Spiral and radial cleavage

In embryonic development, spiral cleavage is one of two types of holoblastic or complete cleavage. At the third cleavage the halves are oblique to the polar axis and typically produce an upper quartet of smaller cells that come to be set between the furrows of the lower quartet. All animal groups showing spiral cleavage are protostomia. Groups such as annelids and mollusks undergo spiral cleavage, in which the cleavage planes are oriented obliquely to the polar axis of the oocyte.

Radial cleavage is a cleavage pattern characteristic of deuterostome animals, for instance vertebrates and echinoderms, in which the spindle axes are parallel or at right angles to the polar axis of the oocyte.