These ranges and ridges are mostly meridionally
aligned, with their orientation strongly influenced by bedding trends.
In embryos of echinoids with typical feeding larvae, the symmetry of cleavage is interrupted at the fourth cell division: the animal pole quartet of cells divides meridionally
and equally, whereas the vegetal quartet of cells divides equatorially and unequally to yield a quartet of small daughter cells (micromeres) at the vegetal pole of the embryo.
Also, and possibly because of the different portioning of the cytoplasm in the single-cell zygote prior to its first cleavage, this asynchronous division also occurs along differing planes, called "rotational cleavage," where one cell divides meridionally
(top-to-bottom), while the other divides equatorially (left-to-right).