THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 



299 



(b) 



The ventral part gives rise in the cervical region to the sternohyoideus, 

 omohyoideus, sternothyreoideus and geniohyoideus. In the abdominal 

 region the ventral part gives rise to the rectus abdominis and to the 

 pyramidalis (Figs. 265 and 267). In the thoracic region there are no 

 muscles derived from the ventral part, corresponding to those in the 

 abdominal region. This is probably due to the development of the 

 sternum. 





FIG. 266. Lateral view of a human embryo of n mm. (about 5 weeks). Bardeen and Lewis. 

 The area from which the skin has been removed is drawn from reconstructions. The dorsal mus- 

 culature has been removed from the region of the upper extremity, exposing the 4th to the 

 8th cervical and the ist to the 3d thoracic vertebrae. The dorsal musculature has likewise 

 been removed from the 5th lumbar and first three sacral segments. Segmentation is practi- 

 cally lost in the dorsal musculature in the thoracic region, but is still evident in the lumbar, 

 sacral and coccygeal regions. The ventro-lateral musculature is distinctly separated from the 

 dorsal, and is beginning to differentiate into the muscles of the thorax and abdomen. 



The ventro-lateral portions of the lumbar myotomes and of the first two 

 sacral myotomes, corresponding to the ventro-lateral portions of the thoracic 

 myotomes, apparently do not take part in the production of muscles which be- 

 long to the body wall proper. It is even questionable whether they give rise to 

 any muscles of the lower extremities. The ventro-lateral portions of the third 



