The winners in life's race

by Arabella B. Buckley

Come with me, then, to some stagnant pool in a country lane, towards the end of March, and there we shall no doubt find a whole company of frogs, croaking to their hearts’ content after their long winter sleep in the mud at the bottom of the pond. They are wide awake now, and are actively employed laying their eggs. Look carefully around the edges of the pond, especially in that part where the wind has driven the scum to the side, and you will doubtless find in some still corner a gluey mass (e, [Fig. 15]), which looks like a lump of jelly with dark specks in it. Take this up carefully, for it is frog spawn; carry it home together with some weeds from the pond; put it in a glass bowl with water; and then from day to day you may study the history of a frog’s life.

Fig. 15.

Metamorphosis of the Frog.

e. Eggs. 1. Tadpoles just out of the egg. 2. With outside gills. 3. With gills hidden, and beak-like mouth. 4. Hind legs appearing. 5. All legs grown, but fish-tail remaining. 6. Putting on Frog appearance; tail being absorbed. 7. Young perfect Frog.

That jelly-like mass is a collection of frog’s eggs. When they were laid, each egg was a small round dark body in a gluey covering, and they all fell to the bottom of the pond, where, by degrees, the water oozing through the envelope swelled each egg, till they clung altogether in a mass, and, rising, floated at the top. Then very soon each round dot lengthened out into a long streak, and in a few days an eyeless head appeared at one end with a soft closed mouth under it, and at the other a tail, with a soft fin round it like the tail of the lancelet; so that by the time you find the spawn, you may, most likely, be able to see the tiny creature wriggling every now and then in its watery bed. This will go on for some time, and a week or two may pass before the moving tadpole breaks through its egg skin, and coming out into the world, fastens on to a piece of weed (1, [Fig. 15]) by two little suckers behind its mouth. And now that it is out of the egg the interest begins. Look carefully day after day and you will see some branching tufts (2, [Fig. 15]) growing larger and larger on each side of its head. What are these? We have not seen them in any fish. No! but if you take a young hound-shark out of his leathery egg before his time, you will find that he has outside gills much like these, only he loses them before he comes out into the world, whereas the tadpole keeps them to breathe with a little longer. If you put the tadpole, at this stage, under the microscope, you can see the red blood flowing through these gills to take up air out of the water.

Meanwhile the tadpole’s lips are gradually forming into a round mouth, much like the lamprey’s, and by-and-by the inner part of this mouth is covered with two little horny jaws, forming a sharp beak (3, [Fig. 15]) with which he will nip off pieces of weed for food. Meanwhile, as he grows larger and larger, and eyes, nostrils, and flat ears form in the head, a covering begins to grow back over the sides of the neck, and little by little the branching tufts disappear (3, [Fig. 15]). How, then, can he breathe now? Watch carefully and you will see that he gulps every moment as we saw the minnow doing ([p. 23]). The fact is that the outside tufts have faded away, and under the cover the tadpole has six slits in his throat, like the slits of the lamprey, which are covered in somewhat similar fashion to those of the amphioxus (see [p. 11]), and he breathes through them.

Here is our tadpole, then, to all intents and purposes a fish. He swims with a fish’s tail; he gulps in water at his mouth, passing it out at the slits in his throat after it has poured over his fish’s gills. Moreover, he has a fish’s heart, of two chambers only, like the minnow’s ([p. 23]), which pumps the blood into these gills to be freshened, while, like the lamprey, he has a gristly cord, enlarged at the end to form a gristly skull, a round sucking mouth, and no limbs. All this time, however, though he has a fish’s fin round his tail, he has no arm or leg fins. Wait a while and you will see that under his tender skin far more useful limbs are being prepared. As he grows bigger and more active week by week, wriggling among the weeds and feeding greedily, two little bumps appear one on each side of his now bulky body, just where it joins the tail. These bumps grow larger every day, until, lo! some morning they have pierced through the skin, and two tiny hind legs (4, [Fig. 15]) are working between the body and the tail. The two front legs are longer in coming, for they are hidden under the cover which grew over the gills, but in about another week they too appear, and we have a small four-legged animal with a lamprey’s tail (5, [Fig. 15]). These legs are something far in advance of fish fins, for they have shoulders and thighs, arm and leg bones, wrist and ankle bones, hand and foot bones; and instead of the large number of rays in a fish’s fin they have four fingers on their short front legs, and five toes at the end of long hind ones; the toes being joined together by a web, which helps him wonderfully in striking the water as he swims.

The tadpole has now become fitted to jump and leap on the land or swim by his legs in the water; and, moreover, while these legs have been growing, another change has been taking place. You will notice by careful watching that at first he still gulps in water as he used to do, but he comes more often to the top, and, poising himself so that his mouth is out of the water, gives out a bubble of bad air, draws in some fresh, and goes down again. Why does he do this? Have you any recollection of another fish-like animal which comes up to take in air? Look back at our friends the mud-fishes ([p. 34]), and read how the Ceratodus fills his air-bladder when he is short of good air in the water. When you have re-read this, you will suspect that the tadpole, too, has something like an air-bladder, which he fills from time to time. And so he has. While his legs are growing a bag has been forming inside at the back of his throat, which afterwards divides into two, and he fills these by shutting his mouth, drawing air in at his nostrils, putting up the back of his tongue to shut it in, and then swallowing it down into the lungs; so that he is now a truly double-breathing animal, using his gills when below water and his lungs when above. Moreover, if you could watch inside his body, you would now see that little by little the blood-vessels going to the gills grow smaller and smaller, and those going to the lungs grow larger and larger; while the fish’s two-chambered heart divides into three chambers, one to receive the blood from the body, another to receive it from the lungs, and one to drive this blood back again through the whole animal. And when at last this change is so complete that all the blood goes to the lungs to be freshened, the gills shrivel up and disappear, and our tadpole is a true air-breathing animal.

Notice, though, that he is still cold and clammy, not warm like a mouse or a bird. For his blood still moves slowly, and as he has only three chambers to his heart instead of four, as warm-blooded animals have, the good blood from the lungs and the worn-out blood from his body become mixed each time they come round, so that his breathing work is still of a low kind all his life. And now that he can leap and swim with his legs, his tail is no longer of use to him, and it is gradually sucked in, growing shorter and shorter till it disappears, and the young frog is complete.