starch.]
[Illustration: FIG. 14.--Various shaped wood cells from plant tissue.]
[Illustration: FIG. 15.--A bit of cartilage.]
[Illustration: FIG. 16.--Frog's blood: _a_ and _b_ are the cells; _c_ is
the liquid.]
[Illustration: FIG. 17.--A bit of bone, showing the cells imbedded in
the bony matter.]
It must not be imagined, however, that a very rigid line can be drawn
between the cell itself and the material it forms. The formed material
is in many cases simply a thickened cell wall, and this we commonly
regard as part of the cell. In many cases the formed material is simply
the old dead cell walls from which the living substance has been
withdrawn (Fig. 14). In other cases the cell substance acquires peculiar
functions, so that what seems to be the formed material is really a
modified cell body and is still active and alive. Such is the case in
the muscle. In other cases the formed material appears to be
manufactured within the cell and secreted, as in the case of bone. No
sharp lines can be drawn, however, between the various types. But the
distinction between formed material and cell body is a convenient one
and may well be retained in the discussion of cells. In our discussion
of the fundamental vital properties we are only concerned in the cell
substance, the formed material having nothing to do with fundamental
activities of life, although it forms largely the secondary machinery
which we have already studied.
[Illustration: FIG. 18.--Connective tissue. The cells of the tissue are
shown at _c_, and the fibres or formed matter at _f_.]
In all higher animals and plants the life of the individual begins as a
single ovum or a single cell, and as it grows the cells increase rapidly
until the adult is formed out of hundreds of millions of cells. As these
cells become numerous they cease, after a little, to be alike. They
assume different shapes which are adapted to the different duties they
are to perform. Thus, those cells which are to form bone soon become
different from those which are to form muscle, and those which are to
form the blood are quite unlike those which are to produce the hairs. By
means of such a differentiation there arises a very complex mass of
cells, with great variety in shape and function.
[Illustration: FIG. 19. A piece of nerve fibre, showing the cell with
its nucleus at _n_.]
It should be noticed further that there are some animals and plants in
which the whole animal is composed of a single cell. These organisms
are usually of extremely minute size, and they comprise most of the
so-called animalculæ which are found in water. In such animals the
different parts of the cell are modified to perform different functions.
The different organs appear within the cell, and the cell is more
complex than the typical cell described. Fig. 21 shows such a cell. Such
an animal possesses several organs, but, since it consists of a single
mass of protoplasm and a single nucleus, it is still only a single cell.
In the multicellular organisms the organs of the body are made up of
cells, and the different organs are produced by a differentiation of
cells, but in the unicellular organisms the organs are the results of
the differentiation of the parts of a single cell. In the one case there
is a differentiation of cells, and in the other of the parts of a cell.
[Illustration: FIG. 20.--A muscle fibre. The nucleii are shown at _n_.]
[Illustration: FIG. 21.--A complex cell. It is an entire animal, but
composed of only one cell.]
Such, in brief, is the cell to whose activities it is possible to trace
the fundamental properties of all living things. Cells are endowed with
the properties of irritability, contractibility, assimilation and
reproduction, and it is thus plainly to the study of cells that we must
look for an interpretation of life phenomena. If we can reach an
intelligible understanding of the activities of the cell our problem is
solved, for the activities of the fully formed animal or plant, however
complex, are simply the application of mechanical and chemical
principles among the groups of such cells. But wherein does this
knowledge of cells help us? Are we any nearer to understanding how these
vital processes arise? In answer to this question we may first ask
whether it is possible to determine whether any one part of the cell is
the seat of its activities.
==The Cell Wall.==--The first suggestion which arose was that the cell
wall was the important part of the cell, the others being secondary.
This was not an unnatural conclusion. The cell wall is the most
persistent part of the cell. It was the part first discovered by the
microscope and is the part which remains after the other parts are gone.
Indeed, in many of the so-called cells the cell wall is all that is
seen, the cell contents having disappeared (Fig. 14). It was not
strange, then, that this should at first have been looked upon as the
primary part. The idea was that the cell wall in some way changed the
chemical character of the substances in contact with its two sides, and
thus gave rise to vital activities which, as we have seen, are
fundamentally chemical. Thus the cell wall was regarded as the most
essential part of the cell, since it controlled its activities. This the
belief of Schwann, although he also regarded the other parts of the
cell as of importance.
[Illustration: FIG. 22.--An amoeba. A single cell without cell wall. _n_
is the nucleus; _f_, a bit of food which the cell has absorbed.]
This conception, however, was quite temporary. It was much as if our
hypothetical supramundane observer looked upon the clothes of his newly
discovered human being as forming the essential part of his nature. It
was soon evident that this position could not be maintained. It was
found that many bits of living matter were entirely destitute of cell
wall. This is especially true of animal cells. While among plants the
cell wall is almost always well developed, it is very common for animal
cells to be entirely lacking in this external covering--as, for example,
the white blood-cells. Fig. 22 shows an amoeba, a cell with very active
powers of motion and assimilation, but with no cell wall. Moreover,
young cells are always more active than older ones, and they commonly
possess either no cell wall or a very slight one, this being deposited
as the cell becomes older and remaining long after it is dead. Such
facts soon disproved the notion that the cell wall is a vital part of
the cell, and a new conception took its place which was to have a more
profound influence upon the study of living things than any discovery
hitherto made. This was the formulation of the doctrine of the nature
of _protoplasm_.
Protoplasm.--(a) _Discovery_. As it became evident that the cell wall is
a somewhat inactive part of the cell, more attention was put on the cell
contents. For twenty years after the formulation of the cell doctrine
both the cell substance and the nucleus had been looked upon as
essential to its activities. This was more especially true of the
nucleus, which had been thought of as an organ of reproduction. These
suggestions appeared indefinitely in the writings of one scientist and
another, and were finally formulated in 1860 into a general theory which
formed what has sometimes been called the starting point of modern
biology. From that time the material known as _protoplasm_ was elevated
into a prominent position in the discussion of all subjects connected
with living phenomena. The idea of protoplasm was first clearly defined
by Schultze, who claimed that the real active part of the cell was the
cell substance within the cell wall. This substance he proved to be
endowed with powers of motion and powers of inducing chemical changes
associated with vital phenomena. He showed it to be the most abundant in
the most active cells, becoming less abundant as the cells lose their
activity, and disappearing when the cells lose their vitality. This cell
substance was soon raised into a position of such importance that the
smaller body within it was obscured, and for some twenty years more the
nucleus was silently ignored in biological discussion. According to
Schultze, the cell substance itself constituted the cell, the other
parts being entirely subordinate, and indeed frequently absent. A cell
was thus a bit of protoplasm, and nothing more. But the more important
feature of this doctrine was not the simple conclusion that the cell
substance constitutes the cell, but the more sweeping conclusion that
this cell substance is in _all_ cells essentially _identical._ The study
of all animals, high and low, showed all active cells filled with a
similar material, and more important still, the study of plant cells
disclosed a material strikingly similar. Schultze experimented with this
material by all means at his command, and finding that the cell
substance in all animals and plants obeys the same tests, reached the
conclusion that the cell substance in animals and plants is always
identical. To this material he now gave the name protoplasm, choosing a
name hitherto given to the cell contents of plant cells. From this time
forth this term protoplasm was applied to the living material found in
all cells, and became at once the most important factor in the
discussion of biological problems.
The importance of this newly formulated doctrine it is difficult to
appreciate. Here, in protoplasm had been apparently found the foundation
of living phenomena. Here was a substance universally present in animals
and plants, simple and uniform--a substance always present in living
parts and disappearing with death. It was the simplest thing that had
life, and indeed the only thing that had life, for there is no life
outside of cells and protoplasm. But simple as it was it had all the
fundamental properties of living things--irritability, contractibility,
assimilation, and reproduction. It was a compound which seemingly
deserved the name of "_physical basis of life_", which was soon given to
it by Huxley. With this conception of protoplasm as the physical basis
of life the problems connected with the study of life became more
simplified. In order to study the nature of life it was no longer
necessary to study the confusing mass of complex organs disclosed to us
by animals and plants, or even the somewhat less confusing structures
shown by individual cells. Even the simple cell has several separate
parts capable of undergoing great modifications in different types of
animals. This confusion now appeared to vanish, for only _one_ thing was
found to be alive, and that was apparently very simple. But that
substance exhibited all the properties of life. It moved, it could grow,
and reproduce itself, so that it was necessary only to explain this
substance and life would be explained.
(b) _Nature of Protoplasm_.--What is this material, protoplasm? As
disclosed by the early microscope it appeared to be nothing more than a
simple mass of jelly, usually transparent, more or less consistent,
sometimes being quite fluid, and at others more solid. Structure it
appeared to have none. Its chief peculiarity, so far as physical
characters were concerned, was a wonderful and never-ceasing activity.
