The first sign of their development is the formation of a short branch (Fig. 21, _A_) growing out at right angles to the main filament. This branch becomes club-shaped, and the end somewhat pointed and more slender, and curves over. This slender, curved portion is almost colorless, and is soon shut off from the rest of the branch. It is called an "antheridium," and within are produced, by internal division, numerous excessively small spermatozoids. As the branch grows, its contents become very dense, the oil drops especially increasing in number and size. About the time that the antheridium becomes shut off, a circle of buds appears about its base (Fig. 21, _B_, _og._). These are the young oögonia, which rapidly increase in size, assuming an oval form, and become separated by walls from the main branch (_C_). Unlike the antheridium, the oögonia contain a great deal of chlorophyll, appearing deep green. When ripe, the antheridium opens at the end and discharges the spermatozoids, which are, however, so very small as scarcely to be visible except with the strongest lenses. They are little oval bodies with two cilia, which may sometimes be rendered visible by staining with iodine. [Illustration: FIG. 22.--_A_, non-sexual reproduction in _Vaucheria sessilis_. _B_, non-sexual spore of _V. geminata_, × 50.] The oögonia, which at first are uniformly colored, just before maturity show a colorless space at the top, from which the chloroplasts and oil drops have disappeared (_D_), and at the same time this portion pushes out in the form of a short beak. Soon after the wall is absorbed at this point, and a portion of the contents is forced out, leaving an opening, and at the same time the remaining contents contract to form a round mass, the germ or egg cell (Fig. 21, _E_, _o_). Almost as soon as the oögonium opens, the spermatozoids collect about it and enter; but, on account of their minuteness, it is almost impossible to follow them into the egg cell, or to determine whether several or only one enter. The fertilized egg cell becomes almost at once surrounded by a wall, which rapidly thickens, and forms a resting spore. As the spore ripens, it loses its green color, becoming colorless, with a few reddish brown specks scattered through it (_F_). In some species the sexual organs are borne directly on the filament (Fig. 21, _G_).