Gametogenesis and embryonic development of Clathrina coriacea and C. blanca, two closely related calcareous sponges from Santa Catalina Island, California, are described. Oogenesis is asynchronous in both species. Spermato- genesis was not observed. Cleavage is total and equal, resulting in the formation of a blastula larva. The larva of C. coriacea contains one large posterior granular cell, whereas two posterior granular cells are present in the larva of C. blanca. Migration of the larval blastomeres into the blastocoel begins while the larva is in the tube of the parent sponge.