FISSION is usually longitudinal in the active state; a few exceptions are recorded. Encystment is not uncommon; and in the coloured forms the cyst-wall is of cellulose. Division in the cyst is usually multiple;[122] in the coloured forms, however, vegetative growth often alternates with division, giving rise to plant-like bodies. _Polytoma_ and other Chlamydomonadidae multiply by "brood-formation" in the active state; the blepharoplast, as Dangeard suggests, persisting to continue the motion of the flagella of the parent, while the rest of the plasm divides to form the brood. CONJUGATION has been observed in many species. In some species of _Chlamydomonas_ it takes place after one or both of the two {116}cells have come to rest, but in most cases it occurs between active cells. We find every transition between equal unions and differentiated sexual unions, as we shall see in discussing the Volvocaceae.[123] The "coupled-cell" differs in behaviour in the different groups, but almost always goes to rest and encysts at once, whatever it may do afterwards. The LIFE-HISTORY of many Flagellates has been successfully studied by various observers, and has shed a flood of light on many of the processes of living beings that were hitherto obscure. The first studies were carried through by the patient labours of Drysdale and Dallinger. A delicate mechanical stage enabled the observer to keep in the field of view a single Flagellate, and, when it divided into two, to follow up one of the products. A binocular eye-piece saved much fatigue, and enabled the observers to exchange places without losing sight of the special Flagellate under observation; for the one who came to relieve would put one eye to the instrument and recognise the individual Flagellate under view as he passed his hand round to the mechanism of the stage before the first watcher finally relinquished his place at the end of the spell of work. Spoon-feeding by Mrs. Dallinger enabled such shifts to be prolonged, the longest being one of nine hours by Dr. Dallinger. {117}[Illustration: FIG. 38.—_Bodo saltans._ A, the positions assumed in the springing movements of the anchored form; B, longitudinal fission of anchored forms; C, transverse fission of the same; D, fission of free-swimming form; E^1-E^4, conjugation of free-swimming with anchored form; E^5, zygote; E^6, emission of spores from zygote; F, growth of spores: _c.vac_, contractile vacuole; _fl.1_, anterior; _fl.2_, ventral flagellum; _nu_, nucleus. (From Parker's _Biology_, after Dallinger.)]