306 



TEXT-BOOK OF EMBRYOLOGY. 



pectoralis and latissimus dorsi migrate from the base of the arm to the thoracic 

 wall. Their nerves are naturally pulled with them. The trapezius muscle, 

 which originates well forward in the cervical region, migrates so that it finally 

 reaches as far as the last thoracic vertebra. The sternomastoideus also origi- 

 nates well forward in the cervical region, but finally extends to the clavicle and 

 sternum. The migration of the upper extremity causes the brachial plexus to 

 have a caudal inclination. 



The lower limb buds arise very soon after the upper. As stated on page 1 53, 

 the upper limbs always maintain a slight advance over the lower in develop- 



S P 



6th 

 Co 



rvertebral disk 



FIG. 272. Transverse section through the 8th cervical segment of a human 

 embryo of 7 mm. (about 4 weeks). Lewis. 



ment. As in the case of the upper, the lower limb buds appear as swellings on 

 the ventro-lateral surface of the body, opposite the fifth lumbar and first sacral 

 myotomes. The interior of each swelling is at first composed of closely packed 

 mesenchymal tissue, but whether any part of the myotomes enters it is question- 

 able. At all events several spinal nerves do enter the tissue and supply the 

 muscles. The differentiation of a central core as the anlage of the skeleton, and 

 the differentiation of the surrounding tissue as the premuscle sheath, take place 

 in the same manner as in the upper extremity (p. 305) . From this premuscle 

 sheath all the muscles of the lower extremity are developed. 



