Charles Eussell Bardeen and Warren Harmon Lewis 23 



skeletal mass fades gradually into the undifferentiated mesenchyme of 

 the distal portion of the limb. It. is this skeletal mass which seems to 

 divide the bundles of nerve fibres of the plexus into the four main 

 divisions which form the origin of the four chief nerves of the limb. 

 The main artery and vein of the limb are represented, but smaller in 

 proportion to the other structures than in nature. The border vein at 

 this period is well developed. The conditions just described are well 

 shown in I'ig. C, Plate III, which represents the conditions seen in the 

 limb from the lateral side after removing the ectoderm and the undiffer- 

 entiated mesenchyme. It is to be noted that the myotomes do not 

 extend into the limb-bud. 



The arm is somewhat more advanced in development than the leg. 

 The following description applies to the conditions represented in Fig. 

 C, Plate III. A detailed account of the structure of the arm at this 

 stage is reserved for a later paper. 



Lateral to the myotome system in the arm region is an ill-defined 

 mass of mesenchyme extending from the upper cervical to the 7th 

 thoracic myotomes. At the level of the 1st and 2d ribs it divides into 

 several masses. The first passes ventral to the arm and brachial plexus. 

 The main portion of it joins the arm pre-muscle sheath. From this 

 mass the pectoral muscles develop, hence we may designate it the pec- 

 toral pre-muscle mass. It is continuous ventrally with an in-egular mass 

 of condensed tissue, the ventral neck pre-muscle mass. The second 

 division of the lateral pre-muscle mass passes dorsal to the arm and 

 brachial plexus and joins the arm pre-muscle sheath. It constitutes 

 the latissimus dorsi pre-muscle mass. The third division, parallel to 

 the ventral portion of the cervical myotome column, represents the 

 levator scapulge and serratus anterior pre-muscle mass. Lateral to it is 

 an ill-defined mass of pre-muscle tissue. The fourth and most dorsal 

 division is thinner and less clearly marked than the others. We may 

 call it the rhomboid pre-muscle mass. The caudal limit of the trape- 

 zius pre-muscle mass appears at the upper end of the arm region. It 

 is at the level of the 4th cervical neural processes, and from here the 

 muscle mass extends to the occipital region. Most of the arm pre- 

 muscle sheath which surrounds the skeletal core has been dissected 

 away. Toward the distal end of the arm the skeletal and pre-muscle 

 tissues blend (Fig. C). Beyond this point differentiation is less ad- 

 vanced. The proximal portion of the arm sheath is continuous with 

 that around the scapula. 



Part of the skeletal core is seen projecting caudally beyond the cut 

 edge of the pre-muscle sheath. The lowej end of the humerus, ulna 



