Human Activin protein dorsal/anteriorizes amphioxus embryos. 10 ng/ml human activin applied at the early blastula eliminates expression of the ventral marker Evx and expands those of the anterior marker FoxQ2 and dorsal markers Chordin and Gsc. The entire ectoderm is specified as neural; the forebrain expressing Otx is expanded. (A-C, E-G) Mid-gastrulae; optical cross-sections. (D, H) Mid-gastrulae; side views, blastopore at right. (I-P) Mid-neurulae (15 hr); side view. Anterior at left. (R-U) Cross-sections through levels indicated in I, J, M, N. nc = nerve cord, nt = notochord, s = somites, e = endoderm. Gene markers as indicated.