20 



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



to the cardinal vein. They extend, however, for no considerable dis- 

 tance into the limb-bud. The myotomes do not extend into the limb- 

 bud. 



By following the successive spinal segments from the fourth sacral to 

 the first lumbar in Fig. A, an idea may be gained of the development of 

 segmental structures in the axial region. The scleromeres are repre- 

 sented at the caudal third of each segment. On the left side of the body 

 the myotomes are omitted from the 3d sacral to the 3d lumbar segments. 

 The spinal nerves first appear in the first and second sacral segments. 



Fig. 33. Tangential section tlirougli the leg region of Embryo II. 35 d. 



Fig. 22 shows the general conditions existing in the limb region of 

 Embryo II when seen in section. At the left the leg-bud is shown cut 

 through an area near the distal extremity of the coelom. At the right 

 the cut is more dorsal and extends through the tips of the lumbar spinal 

 nerves. 



In the arm region of Embryo II, the cervico-brachial plexus is formed 

 and limb nerves extend into the limb-bud. The conditions are essen- 

 tially similar to the conditions found in the leg region of Embryo 

 CLXIII and described below. 



