Accelerated 25x.

Lantern of Aristotle.

Ring of mesenchyme cells beginning to secrete the calcareous skeleton.

Regression of larval tissues. Spicules of the larval skeleton are still present in the aboral region.

Eight-cell stage.

Tip of antero-lateral arms.

Esophagus.

Stage with four pairs of arms.

Stage with three pairs of arms.

Stage with four pairs of arms.

Skeleton on the aboral surface.

Ossicles of the lantern of Aristotle in formation at the oral region.

Detail of the vestibule near the left coelom and the central gut during early eight-arm stage.

Lantern of Aristotle in the oral region and detail of a circular calcareous spicule of an ambulacral foot.

Early stage with two arms (post-oral). Detail of a growing arm.

Growth of antero-lateral pair of arms (2nd pair).

Lantern of Aristotle in the oral region.

Stage with four pairs of arms.

First cleavage

Ossicles of the lantern of Aristotle in formation at the oral region.

Intermediate stage between gastrulation and pluteus larva formation.