MUSCULAR SYSTEM 



131 



usually branched cells, while, physiologically, they are involuntary 



in character. 



THE PARIETAL MUSCLES 



After the myotomes are cut off from the rest of the coelomic walls 

 (p. 14) each consists of a closed sac, containing a part of the ccelom 



Fig. 141. — Myotomes of Amblysloma developing into muscle fibres, ec, ectoderm; mc, 

 myocoele; tns, mesenchyme; so, somatic layer which will form corium. 



(myocoele) and an inner (splanchnic) and an outer (somatic) wall. 

 The more dorsal cells of the splanchnic wall (fig. 45) rapidly increase 

 in number and size, thus tending to obliterate the myocoele. At the 

 same time they become rearranged, so that, instead of forming a 

 cubical or columnar epitheliuih, they have their long axis parallel to 



Fig. 142. — Diagram of descending myotomes, c, coelom; ^, gonad; m, splanchnic 

 wall of myotome developing into muscles; tnc, myocoele; p, peritoneum; pd, pronephric 

 duct; so, somatic wall of myotome; v, ventral border of myotome. 



the long axis of the body (fig. 141), each becoming multinucleate. 

 Gradually the mass of the protoplasm becomes converted into con- 

 tractile substance and the cell is converted into a muscle fibre, the 

 nuclei being in the interior in the lower vertebrates, on the surface of 

 the fibres in the mammals. In this way the splanchnic wall of each 

 myotome is converted into a muscle; hence there are as many pairs of 



