156 



The Development of the Arm in Man 



plate system is easily distingnished from the surrounding tissues by its 

 fibrillation. 



Lateral to the muscle-plate system are ill-defined masses of condensed 

 tissue without fibrillation, but from which muscles differentiate. 



Lateral to the anterior six ribs lies the lateral premuscle mass. It 

 occupies most of the space between the costal portion of the muscle-plate 

 system and the integument. It shades off into the surrounding loose 

 mesenchyma everywhere, but at the anterior end, at about the level of 

 the first intercostal space, it splits into four divisions which pass anter- 

 iorlv. 



Fig. 7. Outline of the arm region of embryo CLXIII from Plate III. 

 Bardeen and Lewis, Vol. I, No. 1, this Journal. X 15 diameters. 



The first or dorsal division lies lateral to the muscle-plate column, 

 and extends to the level of the fifth cervical disc. 



The second, third and fourth divisions correspond so closely with 

 the position in which I find certain muscles in the next stage, and as 

 they also have the same nerve supply as the muscles into which I 

 believe they develop, that I have called them in order: The (2) levator 

 scapulae and serratus anterior, the (3) latissimus dorsi and teres major, 

 and the (4) pectoral premuscle masses. 



The second division, the levator scapulce and serratus anterior pre- 

 muscle mass, ventral to the first and opposite the ventral portion of the 

 muscle-plate column is fairly well defined. It extends into the upper 

 cervical region. It lies in a more median plane than the scapula, and 

 at this stage is in no way attached to it. 



