structure of protoplasm—is very easily destroyed. A unicellular
organism is chemically destroyed by crushing or disruption, and the
consequent admixture of an excess of water with its particles, also by a
temperature high enough to cause pain if applied to our skin, but yet
much below that of boiling water, also by strong sun-light, and by very
many varieties of chemical substances, especially acids, even when very
much diluted. Complex animals and plants are liable to have the
protoplasm of essential and important cells of the body destroyed,
whereupon the destruction or death of the other cells, not involved in
the original trouble, frequently and as a rule results. The protoplasm
of the cells of a complex animal is dependent on the proper activity of
many other cells besides those of its own tissue or locality in the
body. If the protoplasm of certain nerve-cells or of blood-cells or of
digestive-cells is poisoned or injured or chemically upset, other cells
lose as a consequence—not at once but after a short interval—their
necessary chemical food, their oxygen, their accustomed temperature, and
so bit by bit the great “body”—the complex organism—ceases to live,
that is to say, its protoplasm undergoes step by step and bit by bit
irrevocable chemical change or breaking down.
When a man enters upon that condition which we call “death,” the general
muscular movements first cease, then the movements of respiration (so
that a mirror held to the mouth was used to test the coming and going of
the breath, and the absence of a film of moisture on the mirror’s
surface was held to be a proof of death), then the movement of the
heart, which is followed by the awful pallor of the bloodless face and
lips, and the chilling of the whole body, no longer warmed by the
blood-stream. But for long after these changes have occurred the
protoplasm of the cells in many parts is not injured. The beard of a
corpse will grow after all the great arrests of movement above noted
have been established for hours. In cold-blooded animals, such as the
frog, the protoplasm of the muscles is still uninjured many hours after
decapitation, and they can be stimulated and made to contract. Death, in
fact, only occurs in the tissues of a multicellular animal, as their
protoplasm becomes chemically destroyed by injurious temperature,
poisonous accumulations, or active bacterial germs, which become
predominant owing to the stoppage of the great mechanisms of breathing,
circulation, and nerve control.
Is it, then, necessary to suppose that a something, an essence, a
spirit, an intangible existence called “life” or “vitality,” or the
“anima animans,” passes away, or, as it were, evaporates from a thing
which was living and is now dead? Assuredly no more than it is necessary
to suppose that an essence or thing called “death” takes possession of
it when it ceases to carry on the changes which we call “living.” It
must not be supposed that we regard the unique and truly awe-inspiring
processes which go on in the protoplasm of living things as something
simple, easily understood and accounted for, because we have given up
the notion that life is an entity which enters into living things from
without and escapes from them at death. The real fact is, that the
notion of “spirits,” whether of a lower or of a higher kind, supposed to
enter into and “affect” various natural objects, including trees,
rivers, and mountains, as well as animals and man, does not help us, and
only stands in the way of our gaining more complete knowledge of natural
processes. When we say that life and even its most tremendous
outcome—the mind of man—are to be studied and their gradual
development traced as part of the orderly unfolding of natural
processes, we are no whit less reverent, in no degree less impressed by
the wonder, immensity, and mystery of the universe, than those who, with
happy and obstinate adherence to primitive conceptions, think that they
can explain things by calling up vital essences and wandering spirits.
FOOTNOTES
[3] Protoplasm is not a single chemical compound; it is the name given
to the soft, slimy substance of cells, and contains many chemical
compounds—proteids, fats, and others; some on the way to assume greater
chemical complexity; others in process of destruction. The critical
highest chemical body concealed in protoplasm has no generally
recognised name. It is a proteid-like body, consisting chiefly of
carbon, oxygen, hydrogen, and nitrogen, with some saline constituents.
This is the real ultimate “living matter,” and I suggested in the
_Encyclopedia Britannica_ (article Protozoa) in 1886 that it should be
called “plasmogen.”
XX
CHEMISTRY AND PROTOPLASM
When the chemist examines living cell-substance or protoplasm—as free
as possible from dead envelopes and products of its own activity—so as
to make out, if he can, what it is chemically, he finds that it consists
of the elements carbon, oxygen, hydrogen, and nitrogen, with some
sulphur. Phosphorus and some potash, soda and lime in small quantity,
are also very usually associated with the elements named. These are
combined in the protoplasm so as to form chemical compounds resembling
and including white of egg, and are called “proteids.” A chemical
compound is a very definite and special thing, and when one says
so-and-so is a definite chemical compound, one means that it is not a
mere “mixture,” but is composed of chemical elements (some out of the
long list of about eighty indestructible, undecomposable, “simple”
bodies—gases, liquids, metallic and non-metallic solids—recognised by
chemists and known as such), peculiarly united to, or “combined” with,
one another in definite proportions by weight.
