Science from an Easy Chair

by Sir E. Ray Lankester

[Transcriber’s Note: The original image is approximately 2¾ inches (7.5cm) high and 1¾ inches (4.5cm) wide in total.]

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).

Fig. 44.—Outline drawings of three European tadpoles of the actual size of nature. A, a full-sized tadpole of the Common Frog, Rana temporaria, one inch and three-tenths long. B, a fair-sized tadpole of the Obstetric Toad, Alytes obstetricans, common near Paris, two inches and four-fifths long. C, a tadpole of the Garlic Toad, Pelobates fuscus, common in France, Belgium, and Germany, four inches and a half long. Specimens of as much as seven inches in length have been captured.