This jellylike material appeared to be endowed with wonderful powers,
and yet neither physical nor microscopical study revealed at first
anything more than a uniform homogeneous mass of jelly. Chemical study
of the same substance was of no less interest than the microscopical
study. Of course it was no easy matter to collect this protoplasm in
sufficient quantity and pure enough to make a careful analysis. The
difficulties were in time, however, overcome, and chemical study showed
protoplasm to be a proteid, related to other proteids like albumen, but
one which was more complex than any other known. It was for a long time
looked upon by many as a single definite chemical compound, and attempts
were made to determine its chemical formula. Such an analysis indicated
a molecule made up of several hundred atoms. Chemists did not, however,
look with much confidence upon these results, and it is not surprising
that there was no very close agreement among them as to the number of
atoms in this supposed complex molecule. Moreover, from the very first,
some biologists thought protoplasm to be not one, but more likely a
mixture of several substances. But although it was more complex than any
other substance studied, its general characters were so like those of
albumen that it was uniformly regarded as a proteid; but one which was
of a higher complexity than others, forming perhaps the highest number
of a series of complex chemical compounds, of which ordinary proteids,
such as albumen, formed lower members. Thus, within a few years
following the discovery of protoplasm there had developed a theory that
living phenomena are due to the activities of a definite though complex
chemical compound, composed chiefly of the elements carbon, oxygen,
hydrogen, and nitrogen, and closely related to ordinary proteids. This
substance was the basis of living activity, and to its modification
under different conditions were due the miscellaneous phenomena of life.
(c) _Significance of Protoplasm_.--The philosophical significance of
this conception was very far-reaching. The problem of life was so
simplified by substituting the simple protoplasm for the complex
organism that its solution seemed to be not very difficult. This idea of
a chemical compound as the basis of all living phenomena gave rise in a
short time to a chemical theory of life which was at least tenable, and
which accounted for the fundamental properties of life. That theory, the
_chemical theory of life_, may be outlined somewhat as follows:
The study of the chemical nature of substances derived from living
organisms has developed into what has been called organic chemistry.
Organic chemistry has shown that it is possible to manufacture
artificially many of the compounds which are called organic, and which
had been hitherto regarded as produced only by living organisms. At the
beginning of the century, it was supposed to be impossible to
manufacture by artificial means any of the compounds which animals and
plants produce as the result of their life. But chemists were not long
in showing that this position is untenable. Many of the organic products
were soon shown capable of production by artificial means in the
chemist's laboratory. These organic compounds form a series beginning
with such simple bodies as carbonic acid (CO_{2}), water (H_{2}O), and
ammonia (NH_{3}), and passing up through a large number of members of
greater and greater complexity, all composed, however, chiefly of the
elements carbon, oxygen, hydrogen, and nitrogen. Our chemists found that
starting with simple substances they could, by proper means, combine
them into molecules of greater complexity, and in so doing could make
many of the compounds that had hitherto been produced only as a result
of living activities. For example, urea, formic acid, indigo, and many
other bodies, hitherto produced only by animals and plants, were easily
produced by the chemist by purely chemical methods. Now when protoplasm
had been discovered as the "physical basis of life," and, when it was
further conceived that this substance is a proteid related to albumens,
it was inevitable that a theory should arise which found the explanation
of life in accordance with simple chemical laws.
If, as chemists and biologists then believe, protoplasm is a compound
which stands at the head of the organic series, and if, as is the fact,
chemists are each year succeeding in making higher and higher members of
the series, it is an easy assumption that some day they will be able to
make the highest member of the series. Further, it is a well-known fact
that simple chemical compounds have simple physical properties, while
the higher ones have more varied properties. Water has the property of
being liquid at certain temperatures and solid at others, and of
dividing into small particles (i.e., dissolving) certain bodies brought
in contact with it. The higher compound albumen has, however, a great
number of properties and possibilities of combination far beyond those
of water. Now if the properties increase in complexity with the
complexity of the compound, it is again an easy assumption that when we
reach a compound as complex as protoplasm, it will have properties as
complex as those of the simple life substance. Nor was this such a very
wild hypothesis. After all, the fundamental life activities may all be
traced to the simple oxidation of food, for this results in movement,
assimilation, and growth, and the result of growth is reproduction. It
was therefore only necessary for our biological chemists to suppose that
their chemical compound protoplasm possessed the power of causing
certain kinds of oxidation to take place, just as water itself induces
a simpler kind of oxidation, and they would have a mechanical
explanation of the life activities. It was certainly not a very absurd
assumption to make, that this substance protoplasm could have this
power, and from this the other vital activities are easily derived.
In other words, the formulation of the doctrine of protoplasm made it
possible to assume that _life_ is not a distinct force, but simply a
name given to the properties possessed by that highly complex chemical
compound protoplasm. Just as we might give the name _aquacity_ to the
properties possessed by water, so we have actually given the name
_vitality_ to the properties possessed by protoplasm. To be sure,
vitality is more marvelous than aquacity, but so is protoplasm a more
complex compound than water. This compound was a very unstable compound,
just as is a mass of gunpowder, and hence it is highly irritable, also
like gunpowder, and any disturbance of its condition produces motion,
just as a spark will do in a mass of gunpowder. It is capable of
inducing oxidation in foods, something as water induces oxidation in a
bit of iron. The oxidation is, however, of a different kind, and results
in the formation of different chemical combinations; but it is the basis
of assimilation. Since now assimilation is the foundation of growth and
reproduction, this mechanical theory of life thus succeeded in tracing
to the simple properties of the chemical compound protoplasm, all the
fundamental properties of life. Since further, as we have seen in our
first chapter, the more complex properties of higher organisms are
easily deduced from these simple ones by the application of the laws of
mechanics, we have here in this mechanical theory of life the complete
reduction of the body to a machine.
==The Reign of Protoplasm.==--This substance protoplasm became now
naturally the centre of biological thought. The theory of protoplasm
arose at about the same time that the doctrine of evolution began to be
seriously discussed under the stimulus of Darwin, and naturally these
two great conceptions developed side by side. Evolution was constantly
teaching that natural forces are sufficient to account for many of the
complex phenomena which had hitherto been regarded as insolvable; and
what more natural than the same kind of thinking should be applied to
the vital activities manifested by this substance protoplasm. While the
study of plants and animals was showing scientists that natural forces
would explain the origin of more complex types from simpler ones through
the law of natural selection, here in this conception of protoplasm was
a theory which promised to show how the simplest forms may have been
derived from the non-living. For an explanation of the _origin_ of life
by natural means appeared now to be a simple matter.
It required now no violent stretch of the imagination to explain the
origin of life something as follows: We know that the chemical elements
have certain affinities for each other, and will unite with each other
under proper conditions. We know that the methods of union and the
resulting compounds vary with the conditions under which the union takes
place. We know further that the elements carbon, hydrogen, oxygen, and
nitrogen have most remarkable properties, and unite to form an almost
endless series of remarkable bodies when brought into combination under
different conditions. We know that by varying the conditions the chemist
can force these elements to unite into a most extraordinary variety of
compounds with an equal variety of properties. What more natural, then,
than the assumption that under certain conditions these same elements
would unite in such a way as to form this compound protoplasm; and then,
if the ideas concerning protoplasm were correct, this body would show
the properties of protoplasm, and therefore be alive. Certainly such a
supposition was not absurd, and viewed in the light of the rapid advance
in the manufacture of organic compounds could hardly be called
improbable. Chemists beginning with simple bodies like CO_{2} and H_{2}O
were climbing the ladder, each round of which was represented by
compounds of higher complexity. At the top was protoplasm, and each year
saw our chemists nearer the top of the ladder, and thus approaching
protoplasm as their final goal. They now began to predict that only a
few more years would be required for chemists to discover the proper
conditions, and thus make protoplasm. As late as 1880 the prediction was
freely made that the next great discovery would be the manufacture of a
bit of protoplasm by artificial means, and thus in the artificial
production of life. The rapid advance in organic chemistry rendered this
prediction each year more and more probable. The ability of chemists to
manufacture chemical compounds appeared to be unlimited, and the only
question in regard to their ability to make protoplasm thus resolved
itself into the question of whether protoplasm is really a chemical
compound.
We can easily understand how eager biologists became now in pursuit of
the goal which seemed almost within their reach; how interested they
were in any new discovery, and how eagerly they sought for lower and
simpler types of protoplasm since these would be a step nearer to the
earliest undifferentiated life substance. Indeed so eager was this
pursuit for pure undifferentiated protoplasm, that it led to one of
those unfounded discoveries which time showed to be purely imaginary.
When this reign of protoplasm was at its height and biologists were
seeking for even greater simplicity a most astounding discovery was
announced. The British exploring ship Challenger had returned from its
voyage of discovery and collection, and its various treasures were
turned over to the different scientists for study. The brilliant Prof.
Huxley, who had first formulated the mechanical theory of life, now
startled the biological world with the statement that these collections
had shown him that at the bottom of the deep sea, in certain parts of
the world, there exists a diffused mass of living _undifferentiated
protoplasm_. So simple and undifferentiated was it that it was not
divided into cells and contained no nucleii. It was, in short, exactly
the kind of primitive protoplasm which the evolutionist wanted to
complete his chain of living structures, and the biologist wanted to
serve as a foundation for his mechanical theory of life. If such a
diffused mass of undifferentiated protoplasm existed at the bottom of
the sea, one could hardly doubt that it was developed there by some
purely natural forces. The discovery was a startling one, for it seemed
that the actual starting point of life had been reached. Huxley named
his substance _Bathybias_, and this name became in a short time familiar
to every one who was thinking of the problems of life. But the discovery
was suspected from the first, because it was too closely in accord with
speculation, and it was soon disproved. Its discoverer soon after
courageously announced to the world that he had been entirely mistaken,
and that the Bathybias, so far from being undifferentiated protoplasm,
was not an organic product at all, but simply a mineral deposit in the
sea water made by purely artificial means. Bathybias stands therefore as
an instance of a too precipitate advance in speculation, which led even
such a brilliant man as Prof. Huxley into an unfortunate error of
observation; for, beyond question, he would never have made such a
mistake had he not been dominated by his speculative theories as to the
nature of protoplasm.