Take, as an example, water. Water is a definite chemical compound,
formed by the chemical union of two pure elements, the gases hydrogen
and oxygen—eighteen ounces of water consist of two ounces of hydrogen
and sixteen ounces of oxygen. At a temperature above that of boiling
water the gases, when they unite, contract to form water-vapour, three
pints of the uniting gases (consisting of two pints of hydrogen and one
of oxygen) forming two pints only of water-vapour. This, when it is
cooled to a temperature below 212 deg. Fahr., suddenly contracts to a
few thimblefuls of pure liquid water. Neither oxygen nor hydrogen
“uncombined” liquefy till far below zero.
A proteid, in the same way, is a chemical combination of the elements
already mentioned—carbon, oxygen, hydrogen, nitrogen, and sulphur—but
the proportions by volume of these elements to each other are
represented by very high figures, not merely by two to one, as in the
case of water. It is the carbon in them that makes “proteids” turn black
when they are destroyed by burning, and it is the sulphur which causes
the smell of rotten eggs. Whilst an ultimate molecule or physical
particle of water consists of two atoms of hydrogen and one of
oxygen—the molecule of the proteid called “albumen” is built up by
seventy-two atoms of carbon, one hundred and twelve atoms of hydrogen,
eighteen atoms of nitrogen, twelve atoms of oxygen, all brought into
relation with one atom of sulphur. Probably in some other proteids the
number of these atoms must all be multiplied by three. The elaborate
“atomic composition” of a molecule of proteid renders it very unstable;
it easily falls to pieces, the elements combining, in other and simpler
proportions, to form less “delicate” bodies. Living protoplasm consists
chiefly of proteids and of compounds which are on the way up, forming
step by step more elaborate combinations till they reach the proteid
stage—and of many others which are degradation products, coming down,
as it were, from the giddy heights of the proteid combination. The
protoplasm of a cell contains finer and grosser granules, which are
these ascending and descending substances; it also contains others in
solution and invisible—for, like a lump of jelly (such as the cook
serves up shaped by a mould and soaked with flavour and colour),
protoplasm can soak up either a large or a small quantity of water, and
with the water (that is the important point) all sorts of chemical
bodies soluble in water. Just as a lump of quivering calves’-foot jelly
(which is a chemical compound of a lower grade than proteids, but like
them), when placed in a shallow dish of water coloured red by carmine,
does not dissolve in the water, but absorbs the water and the carmine,
allowing the coloured water and any chemical bodies in solution in it to
diffuse into and become physically, though not chemically, a part of its
substance, so protoplasm takes up water and the compounds dissolved by
it. Just as a “jelly” of water-holding gelatine can give up its water
and become hard and horny, so is protoplasm capable of gradually giving
up much of its water, and even in some cases of becoming hard and horny,
yet able to return, when remoistened, to its active state. Moreover, a
“jelly” can be made to “soak up” or take into itself water and let it
pass through its substance, so as to wash out from it all soluble
matters. In the same way the protoplasm of a living cell is supplied
with nourishing and oxygenating fluids which diffuse into it, and is
“washed out,” purified, and cleansed of waste or effete chemical
compounds by the water which first permeates it, and then diffuses out
of it into surrounding watery fluids carrying the excess of soluble
chemical bodies with it.
Whilst proteids are the compounds of the highest stage of chemical
complexity recognised in protoplasm, and appear to form the bulk of its
substance, we must carefully avoid the error (which is not uncommon) of
supposing that protoplasm is itself a definite chemical compound. It is
not. Cell-protoplasm includes the nucleus, that denser central body: and
is a structure consisting of “proteids” and of many granules and
dust-like particles, and of more and of less liquid or watery parts
which are less complex in chemical nature than are proteids. Some of the
visible granules and invisible liquids present in protoplasm are being
built up to the proteid stage of elaboration, whilst some are steps in
degradation and decomposition. We have no reason to suppose that the
molecules of any proteid known at present to the chemist really are the
highest degree of chemical complexity attained to in living protoplasm.
Probably there is present a further stage of elaboration, a chemical
body even more complex than is “proteid,” which is continually
attracting the lower chemical compounds to itself and as continually
breaking down. This is the ultimate chemical substance of life. It is
hidden invisibly in the protoplasm, yet all the chemical changes which
go on in the protoplasm of a cell are either leading up to this supreme
life-stuff or are leading downwards from it. This ultimate compound,
which we suppose to exist but have not demonstrated, has been called
“plasmogen.” It is this body in which resides the peculiar property of
living matter, namely, that of attracting to itself substances
containing the so-called “organic” elements—carbon, oxygen, hydrogen,
and nitrogen—and of acting on them in such a way that they “nourish”
it—that is to say, combine chemically with it to form more “plasmogen.”
