MUSCULAR SYSTEM. 



I2 7 



fibrous connective tissue, and forms the ligamentous connections 

 (myosepta, myocommata) between the muscles of a side. This 

 primitive condition is readily recognized in the trunk and tail of the 

 lower vertebrates, and even in the adults of the more modified birds 

 and mammals the original segmentation can be traced in the inter- 

 costal and rectus abdominis muscles. At first the myotomes lie 

 at about the level of the notochord and spinal cord, but with growth 

 they extend upward and to a greater extent downward, insinuating 

 themselves between the skin and the walls of the ccelom and thus 



FIG. 136. Head of embryo dogfish (Acanthias) seen as a transparent object, showing 

 the preotic mesodermal somites, with dotted outlines, as a, i, 2, and 3. b l -b*, gill clefts, 

 the fifth not yet open; e, eye; oc, otic capsule; p, epiphysial outgrowth; s, spiracle; V, tri 

 geminal, VII, facial-acustic; IX, glossopharyngeal; X, vagus nerves. 



forming part of the somatopleure. The downward growth continues 

 until the muscles of the two sides all but meet in the mid-ventral line, 

 the intervening space being occupied by connective tissue, the linea 

 alba of the adult. 



In the fishes the trunk and tail muscles formed in this way become 

 divided horizontally into dorsal and ventral portions, the epaxial and 

 hypaxial muscles, the line of division which follows more or less 

 closely the lateral line, being marked by a partition of connective tissue 

 already mentioned (figs. 30, 131). These plates of muscle do not retain 

 their flat ends in the adult, but one end becomes conical and fits into a 

 corresponding hollow in the next plate. In the tail of the amphibia 



