The early development of the frog’s egg.—A frog’s egg is a little spherical mass about one-tenth of an inch in diameter. When laid, it is covered by a thin gelatinous layer which soon swells up in the water to form a transparent globe of about half-an-inch diameter, in the centre of which the true egg can be seen. This jelly is extremely slippery and difficult to grasp, and is consequently an efficient protection against the attacks of birds, fishes, insect-larvae, etc., and even of parasitic fungi and other small organisms. The jelly also acts as a float. At the time of laying, each egg consists of a black and a white portion. In the lower, or white, part there is a store of food, on which the little embryo subsists [Pg 342] until it acquires a mouth and begins to fend for itself. The development begins in the upper, or black, part of the egg, and may easily be watched with a lens. And to be appreciated properly, the changes should be watched, and not merely read about. The student should get, if possible, some freshly-laid frog’s eggs, and remove the gelatinous investment from one or two. It will require care to do this without injuring the eggs. They should now be put, with a little fresh water, into a watch-glass, and carefully examined at intervals of half an hour or so. Soon a little pit makes its appearance in the middle of the black half, and gradually extends until it becomes a groove, which [Pg 343] little by little reaches quite round the egg (Fig. 218, A). In the meantime another groove begins to form at right angles to the first, and, in its turn, grows down round the egg (B). If we compare the whole egg to the earth, and the middle of the black half to the North Pole, these two grooves may be said to be along meridians at right angles to each other. The third groove (C) may be considered as along a parallel of latitude, but it is somewhat to the north of the Equator. These first three grooves deepen until the whole egg is cut up into eight separate pieces. The sight of the apparently lifeless speck dividing itself up in this regular and orderly manner, “while you wait,” is an intellectual treat which should not be missed. The cleavage of the egg goes on rapidly, but in a less regular manner now, until the whole is cut up into a hollow sphere of segments (F), black and small (mi) in the northern hemisphere, whitish and larger (mg) in the south.