Sentence:  [C]itellate annelids (leeches + oligochaetes) provide solid ground for comparative analyses, because they form a monophyletic group within the paraphyletic polychaetesnb.... ... The unequal <WSD>cleavage</WSD> of the zygote followed by the unequal <WSD>cleavage</WSD> 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 <WSD>cleavage</WSD> and then to the larger D cell at the second asymmetric <WSD>cleavage</WSD>nb.... 
Meaning of the word marked with <WSD> tags:  The repeated division of a cell into daughter cells after mitosis. (from 19th c.)