The internal clock of reaggregating embryonic sea urchin cells

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Abstract
The presence of micromere “a clock” during early cleavage in sea urchin embryos has been shown by several investigators. The suggestion that an internal clock may be operative in later developmental stages up to the pluteus is presented in this study.The development of reaggregating Arbacia cells was followed from dissociation at the 16‐cell stage to pluteus formation, using scanning electron microscopy. After 7 hr of development, the reaggregates were dissociated a second time and were again able to develop into plutei. The morphological development of both reaggregations appears to be identical. In addition, embryos of both reaggregations developed at the same rate as control embryos. Differences in the cells of the first and second dissociations as compared to controls are described, as is the ability of reaggregating cells to catch up to controls during early development. Possible reasons for their ability to make up for lost time are discussed, including non‐randomness of cell behavior, lack of dedifferentiation and the presence of an internal clock.