Cytoplasmic bridges maintain continuity between subsets of sister blastomeres. Shown is the blastocoel-facing surface of four sister blastomeres of an embryo fixed at the onset of the 5th cleavage (16->32 cell stage), with two cleavage furrows indicated. An older, nearly-complete furrow (arrows), produces sister blastomeres 1 and 2. The nearly-closed cytoplasmic bridge remains in the form of a midbody suspended between 1 and 2 across the blastocoel. With the next cleavage cycle, a later cleavage furrow (arrowheads), separates 1′ from 1″ and 2′ from 2″. It is evident that 1″ and 2″ will become topologically isolated from each other, while cytoplasmic continuity can persist between 1′ and 2′ as long as the cytoplasmic bridge remains.