THE EPIMERE (SEGMENTAL OR VERTEBRAL PLATE) 211 



jection. The sclerotome of the two potential posterior vertebrae fuses 

 to form the cartilage and then the bony urostyle which encloses the 

 posterior end of the notochord. 



The dermatome or cutis plate gives rise to the dermal layer of the 

 dorsal and lateral skin, to connective tissue between the myotomes, 

 and to the musculature of the limbs. The dermal layer of the ventral 

 body wall arises from the somatopleure, so that while the dermis be- 

 comes continuous it originates from two sources. 



The myotomes or muscle segments of the early larva arise as rather 

 solid aggregations of cells, concentrated at the level of the spinal 

 cord and notochord. They enlarge at the expense of the contained 

 myocoels. Before the time of hatching, these myotomal cells become 

 elongated and their muscle fibrillae orient their axes in the longitudi- 

 nal direction of the embryo. This indicates a shift in the axes from the 

 original direction in the early myotomes. The myotomes become 

 separated from each other by septa or myocommata (connective 

 tissue sheets). These are derived from the cutis plate, and assume a 

 "V" shape with the apex of the "V" pointing posteriorly. These myo- 

 tomes become the layer of striated or voluntary (voluntarily con- 

 trolled) muscles of the back, limbs, and dorsal body wall. The fibers 

 of many of these muscles are arranged ultimately in a variety of direc- 

 tions to provide for complete body control. 



The muscles of the heart, blood vessels, and viscera arise from the 

 hypomeric splanchnopleure and are known as smooth or involuntary 

 muscles. 



Limb buds develop from the accumulation of loose mesenchyme of 

 adjacent somites surrounded by ectoderm. Within this blastema of 

 somatic mesoderm there arise muscle and bone, developing from the 

 body outwardly. The forelimb muscles develop before those of the 



Relation of the Anterior Somites to Nerves 



