Chapter XII: Development of the Toad

FIELD AND LABORATORY EXERCISE

TECHNICAL NOTE.--As the work of this chapter, or some similar work
in getting acquainted with the postembryonic development of a
many-celled animal, should be done early in the course, and as
most schools open in the fall, it will perhaps be impossible to
make this first study of development from live specimens in the
field. In such case the examination of a series of prepared
specimens, previously obtained by the teacher, must be resorted
to. In the spring the development of several kinds of animals,
including the toad, can be studied from live specimens in the
field or in breeding-cages and aquaria in the laboratory. The eggs
of the toad may be found in April and May (the toads are heard
trilling at egg-laying time) in ponds. The eggs look like the
heads of black pins, and are in single rows in long strings of
transparent jelly, which are usually wound around sticks or
plant-stems at the bottom of the pond near the shore. Bring some
of these strings into the schoolroom and keep them in water in
shallow dishes. Keep them in the light, but not in direct
sunlight. In the dishes put some small stones and mud from the
pond, arranging them in a slope, thus making different depths of
water. Stones with green algæ on should be selected, for algæ are
the food of the tadpoles. The eggs will hatch in two or three
days, and if too many tadpoles are not kept in the dish, and the
little aquarium be well cared for, the whole postembryonic
development of the toad can be well observed. For the study of the
development from prepared specimens the teacher should have a
complete series of stages from egg to adult toad in alcohol. The
specimens may be examined by the students in connection with a
talk from the teacher on the life-history of the toad.

If the study is made from prepared specimens, make drawings of egg-strings, and of a single egg magnified and shaded to indicate its color. Draw each specimen of the series of tadpoles, noting in the youngest the presence of gills and tail and absence of legs and eyes; in the older the appearance of eyes, the shrivelling of the gills, shrinking of the tail and development of legs; in the still older the characteristic shape, in miniature, of the adult toad.

In observing the course of development of the living specimens there should be made, in addition to the drawings, notes showing the duration of the egg stage, and the time elapsing between all important changes (as seen externally) in the body of the young. Observations and notes on the general behavior of tadpoles should also be made; note the swimming, the feeding, the gradual leaving of the water, etc.

In addition to the easily seen external changes in the body, very important ones in the internal organs take place during development. Perhaps the most important of these concerns the lungs. The young gilled toad breathes as a fish does, but gradually its gills are lost, while at the same time lungs develop and the tadpole comes to the surface to breathe air like any lunged aquatic animal. The toad on leaving the water changes its diet from vegetable to animal food; a tadpole feeds on aquatic algæ; a toad preys on insects. Correlated with the change in habit, the intestine during development undergoes some marked changes, becoming relatively diminished in length.

For an account of the development of the toad see Gage's "Life-history of a Toad" or Hodge's "The Common Toad."