Soon after fertilization, the egg becomes irregular in shape, and remains thus until after the first cleavage. Meanwhile, two polar bodies are formed. They are retained for some time under the fertilization membrane, but not in any fixed position. An ovoid, ciliated blastula is formed. It hatches and rotates slowly to the surface.
Gastrulation is by invagination.
The two portions of the hyaline layer can be followed through to late larval stages. The larval gastrula transforms into a young pluteus, with a calcareous skeleton and rudimentary arms. By the fourth day the alimentary tract and primitive coelom are formed, and the arms have increased in size. The developing hydrocoele may be seen after 20 days, and by 30 days the pluteus is full grown. The breeding of animals at Woods Holl during the months of June, July and August.