Practice question

Mammalian cleavage is holoblastic, with first division meridional. Second cleavage exhibits rotational pattern: one blastomere divides meridionally while other divides equatorially, producing blastomeres with different orientations and transiently tetrahedral four-cell embryo. This rotational asynchrony leads to slightly asynchronous divisions and generation of diversity in cell contact history that feeds into fate bias. First cleavage is not rotational, third cleavage shows mix but defining event is second. Rotational cleavage distinguishes eutherian mammals from many amphibians and is associated with regulative development, compaction timing and early determination of inside-outside positioning crucial for blastocyst formation.