The tadpole is half an inch long when hatched; the mouth is distinct, but small and without lips; the gills rapidly enlarge, and when at their maximum development afford beautiful objects for displaying the circulation; the gills soon begin to decrease in size, and are finally withdrawn within the branchial cavity, as in fishes, and concealed by an opercular fold of integument; the eyes are perfectly formed; the mouth acquires movable lips, is placed nearer the end of the head, and is used for the introduction of vegetable food; the caudal fin increases in size, and serves for rapid locomotion. Without any great change in form, the size is rapidly increased; two small tubercles appear near the vent, the rudiments of the posterior legs, which are soon developed into the perfect limbs; the anterior limbs are afterward formed under the skin in a similar manner; as the legs are perfected the tail is gradually absorbed from the tip to the base, and progression is effected by the hind limbs. The lungs are now fitted for the respiration of air, and the little creatures come on land in search of worms and insects, and in such multitudes in damp weather as to give rise to the belief, still popularly adhered to in many places, that it has rained frogs.

They grow rapidly during the summer and autumn, and in winter plunge into the mud to pass their stage of hibernation. In the tadpole state great numbers are devoured by fishes, other reptiles, and by each other; and the adults furnish food for all classes of vertebrata from fishes up to man himself. It is probable that not more than one in a thousand of those which come from the egg in the spring live to reach their winter retreat; if fortunate enough to escape from all enemies, frogs may live many years. Serpents among reptiles, pickerel among fishes, vultures, storks, herons, and cranes among birds, are the worst enemies of frogs; were it not for the storks of Egypt, that country would be overrun with frogs. When it is remembered that each female frog of the hundreds in a single locality may produce 1,000 young, which hide in crevices in the earth and under stones, ready to come forth to enjoy the genial summer showers, there is no necessity for any attempt to explain the appearance of the frog multitudes by supposing them to have fallen from the clouds, as has been believed even from the time of Aristotle, or by the supposition that they have been taken up from some marsh by a whirlwind and let fall during a rain; the latter occurrence, on a small scale, is not impossible, in exceptional cases.

The frogs which thus appear bear marks of their recent metamorphosis, in the remnant of a tail and other organs; crawling as they naturally would into the ground, the swelling of the earth from rain would drive them out by compression. From facts recorded in the Annals and Magazine of Natural History" (1853, pp. 341 and 482), it would seem that frogs and toads may be reproduced without passing through the intermediate stage of tadpole; it is only of late years that many common fishes have been ascertained to be viviparous, and it is not improbable that eggs laid in localities where water cannot be obtained, as in cellars and hot houses and beds, may produce frogs, whose larval form is very soon exchanged for the perfect state, the gills being prematurely cast to enable the animal to accommodate itself to its new circumstances; and it may be, as Mr. Jenyns remarks, that the frogs are hatched on land in the perfect state, the gills either never having existed or having disappeared immediately after birth.

On the other hand, it has been ascertained that the larval or tadpole state may be unnaturally prolonged; Prof. J. Wyman (in the Proceedings of the American Academy of Arts and Sciences," vol. iii., p. 35) experimented on the tadpoles of the common bullfrog, the greater number of which pass the winter without having undergone metamorphosis, not becoming perfect animals until the following spring; he found that the tadpole state, by the influence of darkness and low temperature, could be prolonged certainly from one to two years, and probably much longer; possibly some of the cases referred to by Mr. Jenyns and others may admit of explanation by prolongation rather than an absence of the larval condition, the young frogs having been the result of tadpoles which had passed their larval condition in some other locality, or in the same in a torpid state for a year.-The tenacity of life in frogs is very great; they survive the severest wounds, live a long time after the heart and entrails are removed, and display muscular contractility and the phenomena of circulation in various organs for many minutes and even hours after death has actually taken place.

On this account the frog has from time immemorial been selected as a subject of experiment to ascertain and illustrate the most important phenomena of human physiology, and has in this way been of inestimable advantage to mankind. The change of a fish-like animal, breathing by means of gills in water, to a leaping, air-breathing creature, with the corresponding modifications of food and habits, is well calculated to excite the admiration of a thinking person. The air cells of the frog's lungs, the membrane of its foot, and the delicate fringe of the tadpole's gills, afford admirable and easily obtained tissues for demonstrating under the microscope the circulation in the capillary vessels, with their chains of moving blood globules. The structure of the lungs and the mechanism of their respiration furnished to anatomists and physiologists proof of the changes which the blood undergoes under the influence of the oxygen of the air through the medium of a thin intervening vascular wall. The sensibility of their muscles to the galvanic currents led Galvani and Volta to most important discoveries in electricity and galvanism, whence flowed the great results obtained by Bell, Faraday, and Matteucci in the physiology of the nervous system, and by Davy and others in physics and the chemical constitution of bodies previously supposed simple.