Cilia were isolated from Tetrahymena pyriformis by an ethanol-calcium procedure. Solutions of outer-fiber protein were obtained either by aqueous extraction of an acetone powder of whole cilia, or by dissolving the isolated outer-fibers in 0.6 M KC1. In aqueous solution, the outer-fiber protein has a sedimentation coefficient of 6.0S and a molecular weight of 104,000 : 14,000. In 5 M guanidine hydrochloride solution the molecular weight falls to 55,000 5,000. After reduction and alkylation in 8 M urea, about 95% of the protein migrates as a single band on electrophoresis in polyacrylamide gel at pH 8.9; the migration velocity is identical with that of reduced and alkylated actin. Freshly prepared outer-fiber protein contains about 7.5 sulfhydryl groups per 55,000 g of protein.The amino acid composition of outer-fiber protein resembles that of actin, with such differences as occur being of the same order as those between actins from different species of animal.

The development of basal bodies and flagella in the water mold Allomyces arbusculus has been studied with the electron microscope. A small pre-existing centriole, about 160 mµ in length, was found in an inpocketing of the nuclear membrane in the vegetative hypha. Thus, formation of a basal body does not occur de novo. When the hyphal tip started to differentiate into gametangia, the centrioles were found to exist in pairs. One of the members of the pair then grew distally to more than three times its original length, whereas the other remained the same size. The larger centriole would correspond to the basal body of a future gamete. Gametogenesis was usually induced by transferring a "ripe" culture to distilled water. Shortly after this was done, a few vesicles were pinched off from the cell membrane of the gametangium and came in contact with the basal body. Apparently, they fused and formed a large primary vesicle. The flagellum then started to grow by invaginating into it. Flagellar fibers were evident from the very beginning. As the flagellum grew so did the vesicle by fusion with secondary vesicles, thus coming to form the flagellar sheath. The different stages of flagellar morphogenesis are described and the possible interrelationships with other processes are discussed.

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Flagella from sperm of the sea urchin Strongylocentrotus droebachiensis and cilia from Tetrahymena pyriformis contain guanine nucleotides bound to the outer-fiber fraction in the ratio of one mole of nucleotide per mole of protein subunit.