The ester-hydrolyzing or lipase actions of extracts of whole mice whose ages ranged from approximately 6 days before birth to 1 year 8 months 21 days were tested on ten simple esters by the method described in previous papers. The "pictures" of the relative enzyme actions changed from a type approaching the "embryonic" as found with embryo rats and a number of tissues of rabbit embryo, to a type characteristic of the adult mouse. The mouse embryos corresponded to the rat embryos in type and differed markedly from the mouse carcinomas which have been studied. The relative and absolute enzyme actions are discussed in some detail, and the results compared with the results obtained for the life cycle of the rat.

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