The fifth cell cycle of the mouse embryo is longer for smaller cells than for larger cells.

Newly-formed pairs of 16-cell blastomeres were collected by periodic observation of isolated 8-cell blastomeres. Any pairs formed by division were recovered and classified as being composed of 1/16 blastomeres that differed in size or were of similar size. All of the latter and some of the former were then cultured for periods of up to 20h. The remaining pairs of different-sized blastomeres were disaggregated to larger or smaller cells. Some of these were reaggregated as smaller: smaller or larger: larger pairs, and these, together with the remaining isolated smaller and larger blastomeres were also cultured for up to 20h. At hourly intervals, cultured cells were sampled and analysed for incidence of division. It was found that larger cells divided on average after 12h whereas smaller cells divided on average after 14h.