But although Bathybias proved delusive, this did not materially affect
the advance and development of the doctrine of protoplasm. Simple forms
of protoplasm were found, although none quite so simple as the
hypothetical Bathybias. The universal presence of protoplasm in the
living parts of all animals and plants and its manifest activities
completely demonstrated that it was the only living substance, and as
the result of a few years of experiment and thought the biologist's
conception of life crystallized into something like this: Living
organisms are made of cells, but these cells are simply minute
independent bits of protoplasm. They may contain a nucleus or they may
not, but the essence of the cell is the protoplasm, this alone having
the fundamental activities of life. These bits of living matter
aggregate themselves together into groups to form colonies. Such
colonies are animals or plants. The cells divide the work of the colony
among themselves, each cell adopting a form best adapted for the special
work it has to do. The animal or plant is thus simply an aggregate of
cells, and its activities are the sum of the activities of its separate
cells; just as the activities of a city are the sum of the activities of
its individual inhabitants. The bit of protoplasm was the unit, and this
was a chemical compound or a simple mixture of compounds to whose
combined physical properties we have given the name vitality.
==The Decline of the Reign of Protoplasm.==--Hardly had this extreme
chemical theory of life been clearly conceived before accumulating facts
began to show that it is untenable and that it must at least be vastly
modified before it can be received. The foundation of the chemical
theory of life was the conception that protoplasm is a definite though
complex chemical compound. But after a few years' study it appeared that
such a conception of protoplasm was incorrect. It had long been
suspected that protoplasm was more complex than was at first thought. It
was not even at the outset found to be perfectly homogeneous, but was
seen to contain minute granules, together with bodies of larger size.
Although these bodies were seen they were regarded as accidental or
secondary, and were not thought of as forming any serious objection to
the conception of protoplasm as a definite chemical compound. But modern
opticians improved their microscopes, and microscopists greatly improved
their methods. With the new microscopes and new methods there began to
appear, about twenty years ago, new revelations in regard to this
protoplasm. Its lack of homogeneity became more evident, until there has
finally been disclosed to us the significant fact that protoplasm is to
be regarded as a substance not only of chemical but also of high
mechanical complexity. The idea of this material as a simple homogeneous
compound or as a mixture of such compounds is absolutely fallacious.
Protoplasm is to-day known to be made up of parts harmoniously adapted
to each other in such a way as to form an extraordinarily intricate
machine; and the microscopist of to-day recognizes clearly that the
activities of this material must be regarded as the result of the
machinery which makes up protoplasm rather than as the simple result of
its chemical composition. Protoplasm is a machine and not a chemical
compound.
[Illustration: FIG. 23.--A cell as it appears to the modern microscope.
_a_, protoplasmic reticulum; _b_, liquid in its meshes; _c_, nuclear
membrane; _d_, nuclear reticulum; _e_, chromatin reticulum; _f_,
nucleolus; _g_, centrosome; _h_, centrosphere; _i_, vacuole; _j_, inert
bodies.]
==Structure of Protoplasm==.--The structure of protoplasm is not yet
thoroughly understood by scientists, but a few general facts are known
beyond question. It is thought, in the first place, that it consists of
two quite different substances. There is a somewhat solid material
permeating it, usually, regarded as having a reticulate structure. It is
variously described, sometimes as a reticulate network, sometimes as a
mass of threads or fibres, and sometimes as a mass of foam (Fig. 23,
_a_). It is extremely delicate and only visible under special conditions
and with the best of microscopes. Only under peculiar conditions can it
be seen in protoplasm while alive. There is no question, however, that
all protoplasm is permeated when alive by a minute delicate mass of
material, which may take the form of threads or fibres or may assume
other forms. Within the meshes of this thread or reticulum there is
found a liquid, perfectly clear and transparent, to whose presence the
liquid character of the protoplasm is due (Fig. 23, _b_). In this liquid
no structure can be determined, and, so far as we know, it is
homogeneous. Still further study discloses other complexities. It
appears that the fibrous material is always marked by the presence of
excessively minute bodies, which have been called by various names, but
which we will speak of as _microsomes_. Sometimes, indeed, the fibres
themselves appear almost like strings of beads, so that they have been
described as made up of rows of minute elements. It is immaterial for
our purpose, however, whether the fibres are to be regarded as made up
of microsomes or not. This much is sure, that these microsomes
--granules of excessive minuteness--occur in protoplasm and are closely
connected with the fibres (Fig. 23, _a_).
==The Nucleus.==--(a) _Presence of a Nucleus_.--If protoplasm has thus
become a new substance in our minds as the result of the discoveries of
the last twenty years, far more marvelous have been the discoveries
made in connection with that body which has been called the nucleus.
Even by the early microscopists the nucleus was recognized, and during
the first few years of the cell doctrine it was frequently looked upon
as the most active part of the cell and as especially connected with its
reproduction. The doctrine of protoplasm, however, so captivated the
minds of biologists that for quite a number of years the nucleus was
ignored, at least in all discussions connected with the nature of life.
It was a body in the cell whose presence was unexplained and which did
not fall into accord with the general view of protoplasm as the physical
basis of life. For a while, therefore, biologists gave little attention
to it, and were accustomed to speak of it simply as a bit of protoplasm
a little more dense than the rest. The cell was a bit of protoplasm with
a small piece of more dense protoplasm in its centre appearing a little
different from the rest and perhaps the most active part of the cell.
As a result of this excessive belief in the efficiency of protoplasm the
question of the presence of a nucleus in the cell was for a while looked
upon as one of comparatively little importance. Many cells were found to
have nucleii while others did not show their presence, and microscopists
therefore believed that the presence of a nucleus was not necessary to
constitute a cell. A German naturalist recognized among lower animals
one group whose distinctive characteristic was that they were made of
cells without nucleii, giving the name _Monera_ to the group. As the
method of studying cells improved microscopists learned better methods
of discerning the presence of the nucleus, and as it was done little by
little they began to find the presence of nucleii in cells in which they
had hitherto not been seen. As microscopists now studied one after
another of these animals and plants whose cells had been said to contain
no nucleus, they began to find nucleii in them, until the conclusion was
finally reached that a nucleus is a fundamental part of all active
cells. Old cells which have lost their activity may not show nucleii,
but, so far as we know, all active cells possess these structures, and
apparently no cell can carry on its activity without them. Some cells
have several nucleii, and others have the nuclear matter scattered
through the whole cell instead of being aggregated into a mass; but
nuclear matter the cell must have to carry on its life.
[Illustration: FIG. 24.--A cell cut into three pieces, each containing a
bit of the nucleus. Each continues its life indefinitely, soon acquiring
the form of the original as at _C_.]
Later the experiment was made of depriving cells of their nucleii, and
it still further emphasized the importance of the nucleus. Among
unicellular animals are some which are large enough for direct
manipulation, and it is found that if these cells are cut into pieces
the different pieces will behave very differently in accordance with
whether or not they have within them a piece of the nucleus. All the
pieces are capable of carrying on their life activities for a while. The
pieces of the cell which contain the nucleus of the original cell, or
even a part of it, are capable of carrying on all its life activities
perfectly well. In Fig. 24 is shown such a cell cut into three pieces,
each of which contains a piece of the nucleus. Each carries on its life
activities, feeds, grows and multiplies perfectly well, the life
processes seeming to continue as if nothing had happened. Quite
different is it with fragments which contain none of the nucleus (Fig.
25). These fragments (1 and 3), even though they may be comparatively
large masses of protoplasm, are incapable of carrying on the functions
of their life continuously. For a while they continue to move around and
apparently act like the other fragments, but after a little their life
ceases. They are incapable of assimilating food and incapable of
reproduction, and hence their life cannot continue very long. Facts like
these demonstrate conclusively the vital importance of the nucleus in
cell activity, and show us that the cell, with its power of continued
life, must be regarded as a combination of protoplasm with its nucleus,
and cannot exist without it. It is not protoplasm, but cell substance,
plus cell nucleus, which forms the simplest basis of life.
[Illustration: FIG. 25.--A cell cut into three pieces, only one of
which, No. 2, contains any nucleus. This fragment soon acquires the
original form and continues its life indefinitely, as shown at _B_. The
other two pieces though living for a time, die without reproducing.]
As more careful study of protoplasm was made it soon became evident that
there is a very decided difference between the nucleus and the
protoplasm. The old statement that the nucleus is simply a bit of dense
protoplasm is not true. In its chemical and physical composition as
well as in its activities the nucleus shows itself to be entirely
different from the protoplasm. It contains certain definite bodies not
found in the cell substance, and it goes through a series of activities
which are entirely unrepresented in the surrounding protoplasm. It is
something entirely distinct, and its relations to the life of the cell
are unique and marvelous. These various facts led to a period in the
discussion of biological topics which may not inappropriately be called
the Reign of the Nucleus. Let us, therefore, see what this structure is
which has demanded so much attention in the last twenty years.
