Figure 2
From: Myogenesis in the basal bilaterian Symsagittifera roscoffensis (Acoela)
CLSM micrographs of F-actin labelling in embryonic S. roscoffensis . A+B. Cleavage stages prior to muscle formation. Actin is expressed in the zonulae adhaerentes. A. 5 hours after egg deposition. B. 33 hours after egg deposition. The embryo possesses approximately 370 cells. C. 48 hours after egg deposition. Myogenesis in the anterior pole, facing upwards, precedes the development of muscle fibres in the posterior pole. Circular (cm), longitudinal (lm) and diagonal muscles (dm) are present. D-F. From the third day after egg deposition onwards, the ventral (D) and dorsal (E) sides are distinguishable. The anterior pole (asterisk) is surrounded by a spiral muscle. Additional muscles are formed by branching off from existing muscles (arrowheads). In addition, double-stranded muscles (double arrowheads) are present. F. Ventral anterior half of (D) in higher magnification. Scale bars: 50 μm.