160 The Development of the Arm in Man 



determine this with certainty. Other portions of the arm skeleton are 

 in the precartilage and condensed tissue stages. Both cartilage and 

 precartilage are surrounded in most places by a distinct perichondrium. 

 This takes a very deep stain with the alum carmine. This perichond- 

 rium shades off into the condensed tissue of the carpus, which is like 

 that composing the skeletal core in the preceding stage. This again 

 shades into the even less differentiated tissue of the digits, which is at 

 a.bont the same stage of development as the hand plate of the preceding 

 stage, and it in turn shades off into the surrounding mesenchyma. 



The muscles in the arm region show very different degrees of devel- 

 opment. Those derived from the muscle plate system are in advance 

 of most of the others. The trapezius, levator scapulae and serratus 

 magnus are about as far advanced as those from the muscle plate system, 

 they show distinct muscle fibers and are for the most part quite sharply 

 limited from the surrounding loose mesenchyma. In position they 

 correspond with their premuscle masses of the preceding stage. The 

 pectoral muscle is next in advance and the latissimus dorsi next. 

 These two muscles grow from the humeral region towards their future 

 attachments on the body wall. It is this portion which lies farthest 

 from the humerus which seems to show the most advance in fibrillation 

 and the sharpest limitation from the surrounding mesenchyma. At the 

 humeral end these muscles gradually shade into a condensed mesen- 

 chyma, which fuses with neighboring muscle and skeletal elements. 

 Both muscles correspond in position to their premuscle masses of the 

 preceding stage. As in the preceding stage, embryo CLXIII, the 

 trapezius and serratus premuscle masses were in advance of the pectoral 

 and latissimus; in embryo CIX we find the same relation still continues. 



The remaining muscles of the arm apparently develop in situ from 

 the premuscle sheath and undergo practically no migrations. They do 

 not appear to be as far advanced as any of the above mentioned muscles. 

 Of these muscles developing from, the arm premuscle sheath, the more 

 proximal ones are more developed than the ones more distal. In the 

 scapulo-humeral region most of the muscles show partial fibrillation, 

 while those in the palm of the hand are in about the same condition 

 as the proximal portion of the premuscle sheath in the preceding stage. 



The fibrillation, position and nerve supply have made it possible to 

 determine the presence of most of the muscles of the arm. 



The Skeletal, System. — The Vertebral Column. The inteiTertebral 

 discs are composed of condensed mesenchyma, the cells having a con- 

 centric arrangement about the chorda. The vertebral bodies between 

 the discs are each composed of two masses of cartilage, one on either 



