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 Amoeba 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. 36 B) are called “amoeboid,” because of their identity with an Amoeba 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. 40 A 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. 41Aa). 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.

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.

There is a deep pit on one side of the disc. This is the mouth. It is 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. 40c). 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.

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. 42a), 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. 42b). 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. 42c). 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. 42d). 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. 42e). 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. 42f). 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.