Fig. 1.

The general steps of secondary abdominal wall development. (A) The primary abdominal wall joins in the ventral midline creating the abdominal cavity. (B) The myoblasts migrate out of the somites toward the ventral midline. (C) Secondary structures form including individual muscles and connective tissue. (D) A ventral view of the fully developed secondary abdominal wall structures cut transversely to reveal the orientation of muscle fibers within specific groups. Rectus abdominal muscle fibers are seen running vertically. Transversus abdominis fibers are perpendicular to the rectus. The fibers of the internal oblique are at a 45 degree angle to both of these muscles. The external obliques are also at a 45 degree angle to both the rectus and the transversus but are perpendicular to the fibers of the internal oblique.