The Frog has no neck, the base of his skull coming close to the collar-bones, and there are only a few pairs of very short apologies for ribs between the shoulders and the long pelvis which produces that steep incline at the rear of his back. He is clothed entirely with a smooth, soft skin, which is kept moist by the action of minute mucous glands distributed all over the body. A row of these glands of larger size forms a pale line running back from the eye on either side. The skin plays an important part in the oxygenation of the Frog's blood; and the experimental physiologists have shown that a Frog deprived of its lungs can carry on its respiration for a lengthened period through the skin alone. Owing to the absence of ribs he has to fill his lungs by swallowing air.

The male is less portly than the female, and he is further distinguished by having two pads on the first finger which in the breeding season become large rough cushions enabling him to hold his mate. In his throat there is a pair of vocal sacs enabling him to produce his love songs, and when these are in use their inflation causes a distension of the skin of the throat; but without these adjuncts the female manages to give answering croakings. When these duets are sung under water they produce some curious effects.

When the pairing season arrives—quite early, usually about the middle of March, but sometimes in February—all the Frogs that have just come out of hibernation select their mates. Any pool of water will do, however transient, and they often make mistakes in this matter, their egg-masses being left high and dry when the waters dry up. The eggs are deposited in a mass of a thousand to two thousand at the bottom of the water, and at first they are only about a tenth of an inch in diameter, but the gelatinous covering absorbs so much water that they swell up to a third of an inch. There is a corresponding lightening of the mass, which floats to the surface and is available for observation. Each of the little jelly-spheres is seen to have a black centre—the egg proper—with a white spot on the lower side. If the spring is an average one, in about four weeks' time the black specks will have developed into brown larvæ or tadpoles, and having escaped from the egg these will be clinging to the remains of the jelly mass by means of a pair of suckers on the underside of the head. There are at present no indications of limbs—head, body and tail, like those of a fish, merge one into another. Even the gills are not yet developed, though what we may term the buds of them are seen on the bars separating the slits behind the head on each side. These buds soon expand into gill-plumes through which the blood circulates, taking up oxygen from the water that passes between them. There is as yet no mouth, but this will soon open, and horny plates on its jaws will enable the tadpole to crop soft vegetable matter, upon which it subsists chiefly. Later on, the gill-plumes will be hidden by a flap which grows over them. The full series of stages in this development may easily be watched by keeping a few tadpoles in a glass of water with a little growing pond weed.

Ultimately, the limbs appear. Though all four develop simultaneously, the hind pair appear first, because the forelimbs are at first hidden by the flap which grew over the gills. After the disappearance of the gill-plumes, proper lungs are developed inside the body, and the animal changes from a fish-like water-breather to an air-breather, in preparation for a life on land. When all the legs are well out the form of the tadpole soon changes to that of the Frog, except that it has a long tail. You may read in some books that the tail is shed, but this is a mistake that no one could make who has watched day by day the evolution of the Frog from the tadpole. The tail is absorbed; it gets smaller daily, until finally the hind body is rounded off and there is nothing left to indicate that it once ended in a tail. Ultimately the Frog may attain a length of head and body equal to four inches.

[Pl. 98.]][L 160.
Common Frog.
Rana temporaris.

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