easy to feed this minute “egg” of a creature! A powder of fine
particles—boiled bacteria, in fact, are what I have used—is introduced
into the water between two slips of glass in which the bell-animalcule
is displaying itself under our microscope. We see the particles whirling
about in a vortex, hitting the disc of the bell-animalcule and then
driven into the pit or cavity of the open mouth, whence they sink,
enclosed in a sphere or droplet of water (_f_) into the internal
protoplasm! If the “boiled bacteria,” before they are introduced, are
stained with an alkaline blue, such as blue litmus, they are seen in the
course of a few seconds to turn red—showing that an acid has been
secreted by the protoplasm (probably accompanied by a ferment) into the
little sphere of water, in which the digestion of the boiled bacteria
now goes on. In the course of a few minutes you will see the little
sphere of water dwindling in size—the nourishing liquid being absorbed
by the protoplasm—and then you will see the undigested fragments passed
on by a slow movement to the vestibule or “pit” of the mouth, extruded
through a temporary opening from the protoplasm, and whirled away by the
water currents! If you colour the “boiled bacteria” with water-soluble
anilin-blue—as I did many years ago—you will see that the colour
vanishes from the particles taken into the bell-animalcule’s protoplasm,
and presently an independent sphere of bright blue liquid begins to form
in the protoplasm. This sphere or globule is the renal organ mentioned
above—here very simple and single (Fig. 40_c_). It is called the
pulsating chamber or “contractile vacuole.” It enlarges rapidly, filling
with blue liquid (when special coloured food has not been supplied the
liquid is colourless), then suddenly contracts, squirting its blue
contents out through a special reservoir (_h_) into the mouth-pit (as
shown by an arrow in the figure).
The nucleus of these unicellular animals is often elongated (_e_), and
shaped according to the general shape of the animalcule; but it is the
same thing as the “nucleus” of all cells, whether of plants or
animals—a denser “kernel” of protoplasm, limited by its own delicate
sheath or membrane. It shows, like the cell-nucleus of ordinary cells, a
special affinity for certain dyes, which do not stain the rest of the
cell, so that it can be made very obvious and clear when the animalcule
is killed by alcohol, picric acid, or other preservative solutions, and
then stained; and it shows a curious breaking-up of its substance into
thread-like fibres when the animalcule is about to divide into two—as
is seen also in all cells when the regular process of division of one
cell into two commences. The larger animalcules have enabled us to find
out what are the special properties of the nucleus of cells, as
contrasted with those of the rest of the protoplasm. The trumpet
animalcule (Stentor) is a single cell, and though only one-thirtieth of
an inch long, is large enough to be cut into pieces by very skilful use
of a fine blade. It is found that, if we cut the Stentor into four or
five bits, all continue to “live”; that is to say, to swim about by the
vibration of the hair-like cilia on their surface. But those bits which
have no part of the nucleus in them die after a few hours. They cannot
take nourishment nor grow. On the other hand, all the bits which
comprise a slice of nucleus commence to contract, and shape themselves
like the original Stentor, then form a mouth, and take nourishment, and
grow up to be fully-sized, complete Stentors—animalcules like that by
the cutting-up of which they were formed. This and similar experiments
are held to prove that the processes of nutrition, growth, and
production of specific form are dependent on the nucleus. In its
absence, you may have contractility and active movement for a time, but
no repair, no building-up of new material, no directed or seemingly
“purposive” movement. Such movements, viz., advance in one direction,
arrest, hesitating, or exploring movement to the right and left,
followed by rapid retreat or advance in a straight line, are often
exhibited by these minute animalcules, and cannot be distinguished in
character from those, say, of a fly or even of a mouse.
These facts throw a great light on the significance of the structure of
the protoplasmic corpuscle which we call a “cell,” and show that the
universal presence of the nucleus in every “cell” is due to the fact
that it plays the most important part in the life of the cell. It is the
seat of control, and contains substances in virtue of which the changes
which constitute growth and form-production take place, and in the
absence of which the rest of the protoplasm cannot “carry on,” although
for a time it lives; that is to say, remains chemically undecomposed,
and shows active movement. At the same time, we must not underrate the
importance of the general protoplasm, without the presence of some of
which the nucleus cannot do its work, nor even exist. It is no wonder,
then, that when a cell divides, there are curious and elaborate
proceedings in the nucleus, by which each daughter cell gets its due
half of the all-important nuclear substance.
