“suspended animation.” Wheel-animalcules, and some other minute
creatures which are found living in tiny pools of water, on the bark of
trees, and in the hollows of leaves, naturally dry up when the water
evaporates. You may dry them yourself in a watchglass; they appear as
nothing more than shapeless dust particles mixed with the dried mud of
a drop of dirty water. They may be kept in this state for months—even
years. I do not know that any limit has been ascertained. But when you
add pure rain-water to the dust in the watchglass, it softens, and in
less than an hour the little wheel-animalcules have softened too, and
expanded into life, swimming about whilst the delicate spikes on their
“wheels” vibrate regularly as though they had never ceased to do so, and
as though the animalcules had not for years been dried-up little
mummies.
Of course, the term “suspended animation” has been applied in earlier
times to the often exaggerated stories of “trance” and deathlike sleep
in human beings. But it is now with more justice applied to these
instances of dried animalcules which return to life when wetted, and to
similar cases of prolonged retention of vitality by seeds, since it
would appear that in these dried animalcules life really is actually and
totally suspended, although the mechanism is there which resumes its
life when the necessary moisture is supplied. In cases of trance in man
and hibernation in animals, the heart is still very slowly and feebly
beating, and the breathing is still—almost imperceptibly—at work. The
chemical changes are still very slowly and gently proceeding. The buried
Indian wizard, and the snail, and the Sleeping Beauty are moist, and
chemically active, though feebly so; life is not absolutely suspended.
But in the dried animalcule (though complete chemical desiccation is not
effected), the removal of the water from the body actually arrests the
changes which we call life, just as a needle may arrest the
balance-wheel of a watch. Supply the water, or remove the needle, and
life ceases to be suspended; it goes on once more (as one of the rules
of Bridge ambiguously enacts) “as though no mistake had been made.”
XVIII
THE UNIVERSAL STRUCTURE OF LIVING THINGS
Without doubt, the greatest and most important statement which can be
made about living things is that they are either separate minute
particles of living matter or (more commonly) are built up by thousands
of such minute particles which have in each individual animal and plant
originated from a single such particle (the fertilised germ), by its
division into two, and the subsequent division of these two each into
two, and of the four so produced each into two—and so on, until by
repeated division into two, millions of corpuscles, hanging together as
one mass, are the result.
[Illustration: FIG. 36.—Simple “cells,” consisting of naked protoplasm,
changing shape and taking in solid food particles. A, is a series of
four successive changes of shape of a fresh-water animalcule, the
proteus or amœba; B, is a similar series of three views of a separate
creeping kind of corpuscle found in the blood and lymph-spaces of
animals, and called a “phagocyte.” It is also said to be “amœboid,”
from its resemblance to the amœba or proteus-animalcule. B, is from
the blood of the guinea-pig. It is not a parasite, but one of the
various kinds of cells which build up the animal body, and are derived
from the single original egg-cell (see Fig. 31) by continued division.
The three drawings show three changes of shape occurring in the same
“phagocyte” in a few minutes. It is engulphing a fever-producing
blood-parasite, a spirillum, marked _a_, into its soft, slimy
protoplasm, to be there digested and destroyed. In the same way the
amœba, A, is seen in four stages of engulphing the vegetable
particle, _a_. In the fourth figure the letter _b_ points to water taken
into the amœba’s protoplasm with the food-particle _a_. In all the
figures, _c_ points to the “vacuole” or liquid-holding cavity, which
bursts and re-forms in A; the letter _d_ points to the cell-nucleus.]
The particles of living matter are spoken of as “cells” for a very
curious reason, to which I will revert. The living matter is called
“protoplasm” (primitive or fundamental slime). A “cell” in the language
of microscopists means a corpuscle or more or less rounded or
irregularly shaped particle of protoplasm. Cells commonly vary in size
from 1/5000th to 1/200th of an inch in breadth, and may be much larger.
