Fig. 1

Development of amphioxus rostral somites. a At the early neurula stage, the dorsal roof of the anterior archenteron begins to expand dorsolateral to the ectoderm. b At the mid-neurula stage, the rostral somites swell and form a U-shape that remains part of the dorsal roof of archenteron. c At the late neurula stage, the somites pinch off from the archenteron roof. nc notochord