The duration of the phases of the cell cycle during the 1-, the 2and the 4-cell stage of the Lymnaea egg were determined with [H]thymidine and with Feulgen cytophotometry. The M, S and G2 phases occupy 48, 27 and 25 % of the first three cell cycles. A Gx phase cannot be observed. Only from the 4-cell stage was [H]thymidine readily incorporated into DNA. The theory that an increase in respiration during the S phase of the 4-cell stage is connected with the energy requirements of DNA synthesis is discussed.