Chapter VII: Preface (7)

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.

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. 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.

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.

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.

Even those who know frog-spawn when they see it and something of the history of the growth of the tadpole and its change into the young frog or toad (as the case may be) do not, as a rule, know about the laying of the eggs. In the early spring (end of March) the full-grown frogs and toads which have passed the winter buried in holes and cracks in the ground in a state of torpor wake up and make their way to neighbouring good-sized ponds. In these the eggs are deposited. The male frogs wait for the females whom they seize from behind, placing their arms under hers and round the chest. They hold so firmly that nothing will persuade them to let go. They often retain their hold for days or even weeks. Sometimes by mistake they seize a fish and hold on securely to its head—a fact which has led to the belief among country-folk that the frog is an enemy of the carp, and tries to blind him by forcing his hands into the carp’s eyes. At this season a frog will clasp your finger or the handle of a stick so persistently that you can lift him out of the water. A large pad of a black colour grows in the breeding-season on the inside of the first finger of the frog’s hand, and is richly supplied with nerves. It is this growth which is sensitive and when touched sets up the cramp-like clasping action of the muscles of the arms. The eggs are eventually squeezed from the female’s body, and are fertilised by the spermatic fluid of the male as they pass into the water. They are, when “laid,” covered with only a thin transparent layer of albumen (or white of egg), and it is only after a few hours that this imbibes water and swells up into a ball-like mass around each little black egg.

Years ago I used to collect the spawning toads and frogs at Baden, near Vienna, in order to observe (in the laboratory of the celebrated microscopist, Professor Stricker, the most gifted of his day) the earliest changes in the little black egg, the size of a rape-seed, which follow upon fertilisation. Properly placed in a watch-glass full of water under a low power of the microscope one little egg could be watched for hours. If it had not been fertilised, nothing occurred. But if it had been, then there were strange movements of its surface and a puckering and sinking in along one definite line, coming and going, but at last becoming well marked like a deep furrow. Without actually splitting, the little sphere was divided by the cleft into two halves. Then, at right angles to the first cleft, a second began to form, and so on, until in the course of hours the sphere became divided on its surface like a blackberry. The separate pieces thus marked out are the first “cells,” or units, of living protoplasm of the young tadpole. They continue to divide and to chemically convert the granular matter with which they are charged into living material whilst the mass slowly, in the course of days (taking up water for its increase in actual size), becomes elongated, and shows the rudiments of head, eyes, ears, spinal cord, and projecting tail. It is a fascinating task to watch this gradual development—and a difficult, but necessary, one (which has now been carried out in the minutest detail by patient students), to harden with chemical solutions the growing embryos taken at successive stages, to embed them in wax or paraffin (as Stricker was the first to do), and to cut them into the finest slices, then to clarify these slices in balsam-varnish, examine them with the microscope, and record and draw every “cell,” every constituent unit, as they increase in number and complication of arrangement. That wonderfully difficult feat has now been carried out not only in the case of the frog and toad, but in the case of hundreds of different kinds of animals of all sorts. Thus we know the history of the growth from the egg in its minutest details in every kind of animal—the “cell-lineage” of the tissues of the full-grown animal traced back to the single original egg-cell.

The egg of animals is always originally a single “cell”—that is to say, a minute corpuscle of slimy consistence, with a dense capsulated kernel or “nucleus” within it. The kernel or nucleus divides into two, and the cell itself divides; each of the daughter cells again divides, and so the process continues, until thousands, and in larger animals millions, of cells are the result, as the mass of cells takes up nourishment and increases in volume. When (as is the case in many animals, _e.g._ starfishes, worms, and mammals) there is only a little granular food-material mixed in with the protoplasm of the egg-cell, that cell is of small size, only the one two-hundredth of an inch in diameter (see Fig. 31). But in the frog there is much granular food-material, and the egg-cell is distended to the size of a rape-seed. When there is still more, as in the bird and many fishes, the egg-cell does not entirely divide as it does in smaller eggs on commencing growth after fertilisation. The protoplasm collects into a disc incompletely separated from the food-material, and it is the disc only which divides into two, four, eight, and ever so many more cells. Some of the cells resulting from the division of the disc form the embryo’s body, and others spread, as they multiply, all over the rest of the egg-ball from its edges so as to enclose the granular food-material in a sac, called the yelk sac. In the frog, on the contrary, the protoplasm does not separate as a disc: the whole egg-cell or ball divides to form the embryo-cells, and the food granules are included in the substance of the dividing cells. “Growth from the egg” is a long story; we must revert now to the tadpoles and their parents.

