3i8 



THE MORPHOGENESIS OF THE SKELETON AND MUSCLES 



The ventral extensions of the lumbar myotomes (except the first) and of the 

 first two sacral myotomes do not participate in the formation of the body wall. 

 If they persist at all, it is possible that they contribute to the formation of the 

 lower limb. The ventral portions of the third and fourth sacral myotomes give 

 rise to the muscles of the perineal region. 



Fig. 322. — Reconstruction of a 9 mm. human embryo to show tlie partially fused myotomes and 

 the premuscle masses of the limbs (Bardeen and Lewis). X 13. Distally, in the upper extremity, the 

 radius, ulna and hand plate are disclosed; in the lower extremity the os coxs anlage and the border vein 

 show. 



Muscles of the Limbs. — It has generally been believed that the muscles of 

 the extremities are developed from buds of the myotomes which grow into the 

 anlages of the limbs. In sharks this is clearly the case, and in man the segmental 

 nerve supply is suggestive, but not proof, of a myotomic origin. According to 