[Illustration: FIG. 42.—Six successive stages in the division of a
“cell,” to show the appearance of the V-shaped filaments of colourable
matter or chromatin. _a_, resting-cell, with chromatin dispersed as fine
irregular filaments in the nucleus; _b_, the chromatin takes the form of
a wreath with twelve loops—it lies horizontally across the cell; _c_,
the loops break from one another, and form twelve separate V-shaped
pieces; _d_, each of the twelve pieces divides along its length into two
parallel V-shaped pieces; _e_, the divided pieces now separate from one
another, so as to form two wreath-like groups of twelve V-shaped pieces
at each end of the cell; _f_, the wall of the cell forms across between
the separated groups of V-shaped pieces, which lose their regular
arrangement. Each group becomes enclosed in a capsule, and is the
nucleus of a new cell. This is the regular process of cell-division, and
the mode in which the chromatin of the nucleus is broken up, so as to be
equally shared by the two daughter cells. In some species of animals the
cells have as many as thirty-six V-shaped chromatin bodies; in others as
few as two. Different plants also show a similar difference in the
number of chromatin bodies characteristic of the species.]
When a cell divides the fission or splitting of the cell is preceded by
peculiar changes in the nucleus. There is a material in the nucleus of
every cell—of those which are simple animalcules, as well as of those
which are germ-cells and sperm-cells, and of those which form, heaped up
in enormous numbers, the living substance of larger animals and
plants—a material which is an elaborated sort of proteid (see p. 185)
and stains strongly with carmine, logwood, and such dyes, and is called
“chromatin.” It exists often in the shape of minute granules and
filaments (Fig. 42_a_), but always takes on, sooner or later, the form
of an irregularly undulated thread or threads. When the cell is about to
divide into two—as all growing and active cells do—the thread arranges
itself like a zigzagging girdle around the equator of the globular
nucleus (Fig. 42_b_). The margin of the nucleus then seems to melt away
into the general protoplasm, and the zigzag bits of the stainable thread
break from each other, forming a ring-like group of V-shaped pieces
(Fig. 42_c_). There is a remarkable fact as to the number of these
V-shaped pieces. They are identical in number in all the cells of one
species or kind of animal or plant, but may be of a different number in
allied species. The salamander has twenty-four of them; some worms have
only two, some insects thirty-six, some plants eight, others twelve, and
so on. When the V-shaped pieces have thus taken up their position in the
dividing cell, each splits longitudinally, so as to form two V-shaped
pieces lying one over the other (Fig. 42_d_). Then the halves separate
and travel away from each other. In this way two circlets, each made up
by the correct number of V-shaped pieces, come into place at opposite
sides of the cell (Fig. 42_e_). After this the protoplasm becomes nipped
in between the two circlets so as to separate the cell into two halves,
each with its circlet of exactly the correct number of V-shaped pieces
of “chromatin” formed by the splitting of those of the parent cell (Fig.
42_f_). It is in this way that the nuclei of the new cells are
accurately provided with not merely half of the nuclear chromatin of the
mother cell, but with half taken from all parts of it, owing to the
thread-like form of the chromatin and the longitudinal splitting of the
thread.
Fertilisation of the egg-cell by the sperm-cell consists essentially in
the junction or fusion of the nuclear chromatin threads of the egg-cell
with the nuclear chromatin threads of a single sperm-cell or
spermatozoon, which sinks into the egg-cell and fuses with it. This has
been witnessed and studied with the greatest care. The leading fact of
interest is that the egg-cell and the sperm-cell have only half the
number of V-shaped nuclear pieces which the ordinary cells of the same
animal or plant possess. Thus a salamander’s ripe egg and ripe
spermatozoid have each only twelve V-shaped pieces—not twenty-four.
