The egg of animals is always originally a single “cell”—that is to say, a minute corpuscle of slimy consistence, with a dense capsulated kernel or “nucleus” within it. The kernel or nucleus divides into two, and the cell itself divides; each of the daughter cells again divides, and so the process continues, until thousands, and in larger animals millions, of cells are the result, as the mass of cells takes up nourishment and increases in volume. When (as is the case in many animals, _e.g._ starfishes, worms, and mammals) there is only a little granular food-material mixed in with the protoplasm of the egg-cell, that cell is of small size, only the one two-hundredth of an inch in diameter (see Fig. 31). But in the frog there is much granular food-material, and the egg-cell is distended to the size of a rape-seed. When there is still more, as in the bird and many fishes, the egg-cell does not entirely divide as it does in smaller eggs on commencing growth after fertilisation. The protoplasm collects into a disc incompletely separated from the food-material, and it is the disc only which divides into two, four, eight, and ever so many more cells. Some of the cells resulting from the division of the disc form the embryo’s body, and others spread, as they multiply, all over the rest of the egg-ball from its edges so as to enclose the granular food-material in a sac, called the yelk sac. In the frog, on the contrary, the protoplasm does not separate as a disc: the whole egg-cell or ball divides to form the embryo-cells, and the food granules are included in the substance of the dividing cells. “Growth from the egg” is a long story; we must revert now to the tadpoles and their parents.