There is a tradition that Dr. Edwards, the father of Henri and grandfather of Alphonse Milne Edwards, directors of the Natural History Museum of Paris, kept some tadpoles in a sort of cage sunk in the Seine, so that they could not come to the surface to breathe air nor escape on to the land, and that they grew to be very big tadpoles, much larger than the size at which tadpoles usually change into frogs. I tried to repeat this experiment when I was a boy—without success—and I have never heard of any one having succeeded with it.[4] It is not cited or credited at the present day. But some thirty years ago it was discovered that something of this kind happens in the case of the Mexican salamander. The English “newts” and the so-called salamanders are creatures of lizard-like shape, which are closely related to frogs and toads. They lay eggs in the water, and the young are tadpoles, with beautiful large plume-like gills on each side of the head. The tadpole of the common English newt may either lose its gills and leave the water in the summer, if it was hatched early in the season, or may remain longer in the gilled condition, and grow to more than two inches in length, if it was hatched late. In certain lakes in Mexico there is a tadpole-like creature with gill-plumes, which grows to eight inches or more in length, and becomes adult and breeds when in that condition. It is known as the “axolotl,” and was considered to be a distinct kind of gill-bearing adult tadpole-like animal similar to some few others which are known (Siren and Necturus). When, however, they were brought to Europe and kept in a cage with only a small provision of water, some of these axolotls were found to leave the water, lose their gills, change their colour and shape in several respects, and become, in fact, transformed into a terrestrial salamander, of a kind already known in North America. It was thus established that the axolotl of the Mexican lake is nothing more nor less than the tadpole of a species of salamander or newt, which has “given up” the habit of leaving the water, and actually grows to full size, and lays its eggs without becoming converted into a gill-less land-dwelling creature! The greatest interest was excited forty years ago, when the discovery was made that, by gradually drying up the water in which the axolotl is kept, it can be induced to resume its transformation, and become changed into a salamander. Thus, the notion of converting the tadpoles of the common frog into very big tadpoles by preventing them from leaving the water, seems not to have been an unreasonable one.

There are some very big kinds of tadpoles, which are the young of toads of other kinds than our British species. In England we have only two kinds of frogs—the common frog and the edible frog—and two kinds of toads, the common toad and the natter-jack or crawling toad (distinguished by the pale line along the middle of his back). But on the Continent of Europe there are others besides those which we have. There is the beautiful little green tree-frog, and there are the fire-bellied toad, and the obstetric toad (the male of which carries the eggs after they are laid, coiled in a string around his hind legs); and then there is the little spur-heeled toad (_Pelobates fuscus_), which smells like garlic, and is remarkable for having a broad, horny claw on his heel. This toad is only about two inches and a half long (measured from snout to vent) when full grown, but its tadpole often exceeds four inches in length, and in rare cases attains the gigantic size of seven inches, so that it actually shrinks in size when it ceases to be a tadpole, and takes on the adult form. Many years ago I found some of these huge tadpoles in a pond near Antwerp, and thought they must be a realisation of Dr. Edwards’ experiment. They were enormous, and it was only on bringing them home that I heard for the first time of the spur-heeled toad and its gigantic tadpoles (Fig. 44 C).

