Within-tier division asynchronies in Ilyanassa, Crepidula, and Bithynia are shown in Figure 25.
In three cases (2m12, 2m21, 3m2) within-tier asynchronies occur with different order between Ilyanassa and Crepidula.
While most within-tier cell cycle diversification is correlated with cell position along the secondary axis, division asynchrony may also occur between left- and right-sided cells within a tier.
Within-tier differences in developmental potential and early mitotic behavior are evidently imposed on a homogeneous ground state by two sources.
A second within-tier difference in division asymmetry occurs in Ilyanassa when 2d1 divides equally and the 2abc1 cells all divide very unequally.
As described below, most within-tier differences in cell division pattern up to the 80+ cell stage are indeed correlated with dorsoventral cell position.
Three tiers in Ilyanassa were found to exhibit quantitative within-tier differences in the geometric asymmetry of cell division; in each case the difference is precisely correlated with cell position along the secondary axis.
Such within-tier differentiation is more widespread, and also tends to be more pronounced, in Ilyanassa compared to the related caenogastropod genera Crepidula [6], [11], [26] and Bithynia [27].
Some taxa, including representatives of two of the most basal gastropod clades, display almost no within-tier differentiation up to the latest stages examined (88+ cells)[29], [46], [47].
Thus, both lineage and position contribute to within-tier differentiation; both mechanisms are expected to generate differences correlated more or less exactly with cell position along the secondary axis.
