[C]itellate annelids (leeches + oligochaetes) provide solid ground for comparative analyses, because they form a monophyletic group within the paraphyletic polychaetes[…]. […] The unequal cleavage of the zygote followed by the unequal cleavage of the CD cell is critical for D-quadrant specification since the cytoplasmic determinants present in teloplasm are segregated to the larger CD cell at first asymmetric cleavage and then to the larger D cell at the second asymmetric cleavage[…].