Among frogs and toads from distant lands are some which bring forth their young alive, the female retaining the eggs in her body instead of laying them in water. The black-and-yellow salamander of Europe (which, like the common toad, has a highly poisonous secretion in the skin) retains its eggs inside its body until the tadpoles are well advanced in development, when they pass from her—about seventy in number—into the water. In the closely allied black Alpine salamander only two, out of thirty or more eggs produced, develop. These two remain inside their mother until they have ceased to have gills and have become terrestrial air-breathing young salamanders like their mother. The Alpine salamander lives where there are no pools suitable for the tadpoles, and so they never enter the water, but remain inside the mother’s body. Some experiments have recently been made with these two species of salamander by varying the conditions as to moisture in which the young grow to maturity, and results of considerable interest have been obtained. One of the most curious arrangements in regard to the young is seen in the Surinam toad, of which we had living specimens five or six years ago in the London Zoological Gardens. In this toad the skin of the female’s back becomes very soft and plastic at the breeding-season. As she lays the eggs the male takes them one by one and presses them into the soft skin of her back, into which they sink. The eggs are thus embedded separately to the number of fifty or sixty, each in a little pit in the mother’s back. They slowly develop, each in its “pit,” the orifice of which is closed by a sort of lid. When the young have grown to the condition of little toads, they push open the lids of the pits and swim out of their mother’s back. Specimens of these toads, with the eggs and young, in various stages, embedded in their mother’s back, are to be seen in most museums of natural history. Toads and frogs catch their prey by throwing forward the sticky tongue which is attached near the front of the lower jaw, and so lick up their victim with startling abruptness. The Cape frog of South Africa (_Xenopus_), like the Surinam toad (_Pipa_), has no tongue, and is also remarkable for possessing hard, pointed ends to its toes. It rarely, if ever, leaves the water.

FOOTNOTES

[4] I am told by Mr. Boulenger, of the Natural History Museum, who is the greatest authority on these animals, that the explanation of this is that unawares Dr. Edwards made use of the young tadpoles of the obstetric toad (_Alytes_), which is very common near Paris, though it does not occur in England. These tadpoles regularly grow to be three inches and more in length (see Fig. 44 B). Dr. Edwards thought he had used the tadpoles of the common frog, but had, by accident, got hold of those of _Alytes_.

XXIII

ABOUT THE STARS

The young astronomer in _Two on a Tower_—that bitter-sweet story in which our great novelist Hardy tells of the weird fascination with which the study of the stars appeals to a sensitive nature, exclaims: “The imaginary picture of the sky as the concavity of a dome whose base extends from horizon to horizon of our earth, is grand, simply grand, and I wish I had never got beyond looking at it in that way. But the actual sky is a horror.” “There is,” he continues, “a size at which dignity begins; further on there is a size at which grandeur begins; further on there is a size at which solemnity begins; further on a size at which awfulness begins; further on a size at which ghastliness begins. That size faintly approaches the size of the stellar universe.” “If you are cheerful and wish to remain so,” he concludes, “leave the study of astronomy alone. Of all the sciences, it alone deserves the character of the terrible. If, on the other hand, you are restless and anxious about the future, study astronomy at once—your troubles will be reduced amazingly. But your study will reduce them in a singular way, by reducing the importance of everything, so that the science is still terrible, even as a panacea.” The facts revealed by the study of astronomy which have this feature of ghastliness and terror relate to the enormous distances in space at which the stars are placed, and to their enormous number.

One may sometimes see on the coast or in some marshland a “pile-driver” at work. At a quarter of a mile distance you can see the great weight hoisted up by cranks and chains above the “pile,” which stands upright but not yet driven very far into the ground. You see the weight let go; it drops vertically on to the pile, and you watch it rising some two or three feet on its return journey upwards, when suddenly you hear the sound of a sharp blow, and only after an effort realise that the sound was made more than a second ago, and that the workmen have had time to raise the weight 3 ft. before the sound travelled to you. Sound travels less than a quarter of a mile in a second. Light also takes time to travel, but it advances ever so much more quickly than sound, namely, 186,000 miles (and a bit more) in a second. It is, therefore, easy to calculate the number of miles traversed by light in a minute or in a year. There are thirty million seconds in a year. The light of the sun takes eight minutes to reach the earth, so, instead of stating the number of miles of this distance, we may say that the sun is eight “light-minutes” distant from the earth (about 89,000,000 miles). This is an enormous figure. The sun and his planets may be represented proportionately by a golden ball a foot in diameter, and a number of little spheres varying in size from that of a dried pea to a boy’s marble, placed at distances from the golden ball varying from 50 ft. to 200 ft. Such a model is shown in the Museum of Practical Geology in Jermyn Street, London. Minute and scattered far apart as the planets of the solar system appear when thus represented, yet the solar system is a compact little group when we come to consider the distance from it of the other suns—the “fixed stars,” which exist literally in millions beyond it. The nearest of these stars (its name is Alpha Centauri) is no less than three light-years distant from us. A light-year is five and a half billion (that is, five and a half million million) miles. The nearest sun to us after our own sun is, therefore, about sixteen billion miles away, and if its light were suddenly extinguished, we should not know of its extinction for three years.