TH XXII TADPOLES AND FROGS HE 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 1 8 FIG. 43.-Stages in the growth from the egg of the common 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. 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 An 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 |