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



both pectoralis major and minor muscles. This mass extends from the 

 level of the third rib to the humerus and clavicle. There is no attach- 

 ment to the ribs. The intercostal muscles have been dissected away 

 in Plate V to show its costal end. The second division of the lateral 

 pre-muscle mass has developed into the latissimus dorsi and teres 

 major muscle mass. It extends, from the level of the 4th rib to the 

 humerus, where it blends with the scapulo-humeral mass. Its develop- 

 ment and differentiation have not proceeded so far as the pectoral mass. 

 The third division of the lateral pre-muscle mass has developed into a 

 long muscle extending from the 1st cervical vertebra to the 9th rib, 

 Digitations extend to the transverse processes of the cervical ver- 

 tebra and to the cephalic 9 ribs. The muscle lies in a more median 

 plane than the scapula and has as yet no attachment to it. It represents 

 the levator scapula? and serratus anterior muscles. The trapezius mass 

 has extended to a lower level than found in CLXIII. There is no 

 scapular attachment. Only the ventral half of the mass pictured in 

 Plate IV consists of muscle fibres, the remaining connective tissue por- 

 tion connects with the dorsal ends of the neural processes. 



Considerable differentiation in the pre-muscle sheath has taken place. 

 The scapulo-humeral mass is with difficulty separated into the various 

 muscles. These are more blended into a single mass than would appear 

 from Plates IV and V. Here portions of the deltoid and trapezius have 

 been dissected away. 



The extensor mass of the forearm can be separated into three groups 

 which are more or less blended however. The larger superficial mass 

 has been partially dissected away. It represents the extensor digitorum 

 communis, extensor carpi ulnaris, and extensor digiti quinti proprius 

 muscles. The second group arises beneath the first group, taking a 

 course at nearly right angles to it. It represents the deep extensor 

 muscle of the forearm. These two groups fuse distally with the general 

 condensed mesenchyme of the digits, where all traces between pre- 

 muscle and pre-cartilage are lost. The third group represents the 

 brachio-radialis and the extensor carpi radiates longus et brevis. A 

 • portion of the brachialis can be seen in Plate IV. The flexor sur- 

 face of the arm is shown in Fig. E, Plate V. The biceps and coraco- 

 brachialis are obscured by the pectoral mass. The flexor mass of the 

 forearm has split into two layers. It shows less differentiation than the 

 extensor mass. 



The arm skeleton shows considerable advance. The shape of the 

 scapula has changed, both coracoid process and acromion are present 

 and of large size. The clavicle has begun to develop and consists of 