(b) _Structure of the Nucleus_.--At first the nucleus appears to be very
much like the cell substance. Like the latter, it is made of fibres,
which form a reticulum (Fig. 23), and these fibres, like those of
protoplasm, have microsomes in intimate relation with them and hold a
clear liquid in their meshes. The meshes of the network are usually
rather closer than in the outer cell substance, but their general
character appears to be the same. But a more close study of the nucleus
discloses vast differences. In the first place, the nucleus is usually
separated from the cell substance by a membrane (Fig. 23, _c_). This
membrane is almost always present, but it may disappear, and usually
does disappear, when the nucleus begins to divide. Within the nucleus we
find commonly one or two smaller bodies, the nucleoli (Fig. 23, _f_).
They appear to be distinct vital parts of the nucleus, and thus
different from certain other solid bodies which are simply excreted
material, and hence lifeless. Further, we find that the reticulum within
the nucleus is made up of two very different parts. One portion is
apparently identical with the reticulum of the cell substance (Fig. 23,
_d_). This forms an extremely delicate network, whose fibres have
chemical relations similar to those of the cell substance. Indeed,
sometimes, the fibres of the nucleus may be seen to pass directly into
those of the network of the cell substance, and hence they are in all
probability identical. This material is called _linin_, by which name we
shall hereafter refer to it. There is, however, in the nucleus another
material which forms either threads, or a network, or a mass of
granules, which is very different from the linin, and has entirely
different properties. This network has the power of absorbing certain
kinds of stains very actively, and is consequently deeply stained when
treated as the microscopist commonly prepares his specimens. For this
reason it has been named _chromatin_ (Fig, 23, _e_), although in more
recent times other names have been given to it. Of all parts of the cell
this chromatin is the most remarkable. It appears in great variety in
different cells, but it always has remarkable physiological properties,
as will be noticed presently. All things considered, this chromatin is
probably the most remarkable body connected with organic life.
[Illustration: FIG. 26.--Different forms of nucleii.]
The nucleii of different animals and plants all show essentially the
characteristics just described. They all contain a liquid, a linin
network, and a chromatin thread or network, but they differ most
remarkably in details, so that the variety among the nucleii is almost
endless (Fig. 26). They differ first in their size relative to the size
of the cell; sometimes--especially in young cells--the nucleus being
very large, while in other cases the nucleus is very small and the
protoplasmic contents of the cell very large; finally, in cells which
have lost their activity the nucleus may almost or entirely disappear.
They differ, secondly, in shape. The typical form appears to be
spherical or nearly so; but from this typical form they may vary,
becoming irregular or elongated. They are sometimes drawn out into long
masses looking like a string of beads (Fig. 24), or, again, resembling
minute coiled worms (Fig. 21), while in still other cells they may be
branching like the twigs of a tree. The form and shape of the chromatin
thread differs widely. Sometimes this appears to be mere reticulum (Fig.
23); at others, a short thread which is somewhat twisted or coiled (Fig.
26); while in other cells the chromatin thread is an extremely long,
very much twisted convolute thread so complexly woven into a tangle as
to give the appearance of a minute network. The nucleii differ also in
the number of nucleoli they contain as well as in other less important
particulars. Fig. 26 will give a little notion of the variety to be
found among different nucleii; but although they thus do vary most
remarkably in shape in the essential parts of their structure they are
alike.
==Centrosome.==--Before noticing the activities of the nucleus it will be
necessary to mention a third part of the cell. Within the last few years
there has been found to be present in most cells an organ which has been
called the _centrosome._ This body is shown at Fig. 23, _g_. It is found
in the cell substance just outside the nucleus, and commonly appears as
an extremely minute rounded dot, so minute that no internal structure
has been discerned. It may be no larger than the minute granules or
microsomes in the cell, and until recently it entirely escaped the
notice of microscopists. It has now, however, been clearly demonstrated
as an active part of the cell and entirely distinct from the ordinary
microsomes. It stains differently, and, as we shall soon see, it
appears to be in most intimate connection with the center of cell life.
In the activities which characterize cell life this centrosome appears
to lead the way. From it radiate the forces which control cell activity,
and hence this centrosome is sometimes called the dynamic center of the
cell. This leads us to the study of cell activity, which discloses to us
some of the most extraordinary phenomena which have come to the
knowledge of science.
==Function of the Nucleus.==--To understand why it is that the nucleus has
taken such a prominent position in modern biological discussion it will
be only necessary to notice some of the activities of the cell. Of the
four fundamental vital properties of cell life the one which has been
most studied and in regard to which most is known is reproduction. This
knowledge appears chiefly under two heads, viz., _cell division_ and the
_fertilization of the egg_. Every animal and plant begins its life as a
simple cell, and the growth of the cell into the adult is simply the
division of the original cell into parts accompanied by a
differentiation of the parts. The fundamental phenomena of growth and
reproduction is thus cell division, and if we can comprehend this
process in these simple cells we shall certainly have taken a great step
toward the explanation of the mechanics of life. During the last ten
years this cell division has been most thoroughly studied, and we have a
pretty good knowledge of it so far as its microscopical features are
concerned. The following description will outline the general facts of
such cell division, and will apply with considerable accuracy to all
cases of cell division, although the details may differ not a little.
[Illustration: FIG. 27.--This and the following figures show
stages in cell division. Fig. 27 shows the resting stage with the
chromatin, _cr_, in the form of a network within the nuclear membrane
and the centrosome, _ce_, already divided into two.]
[Illustration: FIG. 28.--The chromatin is broken into threads or
chromosomes, _cr._ The centrosomes show radiating fibres.]
==Cell Division or Karyokinesis.==--We will begin with a cell in what is
called the resting stage, shown at Fig. 23. Such a cell has a nucleus,
with its chromatin, its membrane, and linin, as already described.
Outside the nucleus is the centrosome, or, more commonly, two of them
lying close together. If there is only one it soon divides into two, and
if it has already two, this is because a single centrosome which the
cell originally possessed has already divided into two, as we shall
presently see. This cell, in short, is precisely like the typical cell
which we have described, except in the possession of two centrosomes.
The first indication of the cell division is shown by the chromatin
fibres. During the resting stage this chromatin material may have the
form of a thread, or may form a network of fibres (see Fig. 27). But
whatever be its form during the resting stage, it assumes the form of a
thread as the cell prepares for division. Almost at once this thread
breaks into a number of pieces known as _chromosomes_ (Fig. 28). It is
an extremely important fact that the number of these chromosomes in the
ordinary cells of any animal or plant is always the same. In other
words, in all the cells of the body of animal or plant the chromatin
material in the nucleus breaks into the same number of short threads at
the time that the cell is preparing to divide. The number is the same
for all animals of the same species, and is never departed from. For
example, the number in the ox is always sixteen, while the number in the
lily is always twenty-four. During this process of the formation of the
chromosomes the nucleoli disappear, sometimes being absorbed apparently
in the chromosomes, and sometimes being ejected into the cell body,
where they disappear. Whether they have anything to do with further
changes is not yet known.
The next step in the process of division appears in the region of the
centrosomes. Each of the two centrosomes appears to send out from itself
delicate radiating fibres into the surrounding cell substance (Fig. 28).
Whether these actually arise from the centrosome or are simply a
rearrangement of the fibres in the cell substance is not clear, but at
all events the centrosome becomes surrounded by a mass of radiating
fibres which give it a starlike appearance, or, more commonly, the
appearance of a double star, since there are two centrosomes close
together (Fig. 28). These radiating fibres, whether arising from the
centrosomes or not, certainly all centre in these bodies, a fact which
indicates that the centrosomes contain the forces which regulate their
appearance. Between the two stars or asters a set of fibres can be seen
running from one to the other (Fig. 29). These two asters and the
centrosomes within them have been spoken of as the dynamic centre of the
cell since they appear to control the forces which lead to cell
division. In all the changes which follow these asters lead the way. The
two asters, with their centrosomes, now move away from each other,
always connected by the spindle fibres, and finally come to lie on
opposite sides of the nucleus (Figs. 29, 30). When they reach this
position they are still surrounded by the radiating fibres, and
connected by the spindle fibres. Meantime the membrane around the
nucleus has disappeared, and thus the spindle fibres readily penetrate
into the nuclear substance (Fig. 30).
[Illustration: FIG. 29.--The centrosomes are separating but are
connected by fibres.]
[Illustration: FIG. 30.--The centrosomes are separate and the
equatorial plate of chromosomes, _cr_, is between them.]
During this time the chromosomes have been changing their position.
Whether this change in position is due to forces within themselves, or
whether they are moved around passively by forces residing in the cell
substances, or whether, which is the most probable, they are pulled or
pushed around by the spindle fibres which are forcing their way into the
nucleus, is not positively known; nor is it, for our purposes, of
special importance. At all events, the result is that when the asters
have assumed their position at opposite poles of the nucleus the
chromosomes are arranged in a plane passing through the middle of the
nucleus at equal distances from each aster. It seems certain that they
are pulled or pushed into this position by forces radiating from the
centrosomes. Fig. 30 shows this central arrangement of the chromosomes,
forming what is called the _equatorial plate_.
The next step is the most significant of all. It consists in the
splitting of each chromosome into two equal halves. The threads _do not
divide in their middle but split lengthwise_, so that there are formed
two halves identical in every respect. In this way are produced twice
the original number of chromosomes, but all in pairs. The period at
which this splitting of the chromosomes occurs is not the same in all
cells. It may occur, as described, at about the time the asters have
reached the opposite poles of the nucleus, and an equatorial plate is
formed. It is not infrequent, however, for it to occur at a period
considerably earlier, so that the chromosomes are already divided when
they are brought into the equatorial plate.
