In an accompanying report Black (1962) has shown that considerable changes in the relative activities of several enzymes of the tricarboxylic acid cycle occur during early development of the oyster. Thus, between the blastula and trochophore stages two enzymes, TPN-specific isocitric dehydrogenase and alpha-ketoglutaric dehydrogenase, increase 2-to 3-fold, roughly paralleling the increase in respiration during this same period. Five other enzymes were found not to change appreciably during development to the trochophore. These were: aconitase, succinic dehydrogenase, malic dehydrogenase, DPNH oxidase, and cytochrome oxidase. Following the trochophore stage cytochrome oxidase decreased somewhat in activity, while the other enzymes remained constant.With the exception of isocitric dehydrogenase, most of the enzymes studied are known to be present in considerable concentration (though not usually localized) in the mitochondrial fraction of vertebrate tissues. The relationship of enzymic changes to the possible biochemical differentiation of these respiratory granules must therefore be considered. Previous investigations pertaining to this phenomenon include the finding that the content of cytochrome oxidase increases in mitochondria of differentiating rat muscle (Shen, 1955), the work reported by Weber and Boell (1955) and Boell and Weber (1955) in which an increase in the content of cytochrome oxidase and succinoxidase in mitochondria of Xenopus laevis was found during development, and the investigation of Mahler, Wittenberger and Brand (1958) in which changes in the relative activities of several respiratory enzymes were found to occur in the large granule fraction of homogenates of the chickbetween 2 and 6^ days of development.In eggs of marine invertebrates few studies have been made of the distribution of respiratory enzymes in the various cell fractions which can be obtained from homogenates. In the sea urchin mitochondria isolated in sucrose from unfertilized eggs have been shown to contain succinoxidase and cytochrome oxidase (Maggio and Ghiretti-Magaldi. 1958). The latter enzyme undergoes a 30% increase in the mitochondria at fertilization, but shows no further change until the blastula stage is reached (Maggio, 1959). Cytochemical studies of eggs of various invertebrates, in which respiratory enzymes (usually indophenol oxidase and succinic dehy-