68 



CHORDATE ANATOMY 



and visceral sheets of mesoderm and the myocoeles lose continuity with 

 the permanent coelom (Fig, 59^). Eventually, as the somite differen- 

 tiates, the myocoele is obliterated. 



Fig. 59. — Diagrams, (transverse sections) showing embryonic origin of pronephric 

 tubules. A, earlier stage; B, later, c, coelom; d, pronephric tubule and duct; e, 

 epimere; h, hypomere; m, mesomere (cross-lined); my, myotome; n, nephrostome; so, 

 somatic layer of hypomere; sp, visceral (splanchnic) layer of hypomere. (From Kings- 

 .ey, after Felix.) 



The differentiation of the vertebrate mesoderm is more elaborate 

 than that of Amphioxus, especially in the prospective trunk region. 

 Here, upon each side, early arise three zones of differentiation: the epi- 



FiG. 60. — Diagrammatic transverse section of the body of a vertebrate embryo at an 

 advanced stage. The muscle-forming myotome is beginning to extend into the ventral 

 body-wall of the embryo, c, coelom; g, genital ridge; jh, muscle derived from myotome; 

 7nc, myocoele; p, peritoneum; pd, pronephric duct; so, somatic layer (dermatome) of 

 somite; v, advancing ventral border of myotome; the finely dotted areas are occupied by 

 mesenchyme. (From Kingsley.) 



mere, a dorsal mainly muscle-forming part; the mesomere, a kidney- 

 forming zone situated just below the epimere; and the h5rpomere, the 

 most ventral zone, constituting the somatic and visceral layers of peri- 

 toneum (Figs. 58 and 59^4). 