At some period or other in the cell division this splitting of the
chromosomes takes place. The significance of the splitting is especially
noteworthy. We shall soon find reason for believing that the chromosomes
contain all the hereditary traits which the cell hands down from
generation to generation, and indeed that the chromosomes of the egg
contain all the traits which the parent hands down to the child. Now, if
this chromatin thread consists of a series of units, each representing
certain hereditary characters, then it is plain that the division of the
thread by splitting will give rise to a double series of threads, each
of which has identical characters. Should the division occur _across_
the thread the two halves would be unlike, but taking place as it does
by a _longitudinal splitting_ each unit in the thread simply divides in
half, and thus the resulting half threads each contain the same number
of similar units as the other and the same as possessed by the original
undivided chromosome. This sort of splitting thus doubles the number of
chromosomes, but produces no differentiation of material.
[Illustration: FIG. 31.--Stage showing the two halves of the
chromosomes separated from each other.]
[Illustration: FIG. 32.--Final stage with two nucleii in which
the chromosomes have again assumed the form of a network. The
centrosomes have divided preparatory to the next division, and the cell
is beginning to divide.]
The next step in the cell division consists in the separation of the two
halves of the chromosomes. Each half of each chromosome separates from
its fellow, and moves to the opposite end of the nucleus toward the two
centrosomes (Fig. 31). Whether they are pulled apart or pushed apart by
the spindle fibres is not certain, although it is apparently sure that
these fibres from the centrosomes are engaged in the matter. Certain it
is that some force exerted from the two centrosomes acts upon the
chromosomes, and forces the two halves of each one to opposite ends of
the nucleus, where they now collect and form two _new nucleii_, with
evidently exactly the same number of chromosomes as the original, and
with characters identical to each other and to the original (Fig. 32).
The rest of the cell division now follows rapidly. A partition grows in
through the cell body dividing it into two parts (Fig. 32), the division
passing through the middle of the spindle. In this division, in some
cases at least, the spindle fibres bear a part--a fact which again
points to the importance of the centrosomes and the forces which radiate
from them. Now the chromosomes in each daughter nucleus unite to form a
single thread, or may diffuse through the nucleus to form a network, as
in Fig. 32. They now become surrounded by a membrane, so that the new
nucleus appears exactly like the original one. The spindle fibres
disappear, and the astral fibres may either disappear or remain visible.
The centrosome may apparently in some cases disappear, but more commonly
remains beside the daughter nucleii, or it may move into the nucleus.
Eventually it divides into two, the division commonly occurring at once
(Fig. 32), but sometimes not until the next cell division is about to
begin. Thus the final result shows two cells each with a nucleus and
two centrosomes, and this is exactly the same sort of structure with
which the process began. (_See Frontispiece_.)
Viewed as a whole, we may make the following general summary of this
process. The essential object of this complicated phenomena of
_karyokinesis_ is to divide the chromatin into equivalent halves, so
that the cells resulting from the cell division shall contain an exactly
equivalent chromatin content. For this purpose the chromatic elements
collect into threads and split lengthwise. The centrosome, with its
fibres, brings about the separation of these two halves. Plainly, we
must conclude that the chromatin material is something of extraordinary
importance to the cell, and the centrosome is a bit of machinery for
controlling its division and thus regulating cell division.
==Fertilization of the Egg.==--This description of cell division will
certainly give some idea of the complexity of cell life, but a more
marvelous series of changes still takes place during the time when the
egg is preparing for development. Inasmuch as this process still further
illustrates the nature of the cell, and has further a most intimate
bearing upon the fundamental problem of heredity, it will be necessary
for us to consider it here briefly.
The sexual reproduction of the many-celled animals is always essentially
alike. A single one of the body cells is set apart to start the next
generation, and this cell, after separating from the body of the animal
or plant which produced it, begins to divide, as already shown in Fig.
8, and the many cells which arise from it eventually form the new
individual This reproductive cell is the egg. But before its division
can begin there occurs in all cases of sexual reproduction a process
called fertilization, the essential feature of which is the union of
this cell with another commonly from a different individual. While the
phenomenon is subject to considerable difference in details, it is
essentially as follows:
[Illustration: FIG. 33--An egg showing the cell substance and
the nucleus, the latter containing chromosomes in large number and a
nucleolus]
The female reproductive cell is called the egg, and it is this cell
which divides to form the next generation. Such a cell is shown in Fig.
33. Like other cells it has a cell wall, a cell substance with its linin
and fluid portions, a nucleus surrounded by a membrane and containing a
reticulum, a nucleolus and chromatic material, and lastly, a centrosome.
Now such an egg is a complete cell, but it is not able to begin the
process of division which shall give rise to a new individual until it
has united with another cell of quite a different sort and commonly
derived from a different individual called the male. Why the egg cell is
unable to develop without such union with male cell does not concern us
here, but its purpose will be evident as the description proceeds. The
egg cell as it comes from the ovary of the female individual is,
however, not yet ready for union with the male cell, but must first go
through a series of somewhat remarkable changes constituting what is
called _maturation_ of the egg. This phenomenon has such an intimate
relation to all problems connected with the cell, that it must be
described somewhat in detail. There are considerable differences in the
details of the process as it occurs in various animals, but they all
agree in the fundamental points. The following is a general description
of the process derived from the study of a large variety of animals and
plants.
[Illustration Fig. 34.--This and the following figures
represent the process of fertilization of an egg. In all figures _cr_ is
the chromosomes; _cs_ represents the cell substance (omitted in the
following figures); _mc_ is the male reproductive cell lying in contact
with the egg; _mn_ is the male nucleus after entering the egg.]
[Illustration: FIG. 35.--The egg centrosome has divided, and
the male cell with its centrosome has entered the egg.]
In the cells of the body of the animal to which this description applies
there are four chromosomes This is true of all the cells of the animal
except the sexual cells. The eggs arise from the other cells of the
body, but during their growth the chromatin splits in such a way that
the egg contains double the number of chromosomes, i.e., eight (Fig.
34). If this egg should now unite with the other reproductive cell from
the male, the resulting fertilized egg would plainly contain a number of
chromosomes larger than that normal for this species of animal. As a
result the next generation would have a larger number of chromosomes in
each cell than the last generation, since the division of the egg in
development is like that already described and always results in
producing new cells with the same number of chromosomes as the starting
cell. Hence, if the number of chromosomes in the next generation is to
be kept equal to that in the last generation, this egg cell must get rid
of a part of its chromatin material. This is done by a process shown in
Fig. 35. The centrosome divides as in ordinary cell division (Fig. 35),
and after rotating on its axis it approaches the surface of the egg
(Figs. 36 and 37). The egg now divides (Fig. 38), but the division is of a
peculiar kind. Although the chromosomes divide equally the egg itself
divides into two very unequal parts, one part still appearing as the egg
and the other as a minute protuberance called the polar cell (_pc'_ in
Fig. 38). The chromosomes do not split as they do in the cell division
already described, but each of these two cells, the egg and the polar
body, receives four chromosomes (Fig. 38). The result is that the egg has
now the normal number of chromosomes for the ordinary cells of the
animal in question. But this is still too many, for the egg is soon to
unite with the male cell; and this male cell, as we shall see, is to
bring in its own quota of chromosomes. Hence the egg must get rid of
still more of its chromatin material. Consequently, the first division
is followed by a second (Fig. 39), in which there is again produced a
large and a small cell. This division, like the first, occurs without
any splitting of the chromosomes, one half of the remaining chromosomes
being ejected in this new cell, the second polar cell (_pc"_) leaving
the larger cell, the egg, with just one half the number of chromosomes
normal for the cells of the animal in question. Meantime the first pole
cell has also divided, so that we have now, as shown in Fig. 40, four
cells, three small and one large, but each containing one half the
normal number of chromosomes. In the example figured, four is the normal
number for the cells of the animal. The egg at the beginning of the
process contained eight, but has now been reduced to two. In the further
history of the egg the smaller cells, called _polar cells_, take no
part, since they soon disappear and have nothing to do with the animal
which is to result from the further division of the egg. This process of
the formation of the polar cells is thus simply a device for getting rid
of some of the chromatin material in the egg cell, so that it may unite
with a second cell without doubling the normal number of chromosomes.
[Illustration: FIG. 38--First division complete and first polar cell
formed, _pc'_.]
[Illustration: FIG. 39.--Formation of the second polar cell, _pc"_.]
[Illustration: FIG. 40.--Completion of the process of extrusion of the
chromatic material; _fn_ shows the two chromosomes retained in the egg
forming the female pronucleus. The centrosome has disappeared.]
Previously to this process the other sexual cell, the _spermatozoon_, or
male reproductive cell, has been undergoing a somewhat similar process.
This is also a true cell (Fig. 34, _mc_), although it is of a decidedly
smaller size than the egg and of a very different shape. It contains
cell substance, a nucleus with chromosomes, and a centrosome, the number
of chromosomes, as shown later, being however only half that normal for
the ordinary cells of the animals. The study of the development of the
spermatozoon shows that it has come from cells which contained the
normal number of four, but that this number has been reduced to one half
by a process which is equivalent to that which we have just noticed in
the egg. Thus it comes about that each of the sexual elements, the egg
and the spermatozoon, now contains one half the normal number of
chromosomes.
[Illustration: FIG. 36--The egg centrosomes have changed their position.
The male cell with its centrosome remains inactive until the stage
represented in Fig. 42.]
