126 COMPARATIVE MORPHOLGY OF VERTEBRATES. 
(myoceele) and an inner (splanchnic) and an outer (somatic) wall 
The cells of the splanchnic wall rapidly increase in number and size, 
thus tending to obliterate the myoccele. At the same time they be- 
come rearranged, so that, instead of forming a cubical or columnar 
epithelium, they have their long axis parallel to the long axis of the body 
TOC CE S 
Fic. 134.—Myotomes of Amblystoma developing into muscle fibres. ec, ectoderm; my, 
myoccele; ms, mesenchyme; so, somatic layer which will form corium. 
(fig. 134), each becoming multinucleate. Gradually the mass of the 
protoplasm becomes converted into contractile 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; 
Fic. 135.—Diagram of descending myotomes. c, ccelom; g, gonad; m, splanchnic wall 
of myotome developing into muscles; mc, myoccele; p, peritoneum; pd, pronephric duct; 
so, somatic wall of myotome; v, ventral border of myotome. 
hence there are as many pairs of these primitive muscles as there were 
of myotomes. The somatic wall of the myotome does not participate 
in the muscle formation, but is gradually changed into mesenchyme 
and eventually gives rise to the corium of the skin. Mesenchyme also 
invades the spaces between the successive myotomes, develops into 
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