spiral cleavage defined in 1951 yearspiral cleavage - spiral cleavage;
spiral cleavage - A type of cleavage. First two divisions are longitudinal. Third is latitudinal and unequal, and forms four small cells at animal pole which by an obliquity of their spindles lie above the grooves between the large vegetal cells. These four small cells are usually extruded in a clockwise direction in relation to the large cells. At the next division, the large cells again form four large and four small cells, and the original small quartet divides; again the spindles lie obliquely, but this time the direction in all divisions is anticlockwise. Clockwise and anticlockwise alternate, and a highly characteristic and constant pattern of blastomeres is formed. This peculiar method of cleavage must be an indication of evolutionary relationship between groups possessing it: polyclad Turbellaria, Annelida, Mollusca (except cephalopoda), perhaps rotifers.
near spiral cleavage in Knolik
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