MUSCULAR SYSTEM. 127 
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 
Fic. 136.—Head of embryo dogfish (Acanthias) seen as a transparent object, showing 
the preotic mesodermal somites, with dotted outlines, as a, 1, 2, and 3. 5'-d‘, 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 oné end becomes conical and fits into a 
corresponding hollow in the next plate. In the tail of the amphibia 
