The egg itself, free of the jelly, is yellow due to its yolk content. The resulting perivitelline space is quite large. There is a thinner hyaline plasma layer than in the Arbacia egg, and less regularity of cleavage. However, the cleavage pattern is not markedly different from that of Arbacia. A hollow, ciliated blastula, which rotates slowly within the fertilization membrane, is produced.
Gastrulation is by invagination.
The times are calculated from fertilization. Pigment can be seen at the anal pole. A rudimentary skeleton, primitive alimentary tract, and mouth and anal openings are present. As the larva matures, it becomes more and more helmet-shaped. At four days the oral, or anterior, arms begin to form, and by the end of a week the anterolateral and antero-internal arms have been added.