Protoplasm—the living substance of “cells”—is a slimy body, almost
liquid, but yet tenacious. It is transparent, but clouded by fine
granules, and can often be seen with a very high power of the microscope
to consist of more and of less liquid matter, intermixed like an
emulsion. It often has within it large cavities filled with liquid, and
also often oil drops; in other cases hard concretions or coarse
granules. But apart from other things, the protoplasm of a “cell”
always contains within it a special, firmer, and denser part, enclosed
in an enveloping coat or skin. This dense body is the “nucleus,” or
kernel, and is of the very greatest importance in the chemical changes
and movements which constitute the life of the cell. It is usually
spherical, and in the living state often looks clear and bright. All
cells, whether they are found building up the bodies of plants and
animals like so many living bricks, or living freely and singly as
animalcules, have the essential structure just described—a semi-liquid
yet tenacious material enclosing a globular firmer body, the nucleus.
[Illustration: FIG. 37.—A, cells forming soft vegetable tissue; _a_,
cell-wall; _b_, protoplasm; _c_, liquid-holding cavity in the
protoplasm; _d_, the nucleus. B, a pigment-cell from the frog’s skin,
expanded. C, the same cell contracted. D, a nerve-cell: observe the
nucleus. E, a muscle-cell stretched. F, the same contracted: observe the
nucleus.]
[Illustration: FIG. 38.—Copy of part of Robert Hook’s drawing of a
magnified piece of cork, showing the “cells” so named by him in 1665.]
How did these viscous nucleated corpuscles come to be called “cells”? It
was in this wise. At the end of the seventeenth century Dr. Robert Hook,
secretary of the Royal Society, published a beautiful book of folio
size, entitled _Micrographia_. In this he pictured various minute
insects and various natural products as seen under his microscope. Among
the objects figured and described was a piece of cork (Fig. 38). Hook
showed that it was built up of a number of empty, air-holding, box-like
chambers, less than the hundredth of an inch in length, and these he
called “cells,” comparing them to the “cells” of the bee’s honeycomb.
Later observers found that this “cellular” structure was very common in
plants—but it was not until more than a hundred years later that it was
observed that the “cells” which build up the soft stems and leaves of
plants are not empty or merely air-holding, but contain a liquid or
viscid matter. Robert Browne, a great botanist, who lived within the
memory of some of our older naturalists, first observed and described
the “nucleus,” or kernel, within the cells of some lily-like plants, and
gave it that name (Fig. 37 A, _d_). About the thirties of last century,
by aid of improved microscopes, a structure like that of the vegetable
“cell” and its “nucleus” was discovered in some animal materials, or
“tissues,” as they are termed—for instance, in cartilage (Fig. 39). The
word “tissue” is applied to each of the various layers and masses, such
as epiderm, fibrous tissue, muscle, nerve, cartilage, bone, which can be
distinguished in an animal body and separated from one another, just as
we may separate the “tissues” of a man’s clothes—the leathern, woollen,
silken, cotton, linen: the cords, laces, threads, and pads or stuffing.
The full meaning of this existence of “cells” or “cellular” structure in
the tissue of plants and animals only gradually became evident. A very
remarkable discoverer, Professor Schwann, of Liège (with whom when he
was an old man I spent an afternoon a great many years ago), was the
first to grasp the great facts and to put forward what has been ever
since called “the cell theory” of animal and vegetable structure and
life.
[Illustration: FIG. 39.—A piece of cartilage, showing the cells which
have formed it embedded in the (shaded) firm substance, and connected to
one another by branching processes of protoplasm.]