[Illustration: FIG. 37--Beginning of the first division for removing
superfluous chromosomes.]
Now by some mechanical means these two reproductive cells are brought in
contact with each other, shown in Fig. 34, and as soon as they are
brought into each other's vicinity the male cell buries its head in the
body of the egg. The tail by which it has been moving is cast off, and
the head containing the chromosomes and the centrosome enters the egg,
forming what is called the male pronucleus (Figs. 35-38, _mn_). This
entrance of the male cell occurs either before the formation of the
polar cells of the egg or afterward. If, however, it takes place before,
the male pronucleus simply remains dormant in the egg while the polar
cells are being protruded, and not until after that process is concluded
does it begin again to show signs of activity which result in the cell
union.
The further steps in this process appear to be controlled by the
centrosome, although it is not quite certain whence this centrosome is
derived. Originally, as we have seen, the egg contained a centrosome,
and the male cell has also brought a second into the egg (Fig. 35,
_ce_). In some cases, and this is true for the worm we are describing,
it is certain that the egg centrosome disappears while that of the
spermatozoon is retained alone to direct the further activities (Fig.
41). Possibly this may be the case in all eggs, but it is not sure. It
is a matter of some little interest to have this settled, for if it
should prove true, then it would evidently follow that the machinery for
cell division, in the case of sexual reproduction, is derived from the
father, although the bulk of the cell comes from the mother, while the
chromosomes come from both parents.
In the cases where the process has been most carefully studied, the
further changes are as follows: The head of the spermatozoon, after
entrance into the egg, lies dormant until the egg has thrown off its
polar cells, and thus gotten rid of part of its chromosomes. Close to it
lies its centrosomes (Fig. 35, _ce_), and there is thus formed what is
known as the _male pronucleus_ (Fig. 35-40, _mn_). The remains of the
egg nucleus, after having discharged the polar cells, form the _female
nucleus_ (Fig. 40, _fn_). The chromatin material, in both the male and
female pronucleus, soon breaks up into a network in which it is no
longer possible to see that each contains two chromosomes (Fig. 41). Now
the centrosome, which is beside the male pronucleus, shows signs of
activity. It becomes surrounded by prominent rays to form an aster (Fig.
41, _ce_), and then it begins to move toward the female pronucleus,
apparently dragging the male pronucleus after it. In this way the
centrosome approaches the female pronucleus, and thus finally the two
nucleii are brought into close proximity. Meantime the chromatin
material in each has once more broken up into short threads or
chromosomes, and once more we find that each of the nucleii contains two
of these bodies (Fig. 42). In the subsequent figures the chromosomes of
the male nucleus are lightly shaded, while those of the female are black
in order to distinguish them. As these two nucleii finally come together
their membranes disappear, and the chromatic material comes to lie
freely in the egg, the male and female chromosomes, side by side, but
distinct forming the _segmentation nucleus_. The egg plainly now
contains once more the number of chromosomes normal for the cells of the
animal, but half of them have been derived from each parent. It is very
suggestive to find further that the chromosomes in this _fertilized egg_
do not fuse with each other, but remain quite distinct, so that it can
be seen that the new nucleus contains chromosomes derived from each
parent (Fig. 42). Nor does there appear to be, in the future history of
this egg, any actual fusion of the chromatic material, the male and
female chromosomes perhaps always remaining distinct.
[Illustration: FIG. 41.--The chromosomes in the male and female
pronucleii have resolved into a network. The male centrosome begins to
show signs of activity.]
[Illustration: FIG. 42.--The centrosome has divided, and the two
pronucleii have been brought together. The network in each nucleus has
again resolved itself into two chromosomes which are now brought
together near the centre of the egg but do not fuse; _mcr_, represents
the chromosomes from the male nucleus; _fcr_, the chromosomes from the
female nucleus.]
[Illustration: FIG. 43.--An equatorial plate is formed and each
chromosome has split into two halves by longitudinal division.]
[Illustration: FIG. 44.--The halves of the chromosomes have separated to
form two nucleii, each with male and female chromosomes. The egg has
divided into two cells.]
While this mixture of chromosomes has been taking place the centrosome
has divided into two parts, each of which becomes surrounded by an aster
and travels to opposite ends of the nucleus (Fig. 42). There now follows
a division of the nucleus exactly similar to that which occurs in the
normal cell division already described in Figs. 28-34. Each of the
chromosomes splits lengthwise (Fig. 43), and one half of each then
travels toward each centrosome to form a new nucleus (Fig. 44). Since
each of the four chromosomes thus splits, it follows that each of the
two daughter nucleii will, of course, contain four chromosomes; two of
which have been derived from the male and two from the female parent.
From now the divisions of the egg follow rapidly by the normal process
of cell division until from this one egg cell there are eventually
derived hundreds of thousands of cells which are gradually moulded into
the adult. All of these cells will, of course, contain four chromosomes;
and, what is more important, half of the chromosomes will have been
derived directly from the male and half from the female parent. Even
into adult life, therefore, the cells of the animal probably contain
chromatin derived by direct descent from each of its parents.
==The Significance of Fertilization.==--From this process of fertilization
a number of conclusions, highly important for our purpose, can be drawn.
In the first place, it is evident that the chromosomes form the part of
the cell which contain the hereditary traits handed down from parent to
child. This follows from the fact that the chromosomes are the only part
of the cell which, in the fertilized egg, is derived from both parents.
Now the offspring can certainly inherit from each parent, and hence the
hereditary traits must be associated with some part of the cell which is
derived from both. But the egg substance is derived from the mother
alone; the centrosome, at least in some cases and perhaps in all, is
derived only from the father, while the chromosomes are derived from
_both_ parents. Hence it follows that the hereditary traits must be
particularly associated with the chromosomes.
With this understanding we can, at least, in part understand the purpose
of fertilization. As we shall see later, it is very necessary in the
building of the living machine for each individual to inherit characters
from more than one individual. This is necessary to produce the numerous
variations which contribute to the construction of the machine. For this
purpose there has been developed the process of sexual union of
reproductive cells, which introduces into the offspring chromatic
material from _two_ parents. But if the two reproductive cells should
unite at once the number of chromosomes would be doubled in each
generation, and hence be constantly increasing. To prevent this the
polar cells are cast out, which reduces the amount of chromatic
material. The union of the two pronucleii is plainly to produce a
nucleus which shall contain chromosomes, and hence hereditary traits
from each parent and the subsequent splitting of these chromosomes and
the separation of the two halves into daughter nucleii insures that all
the nucleii, and hence all cells of the adult, shall possess hereditary
traits derived from both parents. Thus it comes that, even in the adult,
every body cell is made up of chromosomes from each parent, and may
hence inherit characters from each.
The cell of an animal thus consists of three somewhat distinct but
active parts--the cell substance, the chromosomes, and the centrosome.
Of these the cell substance appears to be handed down from the mother;
the centrosome comes, at least in some cases, from the father, and the
chromosomes from both parents. It is not yet certain, however, whether
the centrosome is a constant part of the cell. In some cells it cannot
yet be found, and there are some reasons for believing that it may be
formed out of other parts of the cell. The nucleus is always a direct
descendant from the nucleus of pre-existing cells, so that there is an
absolute continuity of descent between the nucleii of the cells of an
individual and those of its antecedents back for numberless generations.
It is not certain that there is any such continuity of descent in the
case of the centrosomes; for, while in the process of fertilization the
centrosome is handed down from parent to child, there are some reasons
for believing that it may disappear in subsequent cells, and later be
redeveloped out of other parts. The only part of the cell in which
complete continuity from parent to child is demonstrated, is the nucleus
and particularly the chromosomes. All of these facts simply emphasize
the importance of the chromosomes, and tell us that these bodies must be
regarded as containing the most important features of the cell which
constitute its individuality.
==What is Protoplasm?==--Enough has now been given of disclosures of the
modern microscope to show that our old friend Protoplasm has assumed an
entirely new guise, if indeed it has not disappeared altogether. These
simplest life processes are so marvelous and involve the action of such
an intricate mass of machinery that we can no longer retain our earlier
notion of protoplasm as the physical basis of life. There can be no life
without the properties of assimilation, growth, and reproduction; and,
so far as we know, these properties are found only in that combination
of bodies which we call the cell, with its mixture of harmoniously
acting parts. _Life, at least the life of a cell, is then not the
property of a chemical compound protoplasm, but is the result of the
activities of a machine._ Indeed, we are now at a loss to know how we
can retain the term protoplasm. As originally used it meant the contents
of the cell, and the significance in the term was in the conception of
protoplasm as a somewhat homogeneous chemical compound uniform in all
types of life. But we now see that this cell contains not a single
substance, but a large number, including solids, jelly masses, and
liquids, each of which has its own chemical composition. The number of
chemical compounds existing in the material formerly called protoplasm
no one knows, but we do know that they are many, and that the different
substances are combined to form a physical structure. Which of these
various bodies shall we continue to call protoplasm? Shall it be the
linin, or the liquids, or the microsomes, or the chromatin threads, or
the centrosomes? Which of these is the actual physical basis of life?
From the description of cell life which we have given, it will be
evident that no one of them is a material upon which our chemical
biologists can longer found a chemical theory of life. That chemical
theory of life, as we have seen, was founded upon the conception that
the primitive life substance is a definite chemical compound. No such
compound has been discovered, and these disclosures of the microscope of
the last few years have been such as to lead us to abandon hope of ever
discovering such a compound. It is apparently impossible to reduce life
to any simpler basis than this combination of bodies which make up what
was formerly called protoplasm. The term protoplasm is still in use with
different meanings as used by different writers. Sometimes it is used to
refer to the entire contents of the cell; sometimes to the cell
substance only outside the nucleus. Plainly, it is not the protoplasm of
earlier years.
