THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 



265 



abdominal musculature, to a part of the muscles of the neck, and to the 

 muscles of the tail region. There is a possibility that they give rise also to the 

 muscles of the tongue. 



As the myotomes continue to develop, they become elongated in a ventral 



FIG. 224. Lateral view of human embryo of 9 mm. (4! weeks). Bardeen and Lewis. 



The area from which the skin has been removed is drawn from reconstructions. The myotomes 

 have fused to a certain extent, so that segmentation is becoming less distinct. Note that the 

 myotomes correspond to the spinal nerves. The developing muscle mass (the myotomes 

 collectively) extends ventrally in the body wall in the thoracic region, and is divided by a 

 longitudinal groove into two parts a dorsal and a ventro-lateral (see text). 



In the region of the upper extremity, dense masses of " premuscle " tissue are represented which 

 later form the muscles. In the region of the forearm and hand the " premuscle " tissue has 

 been removed to disclose the anlagen of the skeletal elements (radius, ulna, and hand plate). 

 In the region of the lower extremity the superficial tissue has been removed to disclose the 

 border vien, the anlagen of the os coxae, and the lumbo-sacral nerve plexus. 



direction. Those of the thoracic region extend into the connective tissue of 

 the somatopleure, or in other words, into the lateral body walls (compare 

 Figs. 224 and 225). During the fifth week the myotomes give rise to a dorso- 

 ventral mass of developing muscle tissue, in which the segmental character 



