Fine Structure of Kappa in Paramecium aurelia

P. aurdia, variety 4, stock 31 killer, mating type VII (51-TK), and the identical organism without kappa, stock 51 sensitive (51.7 S), were grown at 27°C. in a volume of timothy hay infusion inoculated with Aerobacter aerogcne.s; this medium supported two fissions per day. Standard procedures were used to fix, embed, and section the paramecia. In brief, concentrated organisms were fixed by the addition of an equal volume of 2 per cent OsO, in tap water. After dehydration in graded alcohol, they were embedded in a mixture of 75 per cent n-butyl methacrylate and 25 per cent methyl methacrylate polymerized with 2,4-dichlorobenzoyl peroxide for 48 hours, sectioned on a modified Spencer rotary microtome and examined in the electron microscope (RCA model EMU-2B) with intermediate lens and 25/~ objective aperture.