Segregations of specific cytoplasms occur during early cleavage of the ascidian egg (Conklin, 1905) of which the most obvious is the localization of the myoplasm, presumptive for larval musculature, in the posterior cells at the four-cell stage. Cytological (Meves, 1913; Duesberg, 1915; Conklin, 1931) and cytochemical (Ries, 1937) studies indicate the localization of granules, presumably mitochondria, within the myoplasm. Recently Reverberi (1956) followed the distribution of mito chondria during development, using the vital stain, Janus green. The quantitative measurements of cytochrome oxidase in anterior and posterior blastomeres (Berg, 1956) gave a biochemical confirmation of the above studies as regards the localiza tion of mitochondria. The present study is a continuation of quantitative chemical analyses of anterior and posterior blastomeres from the four-cell stage of Ciona, choosing constituents which might be expected to be located on cellular particles. The minute amounts of cytoplasm available require the use of microchemical methods and limit the number of substances which may be studied; however, differences in activities of succinic de hydrogenase, apyrase, acid phosphatase, and ribonucleic acid have been found in homogenates of the two types of cells. METHODS Chorions were digested off unfertilized Ciona eggs in 3 per cent protease in sea water. The “?�naked†eggs were washed thoroughly, fertilized, and transferred to agar-coated dishes. At the end of the first cleavage the blastomeres were separated in large numbers by agitation and as each of these cleaved in turn, the anterior and posterior cells were separated with the tip of a fine braking pipette and segregated into different dishes. The posterior blastomere is recognizable by an elongated shape and a clear cyto plasmic cap (Castle, 18%). These characteristics are transitory and the period for separation is critical; separation before completion of the cleavage furrow results in cytolysis whereas shortly after cleavage identification becomes increasingly diffi cult due to sphering of the posterior cell. This period was extended by lowering the temperature to 8_100 C. after completion of cleavage, thus prolonging the elon gated state of the posterior cell. Furthermore it was discovered that with oblique