Figure 2.

Muscles of the ventrolateral body wall in the vertebrate abdomen derived from the embryonic hypaxial and epaxial masses. (A) In fishes, such as teleosts, hypaxial muscles of the ventrolateral wall are organized into two layers (M. obliquus superioris and M. obliquus inferioris). (B) Amphibians, such as salamanders, have two to four layers (M. obliquus externus, in some species divided into superficialis and profundus portions, M. obliquus internus, and M. transversus abdominis, with scattered subcutaneous skeletal muscle fibers). (C) Mammals, like dogs, consistently have four layers (M. cutaneous trunci, a well-developed subcutaneous muscle layer, M. obliquus externus abdominis, M. obliquus internus abdominis and M. transversus abdominis). Development of the ventral musculature, such as M. rectus abdominis, is regulated by different molecular signals and therefore follows a different organizational pattern and may not differentiate into multiple layers¹².