The RNA-P and DNA-P content of the nucleus and the RNA-P content of the whole cell of the livers of 8- to 20-day chick embryos and of adult fowls have been determined. The DNA-P content of the liver nuclei was slightly higher in the 8- and 10-day embryo than in all the other stages examined. A significant decrease in the RNA content of the cell occurred during embryonic development. The RNA content of the adult cell was the same as that of the 14- to 16-day embryo. The proportion of the cellular RNA contributed by the nucleus also decreased during development. In respect to both nuclear RNA content and distribution of RNA between nucleus and cytoplasm, the adult resembled the 8- to 12-day embryo. Examination of the fine structure of the cell showed that, as development progressed, free ribosomes decreased in number and the rough membranes increased. Slices of 8-, 14-, and 20-day embryonic livers and of adult livers were incubated with 14 C-leucine, and the amount of labeled amino acid incorporated into whole tissue protein and into the proteins of the subcellular fractions was measured. Embryonic liver incorporated 14 C-leucine 15 to 30 times more rapidly than adult liver. The microsomal protein was always more highly labelled than the protein in any other subcellular fraction; however, in the 8-day embryonic and the adult liver the proportion of total counts found in the nuclear fraction was considerably higher than in the 14- or 20-day embryonic liver. The significance of an apparent correlation between the proportion of the cell's RNA contributed by the nucleus and the proportion of total counts in the nuclear fraction is discussed.

Vegetative cells of Saccharomyces cerevisiae were fixed with potassium permanganate followed by uranyl nitrate, embedded in methacrylate, and studied in electron micrographs of thin sections. Details of the structure of the cell wall, cytoplasmic membrane, nucleus, vacuole, and mitochondria are described. Cell membranes, about 70 to 80 A thick, have been resolved into two dense layers, 20 to 25 A thick, separated by a light layer of the same dimensions, which correspond in thickness and appearance to the components of the "unit membrane" as described by Robertson (15). The cell wall is made up of zones of different electron opacity. Underlying the cell wall is the cytoplasmic membrane, a sinuous structure with numerous invaginations. The nucleoplasm, often of uneven electron opacity, is enclosed in a pair of unit membranes in which nuclear pores are apparent. The vacuole, limited by a single unit membrane, is usually irregular in outline and contains some dense material. Rod-shaped mitochondria, 0.4 to 0.6 µ in length and 0.2 to 0.3 µ in diameter, are smaller in size, but similar in structure to some of those described in plant and animal cells. Attempts to use osmium tetroxide as fixative were unsuccessful, a result similar to that obtained by other workers. It is suggested that yeast cells are impermeable to osmium tetroxide, except when grown under specific conditions.