This is brought about by the parent cells, which divide to form the
egg-cell of the female and the spermatozoid of the male, not splitting
their V-shaped nuclear bits; consequently, the number is reduced to
half (that is, twelve) in the daughter cells resulting from the
division. Accordingly, when the fusion of egg-cell and sperm-cell
occurs—each bringing twelve V-shaped pieces—the proper number is
re-established, namely, twenty-four. In the first division of this
fertilised germ-cell—the cell resulting from the fusion of egg-cell and
sperm-cell—the V-shaped nuclear pieces split in the regular way, and
the first two embryo-cells are formed, each with its twenty-four pieces.
Each of these cells undergoes the regular process, and so by continued
growth and division into two an immense series of cells are produced,
which may separate as they form, or in the case of multi-cellular
creatures, remain in continuity with one another as a bulky plant or
animal. Clearly the whole process arises from the value to the growing
mass of protoplasm of having its substance closely sown or dotted with
centres of nuclear matter—that specially active, co-ordinating
material—and of having those centres of equal volume and quality; and,
lastly, of having that nuclear matter equally, or nearly equally,
derived from the male and female parent. It is, however, not certain
from observation of what occurs when the twelve male and twelve female
V-shaped pieces (or whatever the number may be in any given animal or
plant which have become grouped together in the fertilised germ-cell)
split and separate to form the nucleus of two new cells—that exactly
twelve male and twelve female pieces go into each of the new cells. It
is certain that twenty-four pieces go into each, but although it is
possible that exactly half of them are male and half female in origin,
it is not certain from observation that this is necessarily so.
Supposing different proportions to obtain in each of the two first
embryo-cells, it would help to account for the facts that offspring are
not an exact blend of their parents in all their qualities, and that
all the offspring of the same two parents are not exactly alike, but
often very different from one another.
Some of the simplest living things, consisting of but one microscopic
cell, are animals, and some are plants. The essential difference between
an animal and a plant is shown very clearly by some of these microscopic
creatures. Animals feed on the flesh or “proteid” substances
manufactured by other animals or by plants; they also feed on oils or
fats, and on the sugar and starch manufactured by other animals or by
plants. But they cannot construct these “foods” themselves from the
simpler stable chemical compounds called “mineral bodies,” which,
nevertheless, contain the elements they require—carbon, nitrogen,
hydrogen, and oxygen. Such stable mineral bodies are carbonic acid,
ammonia, and water. In fact, ordinary “smelling salts” (which is
chemically carbonate of ammonia) dissolved in water, if we add to it a
trace of phosphates, sulphates, and chlorides of potash, soda and lime,
contain all the actual chemical elements that an animal needs. Yet no
animal can be nourished by such a “mineral” soup.
On the other hand, it is the special distinction of plants—of green
plants, be it noted—that they can feed on this simple diet, and,
moreover, cannot feed on anything else. The green colouring matter which
gives its beautiful tint to the grass and weeds and the leaves of the
big trees which clothe the earth is absolutely essential in this
process; so also is sunlight. The living protoplasm of the
green-coloured parts of plants is crowded with microscopic discs or
plates of a brilliant transparent green colour. The peculiar substance
causing the colour is called “leaf-green,” or “chlorophyll.” It can be
dissolved out of a leaf, not by water, but by spirit or by ether, and
separately studied. It may be seen in solution (to cite a commercial
instance) in the liqueur known as “crême de menthe,” being used to give
its fine green colour to that preparation. Sunlight shining on to the
green parts of plants is “screened” or “strained” by the leaf-green, so
that only some of the coloured rays pass through it, and it is only by
this peculiarly “strained” green sunlight that the protoplasm of the
cells of the leaf is stimulated to its remarkable chemical activity. The
carbonic acid in the air or in the water in which the green plant is
living is taken up by the protoplasm. Carbonic acid consists of oxygen
and of carbon. The protoplasm, when the green sunlight acts on it,
actually takes out of carbonic acid and throws off as a gas (seen as
bubbles in the case of a water plant) some of its constituent oxygen,
thus keeping up the supply of free oxygen in air and water. Then at the
same time it combines the carbon and the rest of the oxygen with water
(hydrogen and oxygen) inside itself, forming solid starch, which, with
the microscope, we can see actually manufactured as little oblong grains
in the green cells. Not only this, but the element nitrogen is, so to
speak, “forced” in other cells of the plant to combine with the three
elements of the newly-formed starch (carbon, hydrogen, and oxygen), and
thus the first steps leading to the building up of those wonderful
bodies, the proteids, are passed. Nothing of the sort can be done by the
protoplasm of an animal cell.
