Spiralian Development: A Perspective

Abstract

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