[Illustration: Development of a frog. 1, two cell stage; 2, four cell stage; 3, 8 cells are formed, notice the upper cells are smaller; in (4) the lower cells are seen to be much larger because of the yolk; 5, the egg has continued to divide and has formed a gastrula; 6, 7, the body is lengthening, head is seen at the right hand end; 8, the young tadpole with external gills; 9, 10, the gills are internal, hind legs beginning to form; 11, the hind legs show plainly; 12, 13, 14, later stages in development; 15, the adult frog. Figures 1, 2, 3, 4, 5, 6, and 7 are very much enlarged. (Drawn after Leukart and Kny by Frank M. Wheat.)] The eggs of this frog are laid in shallow water in the early spring. Masses of several hundred, which may be found attached to twigs or other supports under water, are deposited at a single laying. Immediately before leaving the body of the female they receive a coating of jellylike material, which swells up after the eggs are laid. Thus they are protected from the attack of fish or other animals which might use them as food. The upper side of the egg is dark, the light-colored side being weighted down with a supply of yolk (food). The fertilized egg soon segments (divides into many cells), and in a few days, if the weather is warm, these eggs have each grown into an oblong body which shows the form of a tadpole. Shortly after the tadpole wriggles out of the jellylike case and begins life outside the egg. At first it remains attached to some water weed by means of a pair of suckerlike projections; later a mouth is formed, and the tadpole begins to feed upon algae or other tiny water plants. At this time, about two weeks after the eggs were laid, gills are present on the outside of the body. Soon after, the external gills are replaced by gills which grow out under a fold of the skin which forms an operculum somewhat as in the fish. Water reaches the gills through the mouth and passes out through a hole on the left side of the body. As the tadpole grows larger, legs appear, the hind legs first, although for a time locomotion is performed by means of the tail. In the leopard frog the change from the egg to adult is completed in one summer. In late July or early August, the tadpole begins to eat less, the tail becomes smaller (being absorbed into other parts of the body), and before long the transformation from the tadpole to the young frog is complete. In the green frog and bullfrog the metamorphosis is not completed until the beginning of the second summer. The large tadpoles of such forms bury themselves in the soft mud of the pond bottom during the winter.