EMBRYOLOGY 



15 



myotomes appear as square boxes, and early students, thinking that 

 they gave rise to the vertebrae, called them protovertebrae, a term 

 still in use to some extent. 



After separation from the rest of the mesothelium each myotome 

 grows dorsally along the side of the spinal cord, and to a greater 

 extent ventrally, insinuating itself between the ectoderm and the 

 somatic wall of the hypomere (fig. 9, in the direction of the arrows). 



Fig. 9. — Diagrammatic transverse section of a vertebrate to illustrate mesenterie«, 

 omentum and downward growth of the myotomes, al, alimentary tract; ao, aorta; c, 

 coelom; ec, ectoderm; dmes, dorsal mesentery; my, myotome; nc, notochord; neph, 

 nephrotome; 0, omentum; sc, spinal cord; so, sp, somatic and splanchnic layers of 

 mesothelium; vmes, ventral mesentery. 



^Each myotome has a somatic and a splanchnic wall. Later the 

 somatic wall breaks up, its cells spreading beneath the ectoderm and 

 furnishing the mesoderm immediately beneath the ectoderm (epi- 

 dermis). The upper part of the splanchnic wall is gradually trans- 

 formed into the body (somatic) musculature, while the ventral por- 

 tion usually buds cells which take part in the formation of the axial 

 skeletal tissue./ 



The mesomeral (middle plate) part of the fingers is largely con r 

 cerned m the formation of the excretory (nephridial) system, but 



