[Illustration: After Schewiakoff, from Lankester's _Treatise on Zoology_. FIG. 9A.--Longitudinal section of a Gregarine in the region of the septum between protomerite and deutomerite. Pr, Protomerite. De, Deutomerite. s, Septum. en, Endoplasm. sc, Sarcocyte. c, Cuticle. m, f, Myocyte fibrils (cut across). g, Gelatinous layer.] [Illustration: FIG. 9B.--_Gregarina munieri_, showing the network of myocyte fibrillae.] The nucleus is always lodged in the endoplasm, and, in the septate forms, in the deutomeritic half of the body. It is normally spherical and always limited by a distinct nuclear membrane, which itself often contains chromatin. The most characteristic feature of the nucleus is the deeply-staining, more or less vacuolated spherical karyosome (consisting of chromatin intimately bound up with a plastinoid basis) which is invariably present. In one or two instances (e.g. _Diplocystis schneideri_) the nucleus has more than one karyosome. All the chromatin of the nucleus is not, however, confined to the karyosome, some being in the form of grains in the nuclear sap; and in some cases at any rate (e.g. _Diplodina, Lankesteria_) there is a well-marked nuclear reticulum which is impregnated with granules and dots of chromatin. [Illustration: From Wasielewski, after A. Schneider. FIG. 10.--Schizogony in _Ophryocystis francisci_. a, Rosette of small individuals, produced from a schizont which has just divided; b, A later stage, the daughter-individuals about to separate and assuming the characters of the adult.] Life-history. A sexual multiplication (schizogony) is only known certainly to occur in a few cases, one being in a Monocystid form, a species of _Gonospora_, which is for a long time intracellular (Caullery and Mesnil [4]), the rest among the _Schizogregarinae_, so named for this reason, in which schizogonous fission takes place regularly during the free, trophic condition. Usually, the body divides up, by a process of multiple fission (fig. 10), into a few (up to eight) daughter-individuals; but in a new genus (_Eleutheroschizon_), Brasil (3) finds that a great number of little merozoites are formed, and a large amount of vacuolated cytoplasm is left over unused.