Chris Schoff and Michael Boyle made this specimen when they were in my embryology class; I scanned it and used Voxx to make this 3D rendering which illustrates the spiral cleavage pattern at the 16-cell stage in the limpet we used to call Tectura scutum.

Spiral cleavage is characterized by an alternation in the orientation of the mitotic apparatus just before cytokinesis.  Instead of aligning with the longitude lines along the embryo, the spindle is tilted to one side or the other.  All of them exhibit the same tilt direction (clockwise or counter-clockwise) at each successive cleavage, giving rise to the eponymous spiral arrangement.

In this limpet the cells are not greatly different in size in the early embryo, so divisions remain roughly synchronous, making it easy to tell, in a preparation like this one, which cells are sisters and cousins.  In this movie we are looking straight down on the animal pole, and the projection goes all the way through to the vegetal pole, although this is much dimmer because of the depth-cueing used in the projection.

— text by George von Dassow