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. LANG, A., 1884. Text-Book of Embryology. Macmillan and Co., Ltd., London, pp.