Schematic diagram showing two neighboring cells in the developing embryo of a sea squirt. (A) Before the cells divide an invagination forms in the membrane at the rear (posterior) side of both cells, while a finger-like protrusion forms on the front (anterior) side of each cell and inserts itself into the invagination of the cell in front of it. The tip of the invagination is attached to the centrosome by microtubules. Specifically, the invagination attaches to one of the two centrioles that make up the centrosome – the same centriole that also grows a primary cilium. (B) When the cells begin to divide, the invagination/protrusions and primary cilia have disappeared and the mitotic spindles are oriented along the anterior-posterior axis. Negishi et al. propose that the membrane invagination holds the centrosome to orient the spindle.