eighth of an inch in diameter. These are the egg masses and eggs of a
common frog. Watching them day by day we see the small one-celled egg
spheres divide into more and more numerous portions which are the
daughter-cells, destined to form by their products the many varied tissues
and organs of the developing larva and adult frog. After three or four
days the egg changes from its globular form into an oval or elliptical
mass, and from one end of this a small knob projects to become a flattened
waving tail a few days later. On the sides 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