IIO MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



of the hypaxial system are to be found, greatly modified, in the 

 pelvic and neck regions. 



The subsequent modifications of the primitive musculature 

 in the higher groups cannot be traced here in detail, even were 

 it better known. Only the origin of the limb muscles can be 

 referred to. This is best known in the fishes, there being only 



FIG. 119. Section 

 through the tail of Am- 

 blystoma, showing (<?) 

 epiaxial and (/y) hypaxial 

 muscles; h, haemal arch; 

 n, notochord; r, ribs. 



FIG. 1 20. Developing fin of trout, after Corning, f, 

 fin ; m, myotomes; n, notochord. Strands of cells cart 

 be traced from several myotomes into the fin. 



scattering observations relating to the air-breathing vertebrates ; 

 but these few accounts justify us in the assumption that in the 

 amniotes the phenomena are essentially the same. In the fish- 

 like forms several of the somites almost immediately behind 

 the head bud from their lower surfaces cords of cells which 

 extend out laterally, lose their distinctness, and form a common 

 matrix out of which the definitive muscles are later developed 

 (Fig. 120). In the amniotes more somites intervene between the 



