Fig. 1

Schematics and nomenclature of the spiral quartet cleavage as found in polyclad flatworms. Micromere and macromere quartets (q and Q, respectively) are colour-coded. a The third cleavage (four- to eight-cell stage) is unequal and asymmetric. The eight-cell stage embryo consists of four larger vegetal macromeres 1Q, and four smaller animally positioned micromeres 1q sitting skewed to one side of their sister macromere, above the macromeres’ cleavage furrows. The typical spiral deformations (SD) of macromeres show a helical twist towards one side with respect to the animal–vegetal axis. This is best seen if the embryo is viewed from the animal pole. The resulting spiral shape taken by all four macromeres has been shown to be either clockwise (dexiotropic) or counter clockwise (laeotropic) among different lophotrochozoans. In the polyclad M. crozieri it is dexiotropic. Notably it has been demonstrated that the mechanism of spiral deformations depends on actin filaments rather than on spindle forming microtubules [70]. b, c In subsequent rounds of division, the larger macromeres again divide unequally and asymmetrically sequentially forming the second and then the third quartets of micromeres. During these divisions the spiral deformations appear in alternating dexiotropic/laeotropic directions (the rule of alternation). Up to the 32-cell stage, polyclad flatworms represent a classic example of stereotypic lophotrochozoan spiral quartet cleavage. d The formation of the fourth quartet (4Q and 4q) deviates from the typical pattern seen in other spiral-cleaving embryos insofar as the micromeres 4q become large and the macromeres 4Q diminutive. Q = A, B, C, D; q = a, b, c, d