After various changes, the green matter with several red spots is condensed into the thick round half, while the tapering beaked part is left transparent, being only filled with the watery liquid. Both bodies are then coated with cellulose, and two vibratile filaments called cilia, from their resemblance to eye-lashes, proceed from a point near the beak. The whole of these changes take place while the two bodies are still within the common cellulose covering; the moment they come out of it, by a rupture in the cell-wall, they swim about with the greatest velocity by means of their cilia, which lash the water so rapidly that they are invisible even with a microscope. The activity of these zoospores, as they are called, continues for about an hour and a half; the motion then becomes gradually less rapid; the cilia may now be seen, and soon fall off; then the bodies acquire a firmer coat of cellulose, and sink to the bottom of the water, where they remain at rest as still, or winter spores. There is great variety in the Protococcus, for the matter in the primordial cell sometimes divides not only into two equal and similar parts, but into 4, 8, 16, 32 equal and similar parts consecutively; each brood is developed into zoospores, which ultimately become resting spores. When a spore is to be formed in a primordial cell, the starch and green matter condense into a nucleus in its centre, and a membrane envelopes the liquid and the nucleus within it, so that a spore in its first stage is a free and independent cell containing azotized matter swimming in a formative liquid. If the spore is to be motile it remains of a green colour, and gets cilia; but if it is to be a winter spore, the internal matter forms into granules, mixed with particles of red oil, which coalesce into a drop, and it generally undergoes the same transformations as those which take place after the conjugation or union of two adjacent cells into one as already described. The zoospores may lose their cilia, fall to the bottom of the water as green spores, and reproduce a facsimile of the parent plant as buds do, or they may acquire a cellulose coat, undergo the transformations and change of colour mentioned, and sink to the bottom of the water as red winter spores. Under certain circumstances which do not seem to be perfectly known, it happens that during the formation of some of the zoospores the green matter is gradually changed into a red oily substance; they lose their cilia, acquire by secretion their cell-walls and a mucous envelope, and float on the water as winter spores. Should they be left dry, they may remain in that state for an indefinite length of time without losing their vitality, and as they are extremely small, they are carried by currents of air into the atmosphere, from whence they are brought down in the rain, and having fallen occasionally in places where they were never seen before, have given rise to the idea of spontaneous generation.