1. A simple aquarium.—A simple aquarium, in which the development of the frog from the egg may be watched, is easily made. Obtain a fairly large basin, and cover the bottom with sand, mud, and stones from a pond. Arrange these so that the bottom shall shelve from the surface of the water at one side to a depth of 3” or 4” at the other. Put in some stones covered with green slime, which will almost certainly be found in the pond, plant a few water-weeds, and allow the water to clear.
2. Frog spawn.—Having prepared the aquarium, obtain, towards the end of March (or earlier in a mild spring) a handful of frog spawn from a pond or ditch. It forms a mass of jelly in which the true eggs—small balls about ¹/₁₀” in diameter—are imbedded. Put this in the aquarium and examine it carefully every day, making the observations described below. If possible, obtain a pair of spawning frogs, and place them in a bucket with a little water, so that the earliest stages also may be studied.
3. The eggs before hatching.—Observe the globe of jelly which surrounds each egg. Try to pick it up between your finger and thumb. Do you think it is of any use in protecting the eggs from being eaten by fishes, birds, etc.? Remove the jelly as completely as possible from one egg, put the egg in a watch-glass with a little water, and examine it carefully with a strong lens. To which of the stages shown in [Fig. 218] does it correspond? If possible, treat a newly-laid egg in the same manner and examine it every hour or so through the day. Make notes of the time elapsing between the various stages shown in [Fig. 218].
4. Hatching.—Notice that the developing eggs change from the spherical to the ovoid form. What is the shape when the embryo begins to move? Notice the appearance of a neck and a tail. At this stage the embryo makes its way out of the jelly, or hatches. The jelly may now be thrown away, as it is of no further use. How does the embryo behave immediately after hatching? Put one in a watch-glass and with a lens try to see the sucker by which it attaches itself to water-weeds, etc.
5. Tadpolehood.—Examine the tadpoles at frequent intervals by the help of a lens, and write careful accounts of the changes which take place in them. Notice particularly the fine threads—the external gills—which grow out from the sides of the neck. How many are there? After a time they shrivel up and are replaced by other gills which cannot be seen. Write a description of a tadpole at the time when it begins to feed. How large is it?
Count the tadpoles in the aquarium at intervals of a few days, carefully removing all dead ones. Supply fresh green slime from the pond from time to time. Have you any reason to suppose that tadpoles are cannibals?
How soon do the tadpoles come to the surface to breathe air? Describe the appearance of one when it begins to do this. Has it any legs? How many can you see? In what order do the legs grow out?
Notice now the dwindling of the tail. (It does not drop off, as is so often stated.) The tadpole has become a frog. What percentage of the original eggs have developed into frogs?
