If adult worms are placed in jars of fresh sea water through which an air current is allowed to bubble, they will soon deposit their eggs on the bottom or sides of the dish. The eggs are deposited without any special orientation in tough, gelatinous, spiral capsules. Each capsule contains from 100 to 200 eggs, each of which is surrounded by a membrane. The eggs measure 100 microns in diameter and are densely granular with yolk. Fertilization is internal, although the eggs are still in the germinal vesicle stage when they are laid.
Cleavage is unequal and spiral.
It is peculiar in that, after giving off the fourth quartet of micromeres, the macromeres are very small and eventually degenerate. The fourth quartet of micromeres thus takes over the function of forming both endoderm and mesoderm. Gastrulation is by epiboly. It is oval in shape, bearing at its lower pole an ectodermal stomodeum which leads to an irregular, ciliated, endodermal sac. Eight ciliated lobes can be seen just below the equator, and at the aboral pole a plate of very small cells covers a mass of ganglion cells.