A reexamination of cleavage patterns in eutherian mammalian eggs: rotation of blastomere pairs during second cleavage in the rabbit.

The pattern of cleavage was examined during second and third furrowing of the rabbit egg. Two-cell eggs, collected just prior to onset of second cleavage, were continuously observed in a culture chamber, which was kept at 37 degrees C. Semi-cinematographic techniques were used to photograph progressive stages of cleavage. It was demonstrated that the pattern of cleavage in the rabbit differs from that in the sea urchin, because the blastomeres at the 4-cell stage are arranged crosswise in the former, while they are situated next to each other in the latter. The crosswise arrangement of the blastomeres in the rabbit at the 4-cell stage is a consequence of a 90 degree rotation of the polar axis in one hemisphere of the egg. Subsequently, due to the rotation of the original polar axis in one hemisphere, the third cleavage plane through one half of the egg is transverse to the third cleavage plane through the other half. Evidence is provided to show that the cross wise configuration of blastomeres at the 4-cell stage occurs in other eutherian mammals. It is proposed that this rotational cleavage pattern be recognized as distinct from those of radial, spiral and bilateral.