SUMMARY: Changes in the structure of the myxamoebae involved in the formation of the stalk of the fruiting body, or sorocarp, of were investigated. Observed changes in the ultrastructure relating to cell differentiation include: (1) food vacuoles disappear and autophagic vacuoles take their place after cell aggregation; (2) an inclusion in the mitochondria disappears during the vegetative feeding stages; (3) the decrease in number of an electron-clear vacuole is coincident with the appearance of the ‘prespore’ vacuole in cells of the hind portion of the migrating pseudoplasmodium; (4) while the prespore cells diminish in size and their cytoplasm becomes electron dense during culmination, the myxamoebae destined to form cellular elements of the stalk swell and their cytoplasm finally disintegrates.

The slime coat that encircles the entire pseudoplasmodium remains after fixation and dehydration as a triple-layered membrane resembling cell membranes. At the start of culmination, the initial core of unwalled stalk-forming cells descends through the then vertically oriented cell mass. The polarization of cytoplasm in the horizontally oriented prestalk cells of the apical papilla is described, as well as the formation of the two-layered sheath around the stalk. The mass of so-called rearguard cells of the migrating pseudoplasmodium separates during culmination and a fraction of them form the basal disc while the remainder rise on the stalk just below the prespore mass.


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