Abstract
The cell-lineage of Bithynia tentaculata L. has been studied through the formation of the fourth quartet of micromeres, i.e., the 81-cell stage. Up to the 28-cell stage, the cleavage pattern is radially symmetrical, and the divisions take place synchronously in the quadrants. Bilateral symmetry is established by the formation of the mesentoblast 4d at the future dorsal side. A resting stage preceding the formation of 4d is lacking. The mesentoblast 4d is formed earlier than the other fourth quartet cells, and it divides twice before 4a-4c are formed. Bilateral symmetry in the molluscan cross becomes apparent by different divisions of the tipcell and the basal cell in the dorsal arm. Dorsoventrality is progressively corroborated by the divisions in the second and third quartets of micromeres, which take a different course in the A and B quadrants on the one side and in the C and D quadrants on the other side.
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