Consequently we distinguish among the simplest living things those which
are provided with leaf-green, and feed, as do the larger green plants,
on dissolved “mineral” solids and gases. There are many thousands of
kinds of them—single simple cells. Some are known to microscopists as
Diatoms and Desmids—often of curious spindle or crescent-shape, others
star-like. The diatoms form on their surface a delicate,
wonderfully-sculptured coat of glass-like silica (quartz), which
resists destruction and persists long after the protoplasm is dead and
washed away. They are favourite objects for examination with the
microscope on account of their great beauty and variety.
Those simplest living things which have not got leaf-green to enable
them to feed on mineral food must—unless they are parasites (as many
important kinds are)—get their food, as do bigger animals, by feeding
on the solid substance of other living things. All living things are, in
fact, ultimately dependent on the green plants—whether microscopic or
of larger kinds—not only for food, but for oxygen gas. If you could
take away green plants altogether from the world, the animals would eat
one another and use up the oxygen gas of the atmosphere, and at the last
there would be a few only of the strongest left, like the last survivor
of the shipwrecked crew of the _Nancy Bell_, and even they would be
suffocating for want of oxygen. The single cells, which are independent
animalcules, and feed like animals on whole creatures smaller than
themselves, or on bits of the fresh substance of other animals or of
plants, are of extraordinary diversity of form and activity. Unlike the
unicellular plants, whose food is dissolved in the water in which they
live, the single-cell animals of necessity take their food in “lumps”
into their inside and digest it, and so their cell-protoplasm has either
a soft surface which can take up a food-morsel at any point or it has a
firm surface with a definite mouth, or aperture, in it (see Fig. 41)
where the mouth is marked by an arrow. Many of them, especially those
with soft glutinous protoplasm, which extends from the main-mass in long
threads or branching processes searching for food-morsels, form
marvellous, perforated shells by chemical deposit, either of silica or
limestone (Radiolaria and Foraminifers). The kinds with a firm or tough
surface to the cell-protoplasm and a permanent mouth and gullet leading
into the cell-substance have very usually a single large lashing-whip
(Flagellata), which drives them through the water in search of prey, or
they are clothed with hundreds of such lashing threads of smaller
size—the “cilia” described above (p. 195)—arranged in rows or circles,
whence these animalcules are called “Ciliata.” The ciliates or
one-celled animals are enabled by their cilia to move with all the
grace, variety, facility, and apparent intelligence of the highest
animals, and also to create powerful vortex-currents by which food
particles are driven into the cell-mouth.
It is a most remarkable and thought-stirring fact that here we have
“animalcules” which are no more than isolated units of the kind and
structure which go by hundreds of thousands to build up a larger
animal—just as bricks are units of the kind which to the number of many
thousands build up a house. And yet each of these free-living units has
a complete organisation—mouth, pharynx, renal organ, locomotive organs,
and so on—similar in activity and general shape to the system of large,
capacious organs built up by the agglomeration of millions of cell-units
to form the body of a higher animal. It is as though a single brick were
provided with door, windows, staircase, fireplace, chimneys, and
wine-cellar! It is clear that there is only a resemblance and not an
identity of origin between the organs of the multicellular animal and
those of the single-celled animalcule. The history of the growth of an
animal from the single egg-cell, and also the series of existing
many-celled animals, leading from simple forms to the most complex,
proves this. And in view of that fact the wonderful elaboration of these
diminutive creatures—many of them so small as to be absolutely
invisible to the naked eye—is all the more curious and impressive. We
have, in fact, parallel organisation and elaboration of structures with
special uses, in two absolutely separated grades or strata of living
things—the one grade marked off by the limitation that only a single
cell, a single nucleated corpuscle of protoplasm, is to be the basis and
material of elaboration—the other and higher grade permitting the use
of millions of single cells, of endless variety and plasticity, capable
of hanging together and being grouped in layers and tissues, in such
enormous masses that an elephant or a whale is the result. And we see
that the same needs are met, not actually in the same way, but in the
same kind of way, in the two cases—the food-orifice, the cilia, and the
“pulsating vacuole” of the unicellular animalcule do the same services
as those done by the structurally different mouth, legs, and kidneys of
the elephant.
