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 [Pg 341] 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? Fig. 218.—The early development of the Frog. mi, small black cells; mg, large white yolk-cells; ect, black cells overspreading yolk-cells; yk.pl, yolk-plug; md.gr, groove of commencing nervous system; md.f, right margin of groove; br.cl, depressions marking position of future gill-slits; stdm, pit which will become mouth; t, tail; br.1, br.2, external gills; e, eye; sk, sucker. (× 5.)