The role of the cytoplasm and nucleus in the control of the length of the division cycle was investigated in Pleurodeles waltl eggs. Injection of spermine into enucleated eggs showed that the ability to cleave was not restricted just to the period of normal cytokinesis (T=1.0) but was possible throughout most of the first egg cycle. The cytoplasmic components required for cytokinesis seem to increase progressively during the first division cycle. Nuclear transfer experiments indicated that the timing of cleavage was normal only when the nucleus and egg cytoplasm were reassociated between T=0.0 (activation time) and T=0.50. Delayed associations, after T=0.50, provoked an alteration in the chronology of first cleavage and led to abnormal embryonic development. In the absence of a nucleus, the egg cycle seemed to stop at T=0.50. These different observations suggest that the normal timing of cleavage not only depends on a ‘cytoplasmic clock’ but is also determined by an isochronous nucleocytoplasmic relationship during the early phase of egg development.

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