1. Eggs of Limnaea stagnalis, of ages differing by 15-min. intervals between oviposition and the third cleavage division, have been centrifuged at 640 g. for 10 min.

  2. Centrifugation before first cleavage did not have any effect on development.

  3. Centrifugation at first, second and third cleavage caused a strong morphogenetic effect.

  4. Centrifugation during prophase of second and third division caused delay of cleavage. The mitotic figure is disrupted.

  5. It is suggested that the morphogenetic effect of centrifugation can be ascribed to a disturbance of the normal distribution of morphogenetic ‘substance’ at or in the cell surface, by random insertion or resorption of membrane after stretching the cell.

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