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Using flow cytometry, spherulating nuclei of Physarum isolated at the beginning of spherule wall formation were found to exhibit a DNA content corresponding to the G phase of the cell cycle, although 8% lower. Before the first mitosis after spherule germination, a very slight incorporation of H thymidine into DNA was observed that was too weak to correspond to S phase, strongly suggesting that nuclei are stopped in G phase inside the spherules. The lower value of nuclear DNA content found using flow cytometry of germinating spherules may not be related to DNA quantity, but may be due to a difference in chromatin organization during growth or spherulation, resulting in interference with the staining. [ABSTRACT FROM AUTHOR]