The myobiasts derived from the hypomeres invade the regions ventral to the vertebrae where they form the prevertebral muscles but the hypaxial myobiasts also extend ven-trolaterally and give rise to various muscles of the neck and body wall.
In the neck the ventral extensions of the hypomeres form the gen-iohyoid and scalene muscles. A ven-
tral longitudinal muscle column derived from the cervical hypomeres forms the infrahyoid muscles.
The hypomeres of the thoracic myotomes extend into the lateral and ventral body wall where they form the layered muscles of the chest and abdomen. In the thoracic region the hypaxial ventrolateral musculature splits into three layers which form the external intercostal, internal intercostal and innermost intercostal muscles. Due to the presence of the ribs, the muscles of the thoracic wall maintain their segmental character. However, in the abdominal region the ventrolateral extensions of the hypomeres fuse together to form a muscular sheet which splits into three layers forming the external oblique, internal oblique and the transversus abdominis muscles. In the abdominal region, a longitudinal muscular column arising at the ventral tip of the hypomeres forms the rectus abdominis muscle.
Hypomeres of the lumbar myotomes form the quadratus lumborum muscle, while those from the sacral and coccygeal myotomes give rise to the muscles of the pelvic diaphragm, anus and sex organs.