Cytokinesis in the rabbit zygote: Fine‐structural study of the contractile ring and the mid‐body

The temporal events of cytokinesis were examined with light‐ and electron‐microscopes during first cleavage of the rabbit zygote. When karyokinesis is at the late anaphase‐to‐early‐telophase stage a shallow indentation appears in the vitelline membrane in a plane perpendicular to the spindle. Microvilli are absent in the indentation although present elsewhere. A band of microfilaments is recognizable under the plasma membrane of this region. Sections which pass through the band of microfilaments contain several early mid‐bodies located centrally in the egg. In later stages of furrowing the band of filaments is 0.1 μ wide and is circumferentially arranged. These filaments which underlie the broad leading edge of the furrow are 50–80 Å in diameter and are components of the contractile ring. The furrow cavity is filled with blebs of the plasma membrane. The microvilli of the lateral furrow walls interdigitate and “primitive” intermediate junctions develop between the two blastomeres. During late cleavage the stem‐bodies fuse into a continuous mid‐body which spans two opposite points of the intercellular bridge and fuses with the filaments of the contractile ring.