Fig. 2

Initial processes during Clytia medusa formation show parallels to polyp bud development. F-actin staining, single confocal sections. Earlier stages left, later stages right. a–d’ Polyp budding, mt mouth tube, t tentacle, d, d’ oral view of an adult polyp. f–o’ Medusa budding. e, e’ Oral view of a young medusa, stippled lines outline the canals of the gastro-vascular system. cc circular canal, tb tentacle bulb, rc radial canal, mt mouth tube. Red nuclei (TOPRO3). f–k’ Lateral views, l–o’ oral views. Stippled lines demarcate the ECM between the tissue layers h–k, m–o. The endoderm forms radial tubes (rt) as the entocodon (ec) sinks inwards g, g’ arrows. The radial tubes are covered by a layer of outer ectoderm externally and a layer of entocodon tissue internally h, h’, m, m’. Red arrowhead h Beginning of formation of the velar plate demarcates the future bell rim. i, i’, k, k’ The distal halves of the radial tubes develop into medusa tentacle bulb endoderm (tb). The radial tube portion up to this level gives rise to the radial (rc) and circular canals (cc) of the gastro-vascular system and the plate endoderm (pe) of the bell (stippled red lines). The outgrowth of the circular canals (cc, white arrowhead) starts at the level of the future bell rim (red arrowhead). The mouth tube (m) starts to grow into the entocodon-cavity. n–o’ Oral views of representative buds reveal the formation of the umbrellar plate endoderm from lateral outgrowth and fusion the four radial tubes (white arrowheads). (o, o’) Late medusa bud with completed plate endoderm and radial canals