Common Bullfrog — Part 6

Rana catesbiana Shaw

The lateral-line organs are easily made out because of the large size of the tadpole.

On the second day of our observation (July 4th) the mouth is beginning its transformation. It is much wider, and as it is constantly opened and shut in breathing, the rudimentary tongue is visible on its floor. The fringe of the lower lip is still present, but the horny beak within seems to have disappeared. From this time to the end of the change the tadpole does not eat.

The elbow of the left arm is now visible within the breathing-pore. The skin of the right side has become thinner, so that the movement of the right arm is plainly visible, and before night the movement has worn a hole through the skin, and the right elbow is plainly visible also. We are not surprised in the morning to see the left arm projecting through the breathing-pore. Two hours later the right arm is pushed out so that both arms are now free. These arms begin their growth at the time when the back legs appear, but are concealed within the gill-chambers.

On this day the tadpole breathes with much effort. The mouth, which is still increasing in width and shows the developing tongue within, is opened and shut constantly. The gills have begun to be absorbed, and the lungs, although developed, are not yet fully in use. The left arm blocks up the breathing-pore, so that little of the water sent to the gills can escape there. The water is taken in at the mouth and nostrils to the gills of the throat, and is expelled through the mouth, instead of through the breathing-pore. The lungs are used somewhat; the tadpole very often comes to the surface of the water and gives out a bubble of air from the mouth. In a very few days (by July 9th) a decided change takes place: the gills are wholly absorbed; the lungs are fully in use; the mouth is kept closed, and the tadpole is an air-breather. Now the tadpole spends much time at the surface of the water, sometimes rushing madly from the bottom to exchange a bubble of impure air for fresh. He is very comfortable out of water.

The developing tadpole is white on the under side of the body and yellow on the under side of the legs. The mouth is open back to a point on a line with the front of the eye. The legs are greatly developed, and the tail is becoming shorter and narrower. It is dark in color and almost black at its end. The tail is no longer used in swimming, but the legs do all of the work. The process of absorption has begun by which all the living part of the tail retreats into the body of the tadpole and helps in the formation of new organs. The wandering white-blood corpuscles (phagocytes) accomplish this retreat of the tail substance in such a manner that the external skin of the tail is not broken, and in fact, nothing of the process, except the result, is visible from the outside. These corpuscles move out into the tail and carry the particles one by one into the body to serve as food for the tadpole. It is therefore true that a tadpole "eats" its tail, though not at all according to the customary manner of eating. The decrease in the size of the tail proceeds rapidly until there is a mere black stub left (Fig. 289), and our tadpole looks no longer like a tadpole, but, instead, like a fully formed frog. The skin is more distinctly mottled. That about the eyes and nose is green and iridescent. The eyes are more elevated, showing rounded black centres surrounded by a broad iris, reddish gold in colour. The mouth is open to a point on a line with the back of the eye. The lines where the membrane of the gill-chambers joined with the body wall are still visible around the arms. The head is kept above the water; the nostrils are constantly moving and the throat pulsating.

It is not until four days later that the ear membrane is visible. Now the tail is wholly gone (July 16th). The frog begins to show hiding and burrowing habits. In fact, young frogs at this stage remain concealed under water, mud, or moss most of the time, except at night. During the first summer the young Bullfrog eats insects of all sorts, but by fall it is capable of disposing of small fish. The Bullfrog, whose transformation has been traced, was heard to croak for the first time in early July, one year after its change. At this time the ears had gradually increased in size until they were considerably larger than the eyes. Head and body together measured over four inches in length.