The creature that comes from the egg looks nothing whatever like a frog. It has no limbs whatever, and consists mainly of a bulky head and tail. This is the tadpole stage in the development of the frog. It can exist only in water, breathing air therefrom by means of gills. Like the fish, it has a two-chambered heart. At this stage it has no lungs, and the gills consist of an external (Fig. 17, a) and an internal pair. The mouth is small, with only horny toothless jaws, with no tongue. The creature is herbivorous, living on decaying vegetable matter. The vertebræ of the spinal column are bi-concave, as in fishes. The tadpole is essentially a fish, and would be so classed if it did not develop further. An evolving fish does not go beyond this stage. But the developing frog does go beyond this stage to a higher one. As its evolution proceeds through the multiplication and differentiation of the cells that form its body, limbs begin to bud out, first posteriorly (Fig. 17, b) and then anteriorly (Fig. 17, c). The lungs now begin to develop, and the external gills dwindle more and more until they soon disappear, the internal ones persisting for a while longer. The tongue, at this stage, also makes its appearance. The creature now can breathe both air and water. This is the permanent condition of many adult amphibians belonging to a lower order than the mature frog, such as the _siren_, _menobranchus_, etc. The siren in developing also passes through the fish stage, but does not get beyond the siren stage. But the evolving frog does go beyond this stage, for with the growth of the legs the tail dwindles slowly by its gradual absorption (Fig. 17, d). The internal gills now disappear through absorption, and the lungs develop more thoroughly. Great changes take place in the blood-vascular system, the fish-like, two-chambered heart evolving into the three-chambered, amphibian heart. In spite of its dwindling, the tail is still a very conspicuous organ. In this phase of its development the frog can breathe only air, and must frequently come to the surface of the water for that purpose, and soon leaves the water altogether. Now this stage of the creature’s development corresponds to the permanent adult condition of another order of amphibians, which is higher than that to which the siren belongs but lower than the order of the adult frog. This intermediate order has such creatures in it as the _triton_. The triton in developing passes through the fish and siren stages, but does not get higher than the triton stage. But the evolving frog goes even higher than this triton-like condition. Its tail is more and more absorbed until it finally disappears, and then the young but perfect frog appears (Fig. 17, e). During this period the teeth develop and the creature becomes carnivorous, feeding on insects. It is thus seen that the developing frog passes by small gradations from one class (the fish class) to an altogether different and higher class (the amphibian