The intermediate steps leading up to plasmogen and the products arising
from its incessant breaking down are formed under the influence of this
unique chemical body, and by it alone. Chemists have not yet succeeded
in making them; only the less elaborate kinds have been “artificially”
constructed without the aid of the living plasmogen. To construct
plasmogen itself is a task for the chemists of the distant future. In
early geological ages plasmogen came into being; it has gone on ever
since “nourishing” itself, maintaining itself, growing and spreading
over the earth. It is improbable that the conditions which led to its
formation have ever recurred. All subsequent plasmogen has been formed
by the growth and increase of that first sample of it, which once in a
remote period of the earth’s history was built up by chemical
conditions, which came to an end as soon as they had produced it.
The only process in nature of which we know, which resembles the
“building” action of plasmogen, the ultimate molecule of life, buried in
the cell’s protoplasm, is the selective action of crystals, which draw
to themselves from a solution or magma of all sorts of chemical bodies
those molecules of a chemical nature identical with their own, and build
them up into special and definite crystalline forms. But there is a very
wide gap between this process and even the mere assimilation by living
matter of the organic elements, so as to raise them from a lower to a
higher grade of chemical complexity of combination. And over and above
this we have added, in the case of living material, to the mere power of
assimilation and growth the almost unthinkable complications and
variations of specific form and quality, and yet further of individual
form and quality, which are determined by special complications and
variations of the plasmogen, that unique compound concealed in the
cell-protoplasm.
We cannot at present, if ever, picture to ourselves adequately the
mechanism of plasmogen, though the attempt has been, and must be, made.
But we can watch its workings closely; we can ascertain the conditions
which promote, check, or modify its activity; in fact, we can observe
its output and experiment on it in a thousand ways, and so get more and
more knowledge of it. We are not led to suppose that it is possessed by
a demon, nor that in it resides an elsewhere unknown essence. It is
enough for us to satisfy ourselves that its qualities, whilst they can
be grouped with the chemical and physical qualities of other bodies, so
far transcend them in complexity and in immensity of result—the whole
creation of plant and animal life—that their appearance constitutes in
effect a new departure, a sudden and, to us, unaccountable acquirement.
But then we must remember that it is also an unaccountable thing to us
that water suddenly becomes ice at a low temperature, and suddenly
becomes vapour at a high temperature, even if we are able to imagine the
mechanism which necessitates those changes. We cannot “explain” the
nature of things. Even though we can classify them and arrange them in
order, and more or less satisfactorily guess what their inner mechanism
is, we cannot, in our present state of knowledge, trace them in detail
to a first beginning. Even though we believe that such a history lies
behind us, we ourselves cannot as yet show how exactly every quality and
property and form of matter has developed in due order as a matter of
necessity during the cooling of the cosmic gas. All we can do is to
ascertain, bit by bit, some sequences, some lines of orderly development
and interaction, adding thus step by step to our knowledge of what has
taken place.
XXI
THE SIMPLEST LIVING THINGS
In old times, if one wanted to compare a man to the humblest and
simplest of animals, one called him “a worm.” But really a worm is a
very elaborate creature, with skin, muscles, blood-vessels, kidneys,
nervous system, pharynx, stomach, and an intestine, and is built up by
hundreds of thousands of protoplasmic cells. Shakespeare got nearer the
mark when he made one of his uncompromising professional “murderers”
exclaim, as he stabbed the young Macduff to the heart, “What, you egg!”
An egg is a single cell or corpuscle of protoplasm, and the simplest
living things are of the same structure—mere units, single corpuscles
of protoplasm, often less than the one-thousandth of an inch in
diameter, and invisible except with the microscope, though in some cases
big enough to be seen by the naked eye as they swim or crawl in a glass
of pond-water. Many thousands of kinds of these simplest animals and
plants have been carefully recorded, distinguished from one another, and
named by naturalists.
Many of these unicellular animals (or “Protozoa”) crawl by a curious
irregular flowing movement of the viscid tenacious protoplasm of which
they consist. There is no firm coat or cell-wall, only the thinnest
pellicle on the surface. The Proteus-animalcule (Fig. 36A) is so called
because of its constant change of shape; it is also called Amœba on
this account. It flows out into broad, sometimes elongated, finger-like
processes, of which one or several of different sizes may be formed at
the same time, and then quickly disappear as the whole creature moves.
Solid particles of food—minute unicellular plants—are engulfed by the
moving viscid protoplasm and digested within it—that is to say,
chemically dissolved, just as food is digested in the stomach of a big
animal. The colourless cells of our blood and lymph (Fig. 36B) are
called “amœboid,” because of their identity with an Amœba in shape
and movement and digestive power. In some of these animalcules
(sun-animalcules and others) the processes of the protoplasm are in the
form of very fine, long spreading threads which entangle a food
particle, and then contract, drawing it up into the disc-like central
body.