With this conclusion one of our fundamental questions has been answered.
We found in our first chapter that the general activities of animals and
plants are easily reduced to the action of a machine, provided we had
the fundamental vital powers residing in the parts of that machine. We
then asked whether these fundamental properties were themselves those
of a chemical compound or whether they were to be reduced to the action
of still smaller machines. The first answer which biologists gave to
this question was that assimilation, growth, and reproduction were the
simple properties of a complex chemical compound. This answer was
certainly incorrect. Life activities are exhibited by no chemical
compound, but, so far as we know, only by the machine called the cell.
Thus it is that we are again reduced to the problem of understanding the
action of a machine. It may be well to pause here a moment to notice
that this position very greatly increases the difficulties in the way of
a solution of the life problem. If the physical basis of life had proved
to be a chemical compound, the problem of its origin would have been a
chemical one. Chemical forces exist in nature, and these forces are
sufficient to explain the formation of any kind of chemical compound.
The problem of the origin of the life substance would then have been
simply to account for certain conditions which resulted in such chemical
combination as would give rise to this physical basis of life. But now
that the simplest substance manifesting the phenomena of life is found
to be a machine, we can no longer find in chemical forces efficient
causes for its formation. Chemical forces and chemical affinity can
explain chemical compounds of any degree of complexity, but they cannot
explain the formation of machines. Machines are the result of forces of
an entirely different nature. Man can manufacture machines by taking
chemical compounds and putting them together into such relations that
their interaction will give certain results. Bits of iron and steel,
for instance, are put together to form a locomotive, but the action of
the locomotive depends, not upon the chemical forces which made the
steel, but upon the relation of the bits of steel to each other in the
machine. So far as we have had any experience, machines have been built
under the guidance of intelligence which adapts the parts to each other.
When therefore we find that the simplest life substance is a machine, we
are forced to ask what forces exist in nature which can in a similar way
build machines by the adjustment of parts to each other. But this topic
belongs to the second part of our subject, and must be for the present
postponed.
==Reaction against the Cell Doctrine.==--As the knowledge of cells which
we have outlined was slowly acquired, the conception of the cell passed
through various modifications. At first the cell wall was looked upon as
the fundamental part, but this idea soon gave place to the belief that
it was the protoplasm that was alive. Under the influence of this
thought the cell doctrine developed into something like the following:
The cell is simply a bit of protoplasm and is the unit of living matter.
The bodies of all larger animals and plants are made up of great numbers
of these units acting together, and the activities of the entire
organism are simply the sum of the activities of its cells. The organism
is thus simply the sum of the cells which compose it, and its activities
the sum of the activities of the individual cells. As more facts were
disclosed the idea changed slightly. The importance of the nucleus
became more and more forcibly impressed upon microscopists, and this
body came after a little into such prominence as to hide from view the
more familiar protoplasm. The marvellous activities of the nucleus soon
caused it to be regarded as the important part of the cell, while all
the rest was secondary. The cell was now thought of as a bit of nuclear
matter surrounded by secondary parts. The marvellous activities of the
nucleus, and above all, the fact that the nucleus alone is handed down
from one generation to the next in reproduction, all attested to its
great importance and to the secondary importance of the rest of the
cell.
This was the most extreme position of the cell doctrine. The cell was
the unit of living action, and the higher animal or plant simply a
colony of such units. An animal was simply an association together for
mutual advantage of independent units, just as a city is an association
of independent individuals. The organization of the animals was simply
the result of the combination of many independent units. There was no
activity of the organism as a whole, but only of its independent parts.
Cell life was superior to organized life. Just as, in a city, the city
government is a name given to the combined action of the individuals, so
are the actions of organisms simply the combined action of their
individual cells.
Against such an extreme position there has been in recent years a
decided reaction, and to-day it is becoming more and more evident that
such a position cannot be maintained. In the first place, it is becoming
evident that the cell substance is not to be entirely obliterated by the
importance of the nucleus. That the nucleus is a most important vital
centre is clear enough, but it is equally clear that nucleus and cell
substance must be together to constitute the life substance. The
complicated structure of the cell substance, the decided activity shown
by its fibres in the process of cell division, clearly enough indicate
that it is a part of the cell which can not be neglected in the study of
the life substance. Again the discovery of the centrosome as a distinct
morphological element has still further added to the complexity of the
life substance, and proved that neither nucleus nor cell substance can
be regarded as the cell or as constituting life. It is true that we may
not yet know the source of this centrosome. We do not know whether it is
handed down from generation to generation like the nucleus, or whether
it can be made anew out of the cell substance in the life of an ordinary
cell. But this is not material to its recognition as an organ of
importance in the cell activity. Thus the cell proves itself not to; be
a bit of nuclear matter surrounded by secondary parts, but a community
of several perhaps equally important interrelated members.
Another series of observations weakened the cell doctrine in an entirely
different direction. It had been assumed that the body of the
multicellular animal or plant was made of independent units.
Microscopists of a few years ago began to suggest that the cells are in
reality not separated from each other, but are all connected by
protoplasmic fibres. In quite a number of different kinds of tissue it
has been determined that fine threads of protoplasmic material lead from
one cell to another in such a way that the cells are in vital
connection. The claim has been made that there is thus a protoplasmic
connection between all the cells of the body of the animal, and that
thus the animal or plant, instead of consisting of a large number of
separate independent cells, consists of one great mass of living matter
which is aggregated into little centres, each commonly holding a
nucleus. Such a conclusion is not yet demonstrated, nor is its
significance very clear should it prove to be a fact; but it is plain
that such suggestions quite decidedly modify the conception of the body
as a community of independent cells.
There is yet another line of thought which is weakening this early
conception of the cell doctrine. There is a growing conviction that the
view of the organism, simply as the sum of the activities of the
individual cells, is not a correct understanding of it. According to
this extreme position, a living thing can have no organization until it
appears as the result of cell multiplication. To take a concrete case,
the egg of a starfish can not possess any organization corresponding to
the starfish. The egg is a single cell, and the starfish a community of
cells. The egg can, therefore, no more contain the organization of a
starfish than a hunter in the backwoods can contain within himself the
organization of a great metropolis. The descendants of individuals like
the hunter may unite to form a city, and the descendants of the egg cell
may, by combining, give rise to the starfish. But neither can the man
contain within himself the organization of the city, nor the egg that of
the starfish. It is, perhaps, true that such an extreme position of the
cell doctrine has not been held by any one, but thoughts very closely
approximating to this view have been held by the leading advocates of
the cell doctrine, and have beyond question been the inspiration of the
development of that doctrine.
But certainly no such conception of the significance of cell structure
would longer be held. In spite of the fact that the egg is a single
cell, it is impossible to avoid the belief that in some way it contains
the starfish. We need not, of course, think of it as containing the
structure of a starfish, but we are forced to conclude that in some way
its structure is such that it contains the starfish potentially. The
relation of its parts and the forces therein are such that, when placed
under proper conditions, it develops into a starfish. Another egg placed
under identical conditions will develop into a sea urchin, and another
into an oyster. If these three eggs have the power of developing into
three different animals under identical conditions, it is evident that
they must have corresponding differences in spite of the fact that each
is a single cell. Each must in some way contain its corresponding adult.
In other words, the organization must be within the cells, and hence not
simply produced by the associations of cells.
Over this subject there has been a deal of puzzling and not a little
experimentation. The presence of some sort of organization in the egg is
clear--but what is meant by this statement is not quite so clear. Is
this adult organization in the whole egg or only in its nucleus, and
especially in the chromosomes which, as we have seen, contain the
hereditary traits? When the egg begins to divide does each of the first
two cells still contain potentially the organization of the whole adult,
or only one half of it? Is the development of the egg simply the
unfolding of some structure already present; or is the structure
constantly developing into more and more complicated conditions owing
to the bringing of its parts into new relations? To answer these
questions experimenters have been engaged in dividing developing eggs
into pieces to determine what powers are still possessed by the
fragments. The results of such experiments are as yet rather
conflicting, but it is evident enough from them that we can no longer
look upon the egg cell as a simple undifferentiated cell. In some way it
already contains the characters of the adult, and when we remember that
the characters of the adult which are to be developed from the egg are
already determined, even to many minute details--such, for instance, as
the inheritance of a congenital mark--it becomes evident that the egg is
a body of extraordinary complexity. And yet the egg is nothing more than
a single cell agreeing with other cells in all its general characters.
It is clear, then, that we must look upon organization as something
superior to cells and something existing within them, or at least within
the egg cell, and controlling its development. We are forced to believe,
further, that there may be as important differences between two cells as
there are between two adult animals or plants. In some way there must be
concealed within the two cells which constitute the egg of the starfish
and the man differences which correspond to the differences between the
starfish and the man. Organization, in other words, is superior to cell
structure, and the cell itself is an organization of smaller units.
As the result of these various considerations there has been, in recent
years, something of a reaction against the cell doctrine as formerly
held. While the study of cells is still regarded as the key to the
interpretation of life phenomena, biologists are seeing more and more
clearly that they must look deeper than simple cell structure for their
explanation of the life processes. While the study of cells has thrown
an immense amount of light upon life, we seem hardly nearer the centre
of the problem than we were before the beginning of the series of
discoveries inaugurated by the formulation of the doctrine of
protoplasm.
