132 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



these primitive muscles as there were of myotomes. The somatic 

 wall of the myotome does not usually participate in the muscle 

 formation, but is changed into mesenchyme and eventually gives rise 

 to the corium of the skin. Mesenchyme also invades the spaces 

 between the successive myotomes, develops into fibrous connective 

 tissue, and forms the ligamentous connexions (myosepta, myocom- 

 mata) 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 



•. C>x 



IX 



I ^ ^ 



Fig. 143. — Head of embryo dogfish (Acanihias) seen as a transparent object, show- 

 ing the preotic mesodermal somites, with dotted outlines, as a, i, 2, and 3. b^b*, gill 

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

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



original segmentation can be traced in the intercostal and rectus 

 abdominis muscles. *^t first the myotomes lie at about the level of 

 the notochord and spinal cord, but with growth they extend upward 

 and to a greater extend downward, insinuating themselves between 

 the skin and the walls of the coelom (fig. 142, right side) and thus 

 forming part of the somatopleure. The downward growth continues 

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

 line (fig. 10), 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 



