We owe to Miss Greenwood[163] a full account of the formation and changes of the food-vacuoles in _Carchesium polypinum_. The vacuole passes steadily along the endosarc for a certain time after its sudden admission into it, and then enters on a phase of quiescence. A little later the contents of the vacuole aggregate together in the centre of the vacuole, where they are surrounded by a zone of clear liquid; this takes place in the hollow of the meganucleus, in this species horseshoe-shaped. The vacuole then slowly passes on towards the peristome, lying deep in the endosarc, and the fluid peripheral zone is absorbed. {147}For some time no change is shown in the food-material itself: this is the stage of "storage." Eventually a fresh zone of liquid, the true digestive vacuole, forms again round the food-pellet, and this contains a peptic juice, of acid reaction. The contents, so far as they are capable of being digested, liquefy and disappear. Ultimately the solid particles in their vacuole reach the anal area of the vestibule, and pass into it, to be swept away by the overflow of the food-current. The anus is seated on a transverse ridge about a third down the tube, the remaining two-thirds being the true pharynx. FISSION is usually transverse; but is oblique in the conical Heterotrichaceae, and longitudinal in the Peritrichaceae. It involves the peristome, of which one of the two sisters receives the greater, the other the lesser part; each regenerates what is missing. When there are two contractile vacuoles, as in _Paramecium_, either sister receives one, and has to form another; where there is a canal or reservoir divided at fission, an extension of this serves to give rise to a new vacuole in that sister which does not retain the old one. In some cases the fission is so unequal as to have the character of budding (_Spirochona_). We have described above (p. 144) the relations of the nuclear apparatus in fission. Several of the Ciliata divide only when encysted, and then the divisions are in close succession, forming a brood of four, rarely more. This is well seen in the common _Colpoda cucullus_. In the majority, however, ENCYSTMENT is resorted to only as a means of protection against drought, etc., or for quiet rest after a full meal (_Lacrymaria_). Maupas[164] has made a very full study of the LIFE-CYCLES of the Ciliata. He cultivated them under the usual conditions for microscopic study, _i.e._ on a slide under a thin glass cover supported by bristles to avoid pressure, preserved in a special moist chamber; and examined them at regular intervals. {148}[Illustration: FIG. 52.—_Paramecium caudatum_, stages in conjugation. _gul_, Gullet; _mg.nu_, meganucleus; _Mg.nu_, reconstructed meganucleus; _mi.nu_, micronucleus; _Mi.nu_, reconstructed micronucleus; _o_, mouth. (From Parker and Haswell, after Hertwig.)]