Schwann, in 1836, showed that the important thing about a “cell” is not
the box or cell-wall so much as the viscid contents and the nucleus. But
the name “cell” was (strangely enough) retained for the contents, even
when the box-like chamber was absent—much as we speak of “a bottle of
wine,” meaning the contents of the bottle, and not the glass vessel
holding it. It was shown that the box-like case or cell-wall (the
original “cell” of Hook) is actually formed by the living nucleated
plasm or viscid matter within it, just as a snail forms its shell, by
the separation or “secretion” of a dead, firm, chemical deposit on its
living surface. Schwann showed that all—not merely special exceptional
instances, but all—the tissues of plants and of animals are built up by
nucleated cells, the cell-wall being often not hard and box-like, but
soft, gelatinous, irregular in shape, and sometimes very thin, sometimes
very thick. Every living cell is thus surrounded by the chemical
products of its own activity, or may deposit those products within
itself as in the goblet-cell and the fat-cell seen in Fig. 40, C and D,
and these products differ in different tissues. The cells of a tissue,
using the word to mean the soft nucleated particles or corpuscles of
protoplasm or “cell-substance,” must be regarded as the microscopic
living “weavers” or makers of the tissue. The cells in one tissue may
form a honeycomb of boxes; in another a jelly-like mass or a fibrous
network, with the cell-substance scattered as nucleated particles in it
(Fig. 39). Or the cells may be elongated and contractile (Fig. 37, E,
F). They may be more or less fused with one another, as in flesh or
muscular fibre; but we can always recognise the presence of the
individual cells under the microscope by their distinct and separate
“nuclei.”
[Illustration: FIG. 40.—Three kinds of cells, magnified a thousand
times linear. A, a row of cilia-bearing cells. B, a single detached
ciliated cell: observe the nucleus in each cell. C, a goblet-cell, from
a mucous surface, producing _c_, a slimy secretion; _d_, the wall of the
cell; _b_, the nucleus; _a_, the protoplasm in which the secretion _c_
was accumulated until it burst out at the free end of the cell. D, a
fat-cell; _a_, the nucleus surrounded by protoplasm; _e_, the thin layer
of protoplasm enveloping the great oil drop _f_, which has formed within
it.]
Schwann’s most important conclusion from this universal presence of soft
corpuscles of cell-substance, each with its globular nucleus, in all the
tissues and most varied parts of animals as well as plants, was that the
life of a living thing, the chemical and physical changes which go on in
it from birth to death, consist in chemical and physical changes in each
of these microscopic, nucleated bodies, and that the life of the whole
animal or plant is the sum of the lives of these microscopic units. If
we wish to know more about the real nature of the growth and activities
of living things, said Schwann, we must thoroughly study and ascertain
the chemical and physical changes, and the properties of the
cell-substance in all the different varieties of tissue. That is the
celebrated “cell-theory” of Schwann. And this examination of, and
experiment with, the cells of all kinds of tissues of plants and animals
has been going on ever since Schwann made his historic statement more
than seventy years ago. The branch of science called “histology” is the
outcome of that study.
Microscopes have been immensely improved since Schwann wrote, first in
England by the father of the present Lord Lister, then later in Germany
by Abbé and Zeiss, of Jena. A variety of methods have been devised for
making the “cells” in thick, solid tissues visible. Very thin
sections—thin enough to be transparent—were at first cut from the
fresh tissues, and examined by transmitted light. This did very well in
a rough way, but better results were obtained by hardening the tissues
in alcohol or chromic acid, when wonderfully fine sections could be cut
and rendered translucent by soaking in varnish, in which they were
preserved for study with the microscope, between two plates of glass.
The sections were stained with various dyes, such as carmine, log-wood,
the aniline dyes, etc., and it was found that the nuclei of the cells
and the granules and fibres both in the minute cells and in the
surrounding substance manufactured by them, could be distinguished more
clearly by means of their differing affinity for the dyes. And whilst
endless section-cutting and staining and careful drawing and record of
the structure discovered, was proceeding in hundreds of
laboratories—other observers especially devoted themselves to the
difficult task of seeing the cell-substance or protoplasm and its
nucleus under the highest power of the microscope, whilst still alive!
It would seem a hopeless task to examine with a high-power microscope
the cells (less than a thousandth of an inch broad) inside the solid
stem or leaves of a plant or of an animal’s body without killing the
plant or animal and the cells of which they consist. As most of my
readers know, the front lens (or “glass”) of a high-power microscope has
to be brought very close indeed to any object in order to bring it into
focus—as near as the one twenty-fifth of an inch. Then the object
examined must be very small and transparent, in order that the light may
pass through it, as through the slide-picture in a magic lantern, and so
form a clear, well-defined picture in the focus of the microscope, where
the eye receives it.
