A study comprising both descriptive and experimental data was undertaken. The descriptive portion utilizes transverse, sagittal, and frontal serial sections of 10 through 20 day old rat embryos.
As development proceeds, the somites lengthen along their dorsoventral dimension, and epithelial-lined “buds” (ventral somite buds) extend into the lateral plate somatopleure. Concomitantly epithelium lining the somite “body” is converted to a loosely associated mass of mesenchyme, beginning at the deep medial border and extending around the cephalic and caudal faces of the somite to involve the superficial surface. Lastly, this process transforms the epithelium covering the ventral somite buds into a mesenchymal aggregation. Anlagen of the abdominal ventrolateral muscles arise in the precise position of the mesenchymal aggregation derived from the ventral somite buds.
The experimental section tests the development capacities of isolated portions of the trunk body wall when implanted in the anterior chamber of the eye. Grafts of somite material are highly proficient in forming striated muscle, while grafts of somite-free somatopleure have little ability to form striated muscle. Grafts of ventral body wall become more proficient in forming striated muscle with age. The increase in muscle-forming ability correlates directly with the invasion of material from the ventral somite buds into the ventral body wall. It is concluded that the ventrolateral (abdominal) musculature is of somitic origin.