24 Development of the Limbs, Body-wall and Back in Man 



and radius are represented. The ulna and radius are continuous with 

 the hand plate, which is composed of less differentiated tissue. There 

 is a slight bend at the elbow. The upper end of the humerus is con- 

 tinuous with the scapula. The scapula is a flattened oval mass embedded 

 in the scapular pre-muscle tissue. No coracoid or arromiom processes 

 are present. The clavicula is not present. The skeletal core is com- 

 posed of dense mesenchyme. It shades oft", however, into the surround- 

 ing pre-muscle sheath. 



The brachial plexus is formed by the 5th to 8th cervical and 1st 

 thoracic nerves. The spinal nerves, as well as the plexus they form, 

 have scarcely any caudal inclination, but pass ventrolaterally into the 

 arm. The plexus is fairly well formed. The main nerves arising from 

 it are present. The presence of the condensed skeletal core separates 

 these into two groups. The musculo-spiral and circumflex on the dorsal 

 and the ulnar, median and musculo-cutaneous on the ventral side. In 

 Fig. C only the tips of the musculo-spiral and median nerves are shown. 

 At this stage they have scarcely grown to the elbow. 



Sixth Week. 



During the sixth week the limbs and the body-wall and the back begin 

 to approach the structural conditions characteristic of the adult. 



The conditions present at the end of the fifth week are shown in 

 Embryo CIX; length, 11 mm.; age, about 5 weeks. This embryo is 

 pictured in Figs. D and E, Plates IV and V. These figures, with the 

 exception of the head in Fig. D, are drawn from wax-plate reconstruc- 

 tions. Fig. 24 represents a cross-section through the 6th to 7th thoracic 

 segment of a slightly older embryo, CVI (length, 15.5 mm.; age, about 

 5-| weeks). 



'The typical thoracic segment during the first half of the sixth week 

 exhibits the following conditions: 



The skeletal structures have begun to assume adult form. Between 

 the intervertebral discs the bodies of the vertebrae are now formed of 

 chondrogenous tissue. This may be seen in the dotted area of the 

 scleromere in Fig. 24 and in the darker areas of the spinal column in 

 Fig. E. The chondrogenous tissue extends into the neural and trans- 

 verse processes of the scleromere (see the left side of Fig. 24). Each 

 costal process extends considerably further ventrally than at the stage 

 shown in Fig. 23. Within the dense tissue of the costal process chon- 

 drogenous tissue is formed similar to, but without direct connection with, 

 that of the vertebral body and transverse process. A lateral view of the 