Fortunately, there are some facts about living cells or corpuscles of
protoplasm which enable us to examine living cells, in spite of these
difficulties. In the first place, there are a whole host of minute
animals and plants—of many different kinds—which consist of only one
cell or nucleated corpuscle of protoplasm (Fig. 36 A); they are
transparent, abound in fresh water and sea water, and can be searched
for with the microscope in a drop of water placed on a flat glass plate
and covered with a specially thin glass slip. Many of these have been
studied for hours—and even days—continuously, and the remarkable
internal currents and movements of their viscid “protoplasm,” its
changes of shape, its feeding and growth, and the details of the process
of division into two—by which it multiplies—have been ascertained, as
well as the action upon it of light, heat, electricity, and mechanical
shock, and of all sorts of chemical substances, carefully introduced
beneath the cover-glass. A second fact of great importance is that the
“cells” or protoplasmic corpuscles, which build up a complex plant or
animal, do not die at once when the plant or animal “dies,” that is to
say, the animal or plant may be “killed” and fine bits of transparent
tissue removed from it and placed beneath the microscope, where, with
proper care, the cells may be kept alive for some time. The hairs of
many plants are strings of transparent “cells,” or boxes, containing
living, streaming, active protoplasm. These hairs can be cut off, and
the cells will remain alive for a long time whilst they are under the
microscope (see Fig. 15 _bis_). The transparent wall of the eye—called
the cornea—can be removed from a frog after it has been killed, and the
still-living cells in the delicate glass-like tissue can be studied with
the highest powers of the microscope, and give evidence of their life by
their movements and other changes. Most convenient and important for
this study is the blood—for there the cells are loose, floating in the
liquid. The cells in a minute drop of human blood can be kept alive for
hours, if the glass slide is kept warm, as it easily can be, and I have
seen the cells in a drop of frog’s blood (skilfully treated) still
alive, and exhibiting active movements, a fortnight after the frog, from
which the drop of blood came, was dead and buried. These floating,
moving cells of the blood are the “phagocytes,” which engulf and digest
disease germs and other particles (Fig. 36 B). Other more numerous cells
of the blood are the oxygen-carriers, or red corpuscles, which do not
show any movements or changes of an active kind whilst alive.
XIX
PROTOPLASM, LIFE AND DEATH
The result of the study of living cell-substance, or protoplasm, is to
show that every cell has an individual life, and often makes this
manifest by its movement, change of shape, and internal currents of
granules, as well as by the special chemical substances it produces and
consumes. All depend for their activity upon the presence of free
oxygen; all are killed by heat far less than that of boiling water; they
continually imbibe water charged with the chemical substances which
nourish them and cause them to grow in bulk and to divide into two; and
they manufacture various chemical bodies in the protoplasm and emit
heat, electrical discharges, and sometimes light. Some or other of them,
in fact, do in their small microscopic way all that the complex, big
animal or plant, of which they are constituents, is seen to do. The
cells of the liver manufacture the bile, those of the salivary glands
the saliva, and those of the intestinal wall a mucous fluid, and squeeze
out or eject those products into the adjacent ducts (see Fig. 40 C).
Other cells lay down (as cell-wall or coating) fibrous and hard
substances which form the skeleton; others become converted into horn
and are shed from the surface of the skin in man as “scurf”; others form
the great contractile masses called muscles. One lot are told off to
control the other cells by something resembling a system of electrical
wires and batteries—these are the nerve-cells (Fig. 37 D), with their
fine, thread-like branches, the nerve-fibres, which are long enough to
permeate every part of the body and place it in connection with the
nerve-cells in the great centres called brain, spinal cord, and ganglia.
At one time it was thought that the cells in the tissues of plants and
animals could originate _de novo_ by a sort of precipitation of liquid
matter. But it is now known that every cell has originated by the
division of a pre-existing cell into two, the nucleus of the mother cell
first dividing and then the rest of the cell. “Every cell originates by
the fission of a preceding cell” is the law, and to that is added,
“Every individual organism, plant or animal, itself originates from a
single cell, the fertilised germ-cell.” These are two laws of
fundamental importance in the study of living things. They are true of
man as well as of the smallest worm; of the biggest tree as well as of
the most insignificant moss or water-weed. When the fertilised egg-cell
divides, and its progeny keep on dividing and growing in bulk by the
conversion of nutriment into protoplasm, the dividing cells do not
necessarily become entirely nipped off from one another. In large tracts
of cells (or tissues) we often find that the neighbouring cells are
connected to one another by excessively fine filaments of protoplasm.
