Charles Russell Bardeen and Warren Harmon Lewis 31 



three onter toes have not yet appeared (see Fig. 26, right side). The 

 cartilaginous skeleton of the limb, like that of the spine, is covered by a 

 dense mesenchyme. Torsion has not yet begun at the ankle-joint. 



The musculature of the posterior limb is so far differentiated that all 

 of the individual muscles characteristic of the adult may be distinguished 

 except the lumbricales. The muscles lie in distinct groups, as is shown 

 in the various figures. 



The femoral or extensor group of muscles is shown in Figs. F and H. 

 The groups of muscles belonging to the peroneal nerve and its branches, 

 the gluteal, peroneal, and pedal extensor muscles may be seen in Figs. 

 F and H. The adductor or obturator group of muscles is best seen in 

 Fig. G. The groups of muscles belonging to the tibial nerve may be 

 seen in Fig. I. A detailed account of these muscles is reserved for a 

 subsec{uent article. 



The nerves of the posterior limb, like the muscles, are so well developed 

 that most of them may be readily compared with those of the adult. 

 To reach the adult position a considerable amount of shifting, however, 

 must take place. 



At the period under consideration the blood-vessels of the limb are 

 those characteristic of the adult. The main artery and the chief vein 

 are the femoral. 



An idea of the general condition of the tissues of the limb at this 

 stage may be obtained from Fig. 27, which represents a photograph 

 taken through a section passing through the limb region. 



The anterior limb presents similar conditions of structure. 



The muscular system shows well-marked fibrillation. All of the 

 muscles of the adult arm are present and in about the relative adult 

 positions. 



The bones of the arm are represented by hyaline cartilage except the 

 distal row of phalanges of the 2d to 5th digits. This row is represented 

 by masses of undifferentiated condensed tissue, into which the long ex- 

 tensor and flexor tendons merge. 



The clavicle is well developed and extends to the 1st rib, where it 

 comes in contact with the sternal anlage. The sternal anlage is com- 

 posed of condensed mesenchyme. The ribs, vertebrse and their neural 

 processes are composed of cartilage. All of the cartilages of the arm are 

 surrounded by a condensed mesenchymal sheath, the perichondrium. 



The nervous system presents nearly the adult conditions. It has not 

 been possible, however, to resolve the brachial plexus into its usual 

 distinct cords. They appear from study of sections and the reconstruc- 

 tion to be fused into one mass. 



