Nematostella embryos inhibited in ß-catenin signaling fail to specify endoderm, but undergo initial archenteron invagination. (A-L) Morphological analysis of early (A-F) and late (G-L) gastrula stage embryos that are uninjected controls (A, G), or injected with GFP (B, H), NvDsh-DIX::GFP (C, I), SpAxin (D, J), Xß-cat-Eng (E,K), or LvCadherin (F, L) mRNA. Nuclei are stained with propidium iodide (red), and F-actin with phalloidin (green). Similar to controls (A, B), NvDsh-DIX (C), SpAxin (D) and Xß-cat-Eng (E) overexpressing embryos develop a primary invagination unlike LvCadherin (F) overexpressing embryos. However, while controls develop a normal endodermal epithelium at the late gastrula stage (G, H), NvDsh-DIX (I), SpAxin (J) and Xß-cat-Eng (K) overexpressing embryos have compacted coelenterons. Cadherin overexpressing embryos do not show any primary invagination and never develop an endodermal epithelium (L). (M) Quantification of the different phenotypes in mid and late gastrula stage embryos observed using confocal imaging clearly show that 91.1% (n = 45) of NvDsh-DIX RNA-injected embryos, 88.4% (n = 43) of SpAxin RNA-injected embryos and 85.7% (n = 49) of ß-catEn RNA-injected embryos cannot maintain the gut epithelium in contrast to 90.9% (n = 33) of uninjected and 83% (n = 59) of GFP RNA-injected embryos which show a normal endodermal epithelium. None of the Lvcadherin RNA injected embryos gastrulated (n = 23).