Only twenty years ago it was supposed, whilst the neighbouring cells
were thus connected as a rule in animals, as well as being often
connected to the finest nerve-filaments, yet that in plants the firm,
box-like cases which surround the protoplasm—and when seen dried and
empty by Robert Hook led him to introduce the word “cell” to describe
them—form completely shut cases, so that the living protoplasm of each
plant-cell is entirely cut off from its neighbour. This has now been
found by improved methods of microscopic examination to be a mistake.
The cell-wall in a great many plants, though so firm and cleanly cut in
appearance, is yet perforated by fine threads of the cell protoplasm, so
that each cell is in living communication with its neighbour. Thus, in
plants as well as in animals, the individual cell-units form a more or
less continuous whole of living matter, separated by dead, inert
cell-walls and products of cell activity; but, nevertheless, connected
in definite tracts and regions to one another by continuity of the
living matter in the form of excessively fine threads.
Those animals and plants which are built up of many cells of many
varieties—that is to say, all but the microscopic unicellular
kinds—may be considered as composite organisms—cell-states or
communities in which the individual cells, all derived from one original
mother-cell, are the citizens, living in groups and habitations
(tissues), having their different occupations and capacities, carrying
on distinct operations and working together for the common good, the
“life,” as we call it, of the individual plant or animal which they
constitute. This comparison should serve merely as an illustration of
the individual character and co-ordinated activity of the cells of a
many-celled plant or animal. It must not be forgotten that the separate
cells are all derived by binary division from the original germ-cell,
that they have not come into juxtaposition from distinct sources, but
often are held together by threads of their living material, which
remain after the process of division of one cell into two.
Protoplasm has been called “the physical basis of life.” Since the
activities to which we give the name “life” reside in protoplasm, and
are chemical and physical activities like those of other bodies, even
though more subtle and complicated—we are justified in regarding
protoplasm as the substance in us and other organisms which “lives.”
Death consists in the destruction—the chemical undoing or decomposition
of protoplasm.[3] In simple microscopic unicellular animals and plants,
this is obvious—so long as the protoplasm retains its chemical
structure it is not “dead.” Thus, it is possible with many small simple
organisms—such as animalcules and the seeds of plants—to dry them, and
to expose them to extreme cold, and to deprive them (by aid of a vacuum
pump) of all access of free oxygen or other gases. All chemical change
is thus necessarily arrested. But the atomic structure of the chemical
molecules in the protoplasm is not destroyed. Sir James Dewar, M.
Becquerel, and others have shown this by most carefully conducted
experiments. Seeds of clover, mustard, and wheat so treated do not
“die”; the mechanism remains intact, and when, after many weeks, the
seeds are moistened, warmed, and admitted to contact with the
atmosphere, the mechanism again begins to work, the protoplasm resumes
its activity, the seed “sprouts.” Similarly Dewar has shown that
bacteria are not killed by extreme cold, the temperature of liquid
hydrogen. When thus frozen they remain inert—but are even in this
condition liable to be “killed” by exposure to the blue and ultra-blue
rays of sunlight! Life was defined by Herbert Spencer as “the
‘continuous’ adjustment of internal to external relations,” and this
implied that what is called “suspended animation” was not really a
possible thing, but that there could only be an apparent or approximate
suspension. On the contrary, it seems that just as we may stop a watch
by holding back the balance-wheel with a needle, and yet not “kill” the
watch—for it will resume its movement as soon as the needle is
removed—so the changes of the chemical molecules of protoplasm can be
arrested, but if the chemical “structure” is uninjured the mechanism of
protoplasm can resume its activity when the arresting causes are
removed. The inactive, unchanging protoplasm is not “dead,” it has not
been “killed” so long as its mechanism is intact.