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We have found that inseminated eggs of the chum salmon, Oncorhynchus keta, actively release alkaline substances when they have been stimulated by deionized water; the pH of the egg suspension prepared in the non-buffered Salmon-Ringer solution shifts within at least 1.3 pH units. The nesslerization method and an ammonia gas sensitive electrode measurement reveal that the alkalization of the suspension is partly due to the efflux of ammonia from the stimulated eggs. The increase in the intracellular ammonia level leading to the alkalization of ooplasm may occur at an early stage of the embryonic development. Since the ammonia release is observed also in the eggs stimulated parthenogenetically by deionized water, it can be inferred that the stimulation of the egg triggers a reaction which increases the ammonia level in the ooplasm.

The environmental medium of stimulated eggs contains fairly large amounts of organic compounds. Its UV spectrum shows a peak at 249 nm; the position of the absorption maximum corresponds to that of inosine monophosphate (IMP). This fact suggests that IMP is one of the components of the organic compounds. The presence of IMP in the medium is also shown by thin layer chromatography. The medium contains monosaccharides and peptides. We have explained these results by postulating that an abrupt change in the activity of adenylate deaminase which catalyzes the conversion of adenosine monophosphate to ammonia and IMP occurs at the initiation of the embryonic development in the chum salmon egg.