Samples of mycelium of Neurospora crassa of known age were harvested from agar plates and examined with the electron microscope. The relative volume of the mitochondria was determined for mycelium of different ages. The volume measurements indicated that the mitochondria were dividing synchronously in fronts 6, 13, and 22½ hr behind the growing hyphal tips. The sequence of mitochondrial division is hypothesized to include mitochondrial cupping followed by division which results in closely associated daughter mitochondria. On the basis of percentages of mitochondrial cupping and association, mitochondrial division was postulated to be occurring at 6, 14, and 26 hr. Close agreement between the mycelial mass doubling time and the calculated mitochondrial mass doubling time indicates that synchronous mitochondrial division is sufficient to maintain growth. The possibility that mitochondrial division is due to intercellular regulation of a mitochondrial genetic system is advanced.

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