Fig. 218.—The early development of the Frog. mi, small black cells; mg, large white yolk-cells; ect, black cells overspreading yolk-cells; yk.pl, yolk-plug; md.gr, groove of commencing nervous system; md.f, right margin of groove; br.cl, depressions marking position of future gill-slits; stdm, pit which will become mouth; t, tail; br.1, br.2, external gills; e, eye; sk, sucker. (× 5.)
The early development of the frog’s egg.—A frog’s egg is a little spherical mass about one-tenth of an inch in diameter. When laid, it is covered by a thin gelatinous layer which soon swells up in the water to form a transparent globe of about half-an-inch diameter, in the centre of which the true egg can be seen. This jelly is extremely slippery and difficult to grasp, and is consequently an efficient protection against the attacks of birds, fishes, insect-larvae, etc., and even of parasitic fungi and other small organisms. The jelly also acts as a float. At the time of laying, each egg consists of a black and a white portion. In the lower, or white, part there is a store of food, on which the little embryo subsists until it acquires a mouth and begins to fend for itself. The development begins in the upper, or black, part of the egg, and may easily be watched with a lens. And to be appreciated properly, the changes should be watched, and not merely read about. The student should get, if possible, some freshly-laid frog’s eggs, and remove the gelatinous investment from one or two. It will require care to do this without injuring the eggs. They should now be put, with a little fresh water, into a watch-glass, and carefully examined at intervals of half an hour or so. Soon a little pit makes its appearance in the middle of the black half, and gradually extends until it becomes a groove, which little by little reaches quite round the egg ([Fig. 218], A). In the meantime another groove begins to form at right angles to the first, and, in its turn, grows down round the egg (B). If we compare the whole egg to the earth, and the middle of the black half to the North Pole, these two grooves may be said to be along meridians at right angles to each other. The third groove (C) may be considered as along a parallel of latitude, but it is somewhat to the north of the Equator. These first three grooves deepen until the whole egg is cut up into eight separate pieces. The sight of the apparently lifeless speck dividing itself up in this regular and orderly manner, “while you wait,” is an intellectual treat which should not be missed. The cleavage of the egg goes on rapidly, but in a less regular manner now, until the whole is cut up into a hollow sphere of segments (F), black and small (mi) in the northern hemisphere, whitish and larger (mg) in the south.
It is worth while to pause here to consider how these early changes are assisted by the peculiar condition of the egg at the time of laying. The southern hemisphere of the egg is laden with a store of food. The food is dead, and acts as a mechanical hindrance to the activity of the living matter. In the northern half of the egg but little food is present to impede its activity, and it is plainly important that this half shall receive as much warmth as possible from the uncertain sunshine of early spring. Two circumstances ensure this. In the first place, the food-laden region is the heaviest part of the egg, so that the latter—buoyed up as it is by the jelly—tends to float with its most “alive” part upwards. Secondly, this upper part is coloured black,—a great advantage, since black objects absorb heat readily. Both these peculiarities therefore favour the more rapid development of the “northern” hemisphere. Hence the third cleft is to the north of the egg’s equator, instead of being halfway between the upper and lower poles, and hence, too, at the close of segmentation the northern segments are smaller and more numerous than the southern. In the case of the hen’s egg ([Chapter XVI.]), the amount of stored food contained in the egg (the yolk) is so enormous that segmentation is confined entirely to a small patch on the upper surface. Another result of the relatively small amount of stored food in the frog’s egg is that the tadpole is compelled to turn out and earn its own living at a stage when the chick’s inherited fortune is still considerable.
The tadpole.—The spherical mass soon becomes ovoid, and is divided into head and trunk by a neck-constriction (J). An occasional wriggle shows that the creature is alive. Shortly afterwards a tail grows out from the hinder end of the trunk, giving the animal something of the appearance of a fish (L). In this stage the tadpole makes its way out of the jelly, and thus hatches, about a fortnight after the laying of the eggs. The little creature is quite helpless; it has no mouth (the egg food is not yet exhausted, however), and its attempts at swimming are still feeble and uncertain. In this defenceless condition ([Fig. 219], 1) it will be seen to attach itself to the water-weed of the aquarium by means of a sucker ([Fig. 218], L, sk) on the underside of its head. It is in a very favourable position for examination, and by help of a lens, two—soon there are three—pairs of fine, thread-like outgrowths can be distinguished on the sides of the neck ([Fig. 219], 2 and 2a). These are the external gills. In a few days the mouth breaks through, and the animal begins to nibble at the vegetation with little horny jaws, and soon swims about the aquarium with confidence.
Fig. 219.—The Frog. Stages in the life-history, from the newly-hatched Tadpoles (1) to the young Frog (8). (× 1.) 2a is a magnified view of 2.
The gills are the organs by which the young tadpole breathes, and each little gill-thread is seen, when viewed with a low power of the microscope, to contain tiny loops of blood-vessels. Every time the tadpole uses his jaws, wags his tail, or, in short, does anything at all, he uses up some oxygen and produces some carbon dioxide. The water of the aquarium contains dissolved oxygen—green water-weeds are kept in aquaria for the express purpose of liberating oxygen ([p. 51])—and this oxygen makes its way through the excessively thin membrane which divides the blood of the gills from the water, to be swept away by the current of the blood to the various parts of the body. The carbon dioxide produced is brought to the gills by the blood stream, and passes through the membrane into the water, where it is utilised by the water-weed as food ([p. 51]). Animal and plant are thus mutually serviceable.
A series of slits soon opens through the sides of the neck, and along their margins are formed folds which are usually called the internal gills. The external gills dwindle and shrivel up as the internal gills are being formed, and at the same time a flap of skin grows backwards from each side of the head and covers over the slits so that they cannot be seen. Presently the two flaps fuse at their edges—except at one point on the left side, where a spout is left—and so enclose a chamber. The water which enters the tadpole’s mouth pours through the gill-slits, into the chamber, and out through the spout. As it swills over the folds of the internal gills there is an exchange of carbon dioxide for oxygen in the manner just described.
The tadpole in the meantime is growing strong and active, and the tail has grown out to form a powerful organ, the sinuous motion of which propels the animal with relatively great speed through the water.