Fig. 2. Nematostella development. In Nematostella, the sexes are separate and adults generate either eggs or sperm. The cleavage program is chaotic with no two embryos developing identically. (A) A hollow blastula is formed. Yolk fragments break off from the apical regions of cells and remain trapped in the blastocoel. (B) Gastrulation proceeds by the invagination of the future oral end of the embryo (^*). Cells entering the blastocoel from the prospective oral pole will give rise to endoderm (en) surrounded by outer ectoderm (ec). (C) The planula `larva' is initially shaped like a teardrop and swims with its apical sensory tuft (at) directed forward. The endoderm (en) forms a solid ball of cells at the planula stage. (D) The coelenteron (gut cavity) has formed by the hollowing out of the solid planula endoderm (ct). Tentacle buds (tn) begin to form around the oral opening (^*) in the swimming larva prior to settlement. Note that the pharynx (ph) extends into the coelenteron. (E) The polyp forms 7 days after fertilization. When it first settles onto the substrate, it has four tentacles and eight endodermal mesenteries. Endoderm lines the hollow tentacles. The thickened pharyngeal wall connects the mouth/anus to the coelenteron.