Spiralian Development: A Perspective

Three basic types of spiral cleavage are described: spiral cleavageby quartets, by duets, and by monets. Following F. R. Lillie'sconcept of adaptation in cleavage, the relation of each of thesecleavage modifications to the structure of the larva or juvenile,which develops therefrom, is considered. These comparisons leadto the conclusion that Lillie's concept of adaptation in cleavagecan be extended beyond such details as cell contents, cell size,tempo of division, etc., to include even the oblique characterof spiral cleavage and its general basic form.