Fig. 1. Effect of retinoic acid on whole sponges and primmorphs. (A) Untreated specimens of S. domuncula. (B,C) Induction of gemmules in animals by retinoic acid (50 µmol l^–1). (B) Gemmules formed on a shell of the snail Trunculariopsis trunculus (incubation: 10 days) after retinoic acid treatment. (C) Group of gemmules arranged on the grooves of a shell. (D–G) Induction of canals in primmorphs. (D) Primmorphs formed in the absence of any organic matrix in the culture dish. The 3D-aggregates remained round and without canals. (E) Formation of canals by incubation of cells for 5 days on non-coated plates followed by 10 days on plates coated with galectin matrix. (F) Primmorphs with canals formed after for 5 days on non-coated plates followed by 10 days on poly-L-lysine plates. (G) Higher magnification of the canal system (> <) in the primmorphs developed for 20 days on the galectin matrix following 5 days incubation on non-coated plates. (H–L) Effect of retinoic acid on the canal system in primmorphs. (H,I) Incubation of canal-containing primmorphs (formed after a total incubation period of 15 days), cultured on the galectin matrix for 10 days, + 1 µmol l^–1 of retinoic acid for an additional 5 days. (J–L) Incubation of the canal-containing primmorphs at the higher concentration of retinoic acid (50 µmol l^–1) for the same period of time. For further details, see Materials and methods. Scale bars: 10 mm (except for C and G, 5 mm).