A Fine Structure Study of Physarum polycephalum During Transformation from Sclerotium to Plasmodium: A Six‐Stage Description

.Physarum polycephalum is classified presently as a sarcodinid in the class Eumycetozoea. It produces a sclerotial dormant stage consisting of a crustose deposit containing nucleated spherules of cytoplasm enclosed within a honey‐comb‐like matrix of organic walls. When rehydrated, the sclerotium reverts to a plasmodium: 1) the spherules become increasingly vacuolated, 2) electron‐dense granules become dispersed within expanding vacuoles, and 3) pseudopodial extensions develop from the periphery of the spherule cytoplasm, penetrating the fragmenting walls, and making interconnections with surrounding spherules, eventually leading to a fully reticulated plasmodium. Six stages are identified during reversion from sclerotium to plasmodium in laboratory cultures, and their successive appearance was mapped over time. The six stages are: 1) sclerotial stage with crenulated nuclei, 2) cytoplasmic activation with smooth nuclear envelopes, 3) initiation of pseudopodial protrusions, 4) pseudopodial penetration into or across walls, 5) cytoplasmic interconnections among spherules with wall disintegration, and 6) fully formed cytoplasmic network as plasmodium. Cytochrome c oxidase activity, expressed per unit protein content of the homogenate, remains fairly constant throughout the developmental sequence, whereas acid phosphatase activity, expressed per unit protein concentration, is somewhat lower in the sclerotium than in subsequent stages of development after hydration.