Dev Growth Differ 1971 Dec 01;134:285-302. doi: 10.1111/j.1440-169X.1971.00285.x.

ON THE SYSTEM CONTROLLING THE TIME OF MICROMERE FORMATION IN SEA URCHIN EMBRYOS.

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The previously reported observation that micromere formation after cleavage suppression is not linked with the number of blastomeres present but rather with the time schedule of the fourth cleavage of the normal embryos has been confirmed. A hypothesis is advanced that a rhythmical fluctuation of the sulfhydryl contents of the egg proteins is the clock system, and micromere formation is connected with the fourth SH cycle after fertilization. The hypothesis was tested under 3 conditions: (i)  Conditions which stop the nuclear activities but preserve the SH cycle, followed by a release from these conditions. (ii)  Conditions which "freeze" both nuclear and cytoplasmic rhythms, and later removal of the conditions. (iii)  Conditions which leave nuclear activities intact but prevent the cytoplasmic rhythms, followed by normal culturing. The results came out as anticipated by the hypothesis.

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