Warren Harmon Lewis 183 



evident from the stages we have studied. At a very early stage these 

 masses receive their nerves and later drag them posteriorly. By the 

 seventh week the pectoral mnscles have reached their adnlt positions so 

 far as the thoracic attachments are concerned. The latissimns dorsi, 

 even Ly the end of this week, only extends to the ninth rib. 



Another A'ery important group of muscles migrate from the head and 

 anterior cervical region to the arm and thorax. In an embryo, four 

 and one-half weeks old, the posterior end of the trapezius })remuscle 

 mass lies at the level of the fourth cervical vertebra, at five weeks the 

 muscle fibers extend to the level of the fifth cervical vertebra, and at 

 six weeks to the fifth thoracic vertebra. At this age also the muscle 

 has acquired its attachment to the scapula and clavicle. At seven weeks 

 the muscle extends to the level of the sixth thoracic vertebra. The 

 sp'inal accessory nerve is connected to the premuscle mass as early as 

 the middle of the fifth week, and as the muscle extends posteriorly the 

 nerve is carried along with it. 



The sterno-mastoid muscle originates high up in the neck with the 

 trapezius. It extends posteriorly and ventrally, reaching the clavicle 

 and sternum during the sixth week. 



The infrahyoid muscles also migrate from the anterior neck region, 

 carrying their nerves down with them. 



The rhomboid premuscle mass at 4.5 weeks lies at the level of the 

 fifth cervical vertebra and gets its nerve supply at this time from the 

 fifth cervical nerve. At five weeks it has extended to the sixth cervical 

 vertebra, and at seven weeks it is for the most part in the thoracic 

 region and has acquired its scapidar attachment. 



The serratus anterior premuscle mass at four and one-half weeks 

 already extends into the upper thoracic region and has its cervical nerve 

 supply. It has probably already migrated from the cervical region. 

 At five weeks it has reached its posterior attachment on the thorax, 

 but is not as yet attached to the scapula. This occurs during the sixth 

 week. The serratus anterior muscle is thus one of the first of these 

 migrating muscles to attain its permanent attachments. It is also 

 evident that the various serrations of this muscle are of secondary origin. 



