des of the larger anterior portion
shallow grooves make their appearance and soon break through from the
throat or pharynx to the exterior as gill-slits. Shortly afterwards the
little embryo wriggles out of its encasing coat of jelly, develops a
mouth, and begins its independent existence as a small tadpole, with eyes,
nasal and auditory organs, and all other parts that are necessary for a
free life. Thus the one-celled egg has transformed into something that it
was not at first, and in doing this it has proved the possibility and the
reality of organic reconstruction.

The tadpole breathes by means of its gills, and it is at first entirely
devoid of the lungs which the adult frog possesses and uses. When we speak
of the larval respiratory organs as gills we imply that they are like the
organs of a fish which have the same name; they are truly like those of
fishes, for the blood-vessels which go to them are essentially the same as
in the lower types and they are supported by simple skeletal rods like the
gill-bars of the fish. In a word, they are the same things.

The animal feeds and grows during the months of its first summer, and
hibernates the following winter; with the warmth of spring it revives and
proceeds further along the course of its development. Near the base of the
tail two minute legs grow out from the hinder part of the body, and while
these are enlarging two front legs make their appearance a little behind
the gills. The tadpole now rises more frequently to the surface where it
takes small mouthfuls of air. Meanwhile great changes are effected inside
the body where the various systems of fishlike organs become remodeled
into amphibian structures. A sac is formed from the wall of the esophagus,
and this enlarges and divides to form the two simple lungs. The legs
increase in size, the tail dwindles more and more, the gills close up, and
soon the animal hops out on land as a complete young frog. From this time
on it breathes by means of its lungs instead of gills, even though it
returns to the water to escape its foes, to seek its prey, and to
hibernate in the mud of the lake bed during the winter months.

All these changes are familiar and natural, but until science places them
and similar facts in their proper relations their significance is lost to
us. The tadpole is essentially a fish in its general structure and mode of
life, even though its heritage is such that it can develop into a higher
animal.