Shortly after the legs appear, the gills begin to be absorbed, and lungs take their place. At this time the young animal may be seen coming to the surface of the water for air. Changes in the diet of the animal also occur; the long, coiled intestine is transformed into a much shorter one. The animal, now insectivorous in its diet, becomes provided with tiny teeth and a mobile tongue, instead of keeping the horny jaws used in scraping off algae. After the tail has been completely absorbed and the legs have become full grown, there is no further structural change, and the metamorphosis is complete. [Illustration: At the left is a hen's egg, opened to show the embryo at the center (the spot surrounded by a lighter area). At the right is an English sparrow one day after hatching.] Development of Birds.--The white of the hen's egg is put on during the passage of the real egg (which is in the yolk or yellow portion) to the outside of the body. Before the egg is laid a shell is secreted over its surface. If the fertilized egg of a hen be broken and carefully examined, on the surface of the yolk will be found a little circular disk. This is the beginning of the growth of an _embryo_ chick. If a series of eggs taken from an incubator at periods of twenty-four hours or less apart were examined, this spot would be found at first to increase in size; later the little embryo would be found lying on the surface. Still later small blood vessels could be made out reaching into the yolk for food, the tiny heart beating as early as the second day of incubation. After about three weeks of incubation the little chick hatches; that is, breaks the shell, and emerges in almost the same form as the adult. [Illustration: The embryo (_e_) of a mammal, showing the absorbing organ in black, the branch-like processes which absorb blood from the mother being shown at (_v_); _ct_, the tube connecting the embryo with the absorbing organ or placenta.]