Onset of muscle formation: early first chaetiger ciliary band. Scale bars = 50 μm, anterior is to the left. A. Ventral view, phalloidin staining (red) and anti-acetylated tubulin staining (green). The anlage of the brain (arrow) and the circumoesophageal connectives surround the mouth opening (stom). Prototroch (prot), metatroch (met), the peristomial (pcb) and the first chaetiger ciliary band (1BS) and the telotroch (tel) are stained by the anti-acetylated tubulin antibody. The square marks the region of the close up depicted in C. B. Same picture as A, but only phalloidin staining. At this stage, phalloidin prominently labels the F-actin of the cytoskeleton, marking all cell margins. C. Close up from A, phalloidin staining only. The first discernable muscle fibers of the ventral longitudinal muscle (VLM) become apparent.