First steps of Phoronopsis harmeri myogenesis. In all images, the apical is to the top, and the ventral is to the right, except in B and C where the ventral side is toward the reader. A. Photograph of live mid-gastrula. B. Confocal image of the midgastrula stained with phalloidin; z-projection of 20 middle optical sections; frontal view. C. Mid-gastrula stained with phalloidin, 3D-projection of ventral side viewed from the dorsal side. The muscular cells (arrowheads) spread on the ventral wall of the embryo, above the archenteron (ar). D. Late gastrula with myoepithelial cells (mc), which have immigrated into the posterior part of the embryo and formed two lateral tiers (lt). E. Late gastrula stained with phalloidin, 3D-projection of first muscular system (blue) in the embryo (yellow); lateral view. F. Sagittal semi-thin section of late gastrula, in which hindgut begins to form as a posterior depression (pd) of the ectoderm. G – J. Ultrastructural organization of the first muscle cells, which have immigrated from the archenteron anterior wall; sagittal sections of the embryo. G. Basal processes (closed arrowheads) of muscle cells. H. First myofilaments (arrows) in the basal cytoplasm of muscle cells. I. The cilium with basal body (bb) and striated rootlet (sr) in the muscle cell. J. Part of the lining of the anterior pole of the late gastrula. The lining is formed by myoepithelial cells, which connect to each other via desmosomes (double arrowheads) and have a nucleus (n) with a nucleolus (nu). Abbreviations: ap – apical plate; bc – blastocoels; bp – blastopore; ep – epidermis.