Abstract In normal ascidian development, cuticular fins begin to form at the late tailbud stage and are fully formed at hatching. When one or several neurulae were manually demembranated (follicle cells, vitelline coat and test cells removed) and cultured in seawater they failed to form caudal fins. Fins were normal when the follicle cells alone were removed. The shape of the fins was normal when demembranation was delayed to the late tailbud stage. Does demembranation cause the loss of an essential factor produced by the embryos themselves or do the test cells provide a factor for fin morphogenesis? Demembranated neurulae of Ascidia callosa were cultured in groups ranging in size from 2 to 80 in 1 ml volumes of seawater. The mean lengths of the caudal fins increased with group size. In larger groups, some embryos developed fins that were normal in shape and as long as undemembranated controls. Results were similar with Corella inflata. These experiments suggest that a diffusible substance from the embryos facilitates fin morphogenesis and that test cells are not required. Test cells deposit ‘ornaments’ on the tunic in some species. In other species no ornaments are produced. Ten families are compared. It is proposed that the test cells make the tunic hydrophilic.