Mature animals, however, will spawn spontaneously in the laboratory. No special feeding is necessary, as the larvae are well supplied with yolk. They are free-swimming and should be decanted daily to fresh sea water. It is chocolate brown in color and very opaque, containing a large amount of yolk. Two polar bodies are extruded and lost shortly after the ovum is shed.
Cleavage is total, unequal and spiral.
Gastrulation is by epiboly. The first cleavage occurs about two hours after insemination. By 36 hours, the swimming embryo has a developing shell gland and cerebral ganglion, and by 80 to 105 hours it metamorphoses. The ectodermal cells which form this covering, or test, are arranged in five distinct rows. The two end rows are covered with short, evenly distributed cilia, and each of the intervening rows bears a band of longer, powerful cilia.