141Based on their differential distribution between the animal and vegetal poles in the
zygote, several candidates have been proposed to mediate this cleavage-related
asymmetry (reviewed in Ajduk and Zernicka-Goetz 2015 ). Apart from findings of
asymmetric localization, however, none of these candidate proteins have been
shown to function in the determination of whether a meridional or equatorial divi-
sion occurs at the two-cell stage. Thus, the exact molecular mechanism(s)
Fig. 4.8 Cleavage pattern in the early mammalian embryo. The first division is meridional, along
the animal and vegetal poles, and the second division can be either meridional or equatorial. This
generates four possible cellular arrangements at the four-cell stage with meridional–equatorial and
equatorial–meridional divisions distinguishable by virtue of one of the blastomeres in the two-cell
embryo dividing prior to the other. In embryos that exhibit the equatorial–equatorial pattern, puta-
tive factors localized to the animal and vegetal poles segregate to different daughter cells. See text
for details
4 Vertebrate Embryonic Cleavage Pattern Determination