In characteristic spiralian embryos the mesentoblast is the stem cell of the mesodermal bands. It is a derivative of the dorsal quadrant. At least in gastropod molluscs, the ancestral form for the specification of the dorsal quadrant out of four initially equal quadrants is by centralization of one of the four macromeres after the separation of the presumptive ecto- and entoblast cells. Then this macromere is induced by the animal micromeres to produce the mesentoblast. In this paper it is shown that in the embryo of the polyplacophoran Acanthochiton crinitus, specification of the dorsal quadrant and formation of the mesentoblast exactly follow the same pattern. After deletion of the first quartet of micromeres none of the macromeres is centralized, no mesentoblast is formed, and the embryo remains radially symmetrical. Apparently, the mechanism for the specification of the dorsal quadrant and the formation of the mesentoblast has been conserved during the evolution of the main molluscan taxa. It has been discussed whether this mechanism might be a plesiomorphous property, characteristic of less derived spiralian phyla.