Timing of the phases of the cell cycle during the period of asynchronous division up to the 49-cell stage in Lymnaea Available to Purchase

The duration of the phases of the cell cycle (M-G₁-S-G₂) has been determined from the 8-up to the 49-cell stage in eggs of Lymnaea, using autoradiography and cytophotometry of Feulgen-stained nuclei. Division asynchrony of corresponding cells in different quadrants is primarily caused by unequal lengthening of the G₂ phases. In general it appeared that in the vegetative cells lengthening of the cell cycles is chiefly due to an extension of the G₂ phases, whereas in the cells of the animal half the duration of both the S and the G₂ phases are extended. DNA synthesis is not blocked in cells which stop dividing and start to differentiate. A conspicuous lengthening of the cell cycles is observed in the 16- and 24-cell embryo; this is accompanied with the reappearance of distinct nucleoli. Supporting evidence has been obtained for the assumption that bilateral symmetry at the animal pole of the embryo is induced by cells from the vegetative hemisphere, presumably by the macromere 3D, during the 24-cell stage.