==Fundamental Vital Activities as Located in Cells.==--We are now in
position to ask whether our knowledge of cells has aided us in finding
an explanation of the fundamental vital actions to which, as we have
seen, life processes are to be reduced. The four properties of
irritability, contractibility, assimilation, and reproduction, belong to
these vital units--the cells, and it is these properties which we are
trying to trace to their source as a foundation of vital activity.
We may first ask whether we have any facts which indicate that any
special parts of the cell are associated with any of these fundamental
activities. The first fact that stands out clearly is that the nucleus
is connected most intimately with the process of reproduction and
especially with heredity. This has long been believed, but has now been
clearly demonstrated by the experiments of cutting into fragments the
cell bodies of unicellular animals. As already noticed, those pieces
which possess a nucleus are able to continue their life and reproduce
themselves, while those without a nucleus are incapable of reproduction.
With greater force still is the fact shown by the process of
fertilization of the egg. The egg is very large and the male
reproductive cell is very small, and the amount of material which the
offspring derives from its mother is very great compared with that which
it derives from its father. But the child inherits equally from father
and mother, and hence we must find the hereditary traits handed down in
some element which the offspring obtains equally from father and mother.
As we have seen (Figs. 34-44), the only element which answers this demand
is the nucleus, and more particularly the chromosomes of the nucleus.
Clearly enough, then, we must look upon the nucleus as the special agent
in reproduction of cells.
Again, we have apparently conclusive evidence that the _nucleus_
controls that part of the assimilative process which we have spoken of
as the constructive processes. The metabolic processes of life are both
constructive and destructive. By the former, the material taken into the
cell in the form of food is built up into cell tissue, such as linin,
microsomes, etc., and, by the latter, these products are to a greater or
less extent broken to pieces again to liberate their energy, and thus
give rise to the activities of the cell. If the destructive processes
were to go on alone the organism might continue to manifest its life
activities for a time until it had exhausted the products stored up in
its body for such purposes, but it would die from the lack of more
material for destruction. Life is not complete without both processes.
Now, in the life of the cell we may apparently attribute the destructive
processes to the cell substance and the constructive processes to the
nucleus. In a cell which has been cut into fragments those pieces
without a nucleus continue to show the ordinary activities of life for a
time, but they do not live very long (Fig. 25). The fragment is unable to
assimilate its food sufficiently to build up more material. So long as
it still retains within itself a sufficiency of already formed tissue
for its destructive metabolism, it can continue to move around actively
and behave like a complete cell, but eventually it dies from starvation.
On the other hand, those fragments which retain a piece of the nucleus,
even though they have only a small portion of the cell substance, feed,
assimilate, and grow; in other words, they carry on not only the
destructive but also the constructive changes. Plainly, this means that
the nucleus controls the constructive processes, although it does not
necessarily mean that the cell substance has no share in these
constructive processes. Without the nucleus the cell is unable to
perform those processes, while it is able to carry on the destructive
processes readily enough. The nucleus controls, though it may not
entirely carry on, the constructive metabolism.
It is equally clear that the _cell substance_ is the seat of most of the
destructive processes which constitute vital action. The cell substance
is irritable, and is endowed with the power of contractility. Cell
fragments without nucleii are sensitive enough, and can move around as
readily as normal cells. Moreover, the various fibres which surround the
centrosomes in cell division and whose contractions and expansions, as
we have seen, pull the chromosomes apart in cell division, are parts of
the cell substance. All of these are the results of destructive
metabolism, and we must, therefore, conclude that destructive processes
are seated in the cell substance.
The _centrosome_ is too problematical as yet for much comment. It
appears to be a piece of the machinery for bringing about cell division,
but beyond this it is not safe to make any statements.
In brief, then, the cell body is a machine for carrying on destructive
chemical changes, and liberating from the compounds thus broken to
pieces their inclosed energy, which is at once converted into motion or
heat or some other form of active energy. This chemical destruction is,
however, possible only after the chemical compounds have become a part
of the cell. The cell, therefore, possesses a nucleus which has the
power of enabling it to assimilate its food--that is, to convert it into
its own substance. The nucleus further contains a marvellous
material--chromatin--which in someway exercises a controlling influence
in its life and is handed down from one generation to another by
continuous descent. Lastly, the cell has the centrosome, which brings
about cell division in such a manner that this chromatin material is
divided equally among the subsequent descendants, and thus insures that
the daughter cells shall all be equivalent to each other and to the
mother cell.
We must therefore look upon the organic cell as a little engine with
admirably adapted parts. Within this engine chemical activity is
excited. The fuel supplied to the engine is combined by chemical forces
with the oxygen of the air. The vigour of the oxidation is partly
dependent upon temperature, just as it is in any other oxidation
process, and is of course dependent upon the presence of fuel to be
oxidized, and air to furnish the oxygen. Unless the fuel is supplied and
the air has free access to it, the machine stops, the cell _dies_. The
energy liberated in this machine is converted into motion or some other
form. We do not indeed understand the construction of the machine well
enough to explain the exact mechanism by which this conversion takes
place, but that there is such a mechanism can not be doubted, and the
structure of the cell is certainly complex enough to give plenty of room
for it. The irritability of the cell is easily understood; for, since it
is made of very unstable chemical compounds, any slight disturbance or
stimulation on one part will tend to upset its chemical stability and
produce reaction; and this is what is meant by irritability.
Or, again, we may look upon the cell as a little chemical laboratory,
where chemical changes are constantly occurring. These changes we do not
indeed understand, but they are undoubtedly chemical changes. The result
is that some compounds are pulled to pieces and part of the fragments
liberated or excreted, while other parts are retained and built into
other more complex compounds. The compounds thus manufactured are
retained in the cell body, and it grows in bulk. This continues until
the cell becomes too big, and then it divides.
If a machine is broken it ceases to carry on its proper duties, and if
the parts are badly broken it is ruined. So with the cell. If it is
broken by any means, mechanical, thermal, or otherwise, it ceases to
run--we say it dies. It has within itself great power of repairing
injury, and therefore it does not cease to act until the injury is so
great as to be beyond repair. Thus it only stops its motion when the
machinery has become so badly injured as to be beyond hope of repair,
and hence the cell, after once ceasing its action, can never resume it
again.
There are, of course, other functions of living things besides the few
simple ones which we have considered. But these are the fundamental
ones; and if we can reduce them to an intelligible explanation, we may
feel that we have really grasped the essence of life. If we understand
how the cell can move and grow and reproduce itself, we may rest assured
that the other phenomena of life follow as a natural consequence. If,
therefore, we have obtained an understanding of these fundamental vital
phenomena, we have accomplished our object of comprehending the life
phenomena in our chemical and mechanical laws.
But have we thus reduced these fundamental phenomena to an intelligible
explanation? It must be acknowledged that we have not. We have reduced
them to the action of chemical forces acting in a machine. But the
machine itself is unintelligible. The organic cell is no more
intelligible to us than is the body as a whole. The chemical
understanding which we thought we had a few years ago in protoplasm has
failed us, and nothing has taken its place We have no conception of what
may be the primitive life substance. All we can say is that this most
marvellous of all natural phenomena occurs only within that peculiar
piece of machinery which we call the cell, and that it is the result of
the action of physical forces in that machine. How the machine acts, or
even the structure of the machine, we are as far from understanding as
we were fifty years ago. The solution has retreated before us even
faster than we have advanced toward it.
==Summary.==--We may now notice in a brief summary the position which we
have reached. In our attempt to explain the living organism on the
principle of the machine, we are very successful so far as secondary
problems are concerned. Digestion, circulation, respiration, and motion
are readily solved upon chemical and mechanical principles. Even the
phenomena of the nervous system are, in a measure, capable of
comprehension within a mechanical formula, leaving out of account the
purely mental phenomena which certainly have not been touched by the
investigation. All of these phenomena are reducible to a few simple
fundamental activities, and these fundamental activities we find
manifested by simple bits of living matter unincumbered by the
complicated machinery of organisms. With the few fundamental properties
of these bits of organic matter we can construct the complicated life of
the higher organism. When we come, however, to study these simple bits
of matter, they prove to be anything but simple bits of matter. They,
too, are pieces of complicated mechanism whose action we do not even
hope to understand. That their action is dependent upon their machinery
is evident enough from the simple description of cell activity which we
have noticed. That these fundamental vital properties are to be
explained as the result of chemical and mechanical forces acting through
this machinery, can not be doubted. But how this occurs or what
constitutes the guiding force which corresponds to the engineer of the
machine, we do not know.
Thus our mechanical explanation of the living machine lacks a
foundation. We can understand tolerably well the building of the
superstructure, but the foundation stones upon which that structure is
built are unintelligible to us. The running of the living machine is
thus only in part understood. The living organism is a machine or, it
is better to say, it is a series of machines one within the other. As a
whole it is a machine, and its parts are separate machines. Each part is
further made up of still smaller machines until we reach the realm of
the microscope. Here still we find the same story. Even the parts
formerly called units, prove to be machines, and when we recognize the
complexity of these cells and their marvellous activities, we are ready
to believe that we may find still further machines within. And thus
vital activity is reduced to a complex of machines, all acting in
harmony with each other to produce together the one result--life.