XXII
TADPOLES AND FROGS
The season of tadpoles is not a season recognised by housekeepers and
gourmets (except in France, where frogs are eaten in April), but one
dear to schoolboys and all lovers of Nature. The ponds on heaths and in
the corners of meadows now show great masses of soft jelly-like balls of
the size of a marble, huddled together and marked each by a little black
spot at its centre, as big as a rape-seed. This is the “spawn” of our
common frog. The spawn of the common toad is very similar, but the black
spots are set in long strings of jelly, not in separate balls. The
little black body is precisely the same thing as the yellow part of a
hen’s egg, and the jelly around it corresponds to the “white” of the
bird’s egg; but there is nothing to represent the shell. The “yelk” of
the bird’s egg is, it is true, much larger, but corresponds to the black
sphere of the frog’s egg—the actual germ—and is like the latter a
single protoplasmic cell, distended with nourishing granular matter. It
is the excess of this matter which makes the yellow ball of the bird’s
egg so much bigger than the black or rather deep-brown germ of the frog.
The little black spheres elongate from day to day in the warm spring
weather, and at last the minute tadpoles (see Fig. 43 and its
explanation) break loose from the jelly, hanging on to its surface by
aid of a tiny sucker, and feeding on the minute green vegetable growths
which have appeared all over the jelly-like mass. Their rate of growth
depends very much on the temperature, and is much more rapid in Italy
and the South of France than in England. At first they are so small that
it is difficult to distinguish, except with a pocket-lens, the little
black plume-like gills on each side of the head, and it is only as they
grow bigger and lose these little plumes that the young things assume
the characteristic shape of a rounded head—really head and body—with a
long flattened tail which strikes vigorously to the right and left, and
enables the tadpole to swim like a fish.
[Illustration: FIG. 43.—Stages in the growth from the egg of the common
frog—drawn of the natural size. 1. Egg in its jelly-like envelope. 2.
Very young tadpoles adhering to weed by their suckers (placed just below
the mouth). 3. Very young tadpole, showing two pair of external gills: a
third pair is present, but so small as to be invisible without
magnification. 4, 5, 6. Stages in the later growth of the tadpole: the
external gills have disappeared, but the legs have not yet made their
appearance. 7. Tadpole of full size, with fore and hind legs. 8. The
tadpole has now become a small frog, and has left the water. The tail
has shrunk, but has not entirely disappeared: it remains throughout life
hidden by the skin and the large thighs of the growing frog. This figure
has been kindly supplied by Messrs. Macmillan & Co., from Dr. Gadow’s
volume on the “Amphibia and Reptiles,” in the _Cambridge Natural
History_.]
I suppose that every one, or nearly every one, knows that these swarming
little black tadpoles are the young of frogs and toads. As the season
goes on they grow to as much as an inch and a quarter (sometimes an inch
and three-quarters) in length, and develop a number of golden
metallic-looking spots in the skin, which give them a brownish hue. Both
the fore and the hind limbs have now developed, but are hidden beneath
the skin, and all this time the tadpole is breathing, like a fish, by
means of gills, concealed from view by a fold of skin. Very early it
acquires a pair of lungs, and by the time the legs break through the
skin (the hind legs do so first) the lungs are inflated, and help in
respiration. Now the head becomes modelled like that of a young frog,
the tail ceases to grow, its flat transparent border is absorbed and
eaten by “phagocytes,” and the legs become strong and large. Soon the
gills atrophy, and the young creature crawls out of the water and spends
much of its time in the damp grass and herbage near its native pond,
rapidly assuming the shape of a frog. An interesting fact is that all
the time that it is a tadpole the little animal eats vegetable food or
soft animal food (even other tadpoles), has horny lips, and a very long
intestine, coiled like a watch-spring. But as soon as it leaves the
water it becomes purely carnivorous, feeding on small insects and worms,
and its intestine straightens out and becomes, relatively to the
increased size of the body, quite short.