part of part of immediately preceded by immediately preceded by occurrent late gastrula stage stage 16 early prism stage The embryo has a distinctive, angular shape due to the elongation of the skeletal rods. The body rods have elongated toward the apex (posterior end) of the embryo. The dorsoventral connecting rods are elongated, and the postoral rods are still very short although they already support thickened regions of ectoderm that will later form the distal tip of the postoral arms. The archenteron tip is slightly expanded, and its oral surface is directed toward the oral ectoderm. Ingression of secondary mesenchyme cells (SMCs) is complete. The embryo is reddish in color due to the presence of pigment cells within the aboral ectoderm. The animal pole domain (or apical plate) is prominent and the lateral walls of the embryo have thinned. The coelomic pouches and gut constrictions have not yet appeared. ecao_developmental_stage