A whole group or division of these simplest animals are provided with
special moving or vibrating hair-like extensions of the protoplasm
called “cilia,” that being the Latin name for “eyelashes,” to which they
are compared. These cilia are arranged with great regularity in rows,
circlets, or spirals, on the surface of the “cell.” They are found not
only on cells which are independent unicellular animals and plants, but
also on cells which form the clothing or surface layer of many larger
animals (Fig. 40A and B). Thus, in ourselves, they are found lining the
windpipe, and they also line the internal cavity of the brain and spinal
cord. The gills of the oyster, and such shell-fish, and other parts of
their skin, are paved with ciliated or cilia-bearing cells, set side by
side in thousands. A single “cilium” is like a little lash of a whip,
and is always making its lashing movement. For a fraction of a second it
is straight and upright, then suddenly curves over and bends to one side
with a “flick,” and immediately recovers its upright position (see Fig.
29, p. 131). All the cilia on one cell or one surface “beat” in the same
direction, and with a common rhythm, so that if the cell is a free,
independent animalcule it is driven along through the water by the rapid
strokes of these numberless tiny “oars,” or “paddles.” If the cilia are
on a surface—like the oyster’s gill—they drive the water along and
create a constant current. Each cilium consists of an elastic and a
contractile fibre closely fused together: the contraction of the one
part causes a flick or bending of the hair-like cilium, the elasticity
of the other substance causes it at once to straighten out again.
The ciliated unicellular animalcules (often called the infusoria,
because they flourish in decomposing “infusions”) not only swim by means
of their cilia, but have a definite mouth or opening in the firm outer
layer of the protoplasm of the cell, into which solid particles of food
are driven by whirlpool-like currents set up by special lines of cilia
(Fig. 41A_a_). The mouth leads through a definite “gullet” into the
interior of the cell. Remember that the whole creature is but a single
minute cell or corpuscle of protoplasm! It is only from the hundredth to
the thousandth of an inch long—with nucleus (_e_ in the figure) of
denser structure within—just like, in essential structure and
properties, one single cell of the many thousands which build up the
liver, or are packed in layers to form our outer skin, or are piled side
by side (by self-division) to make the stems and leaves of plants. Yet
here is such a cell—self-sufficing. When it divides (as it does) the
two resulting cells do not remain in contact as they do when a germ cell
(a fertilised egg-cell) divides. They simply separate, and each swims
away, and carries on its own life. Many of them are fitted out with
these cilia as a most serviceable locomotor apparatus, and as producers
of food currents driving the food right in to a permanent,
definitely-shaped mouth. Some have also a separate opening by which the
undigested remains of the food are extruded. They have also a
liquid-holding cavity or series of cavities which, when distended,
contract and discharge their contents to the exterior. This is an
apparatus for “washing out” the protoplasm of the unicellular animalcule
and getting rid of excretory products; it is definitely comparable in
its use, though so different in origin, to the many-celled kidneys and
bladder of higher animals.
[Illustration: FIG. 41.—Two specimens of a bell-animalcule
(_Vorticella_). A, extended. B, with retracted disc and coiled stalk.
_a_, the ciliated disc; _b_, the firm ring behind the disc, called
“peristome”; _c_, the pulsating chamber, called often the contractile
vacuole; _d_, a completely digested particle of food on its way to be
cast out through the gullet; _e_, the sausage-shaped nucleus; _f_, a
particle of food which has just sunk into the protoplasm from the
gullet, and is surrounded by a little water; _g_, the gullet; _h_, the
reservoir leading from the pulsating chamber to the gullet; _i_, the
hollow stalk; _k_, the spirally attached muscle within the stalk; _l_,
the attachment of the stalk to a weed _m_.]
One of the numerous kinds of “bell-animalcules” affords an excellent
example in which we can watch the structures and life-processes in a
single cell (Fig. 41). It is a pear or bell-shaped body, little more
than one-thousandth of an inch broad, supported on a long, hollow stalk
(though sometimes it breaks off from its stalk and swims freely); inside
the stalk is a muscle (_k_), so attached that when it contracts it
shortens the stalk by throwing it into a close-set, corkscrew spiral
(Fig. 41B). The bell-shaped body has a relatively firm surface, beneath
which is soft, viscid protoplasm and a large sausage-like nucleus. The
body can expand itself so as to look like a solid bell or trumpet-shaped
figure, with a flat, disc-like surface where the “hollow” of the bell
should be, or it can draw the edges of the disc together and assume the
shape of a ball. A line of “cilia” is set on the edge of the bell’s disc
(_a_) and takes a spiral course.