714 THE MUSCULAR SYSTEM 



out in the area occupied by the myocommata or connective tissue partitions 

 between the myotomes, and thus ribs and myocommata are correlated inti- 

 mately with myotomic differentiations in all lower vertebrates. However, in 

 reptiles, birds, and mammals, the outgrowing ribs travel downward within the 

 connective tissue between the myotomes, but the development of the mycom- 

 mata are suppressed. 



b) Differentiation of the Myotomes in Fishes and Amphibia. In 

 the fishes, as the ventral myotomic progression occurs, the differentiating 

 muscle fibers become united anteriorly and posteriorly to the myocommata. 

 In Necturus and in amphibian larvae, in general, this relationship also is 

 established, but, in addition, the myotomes become separated into sheets or 

 layers. In the frog during metamorphosis, this splitting of myotomes and 

 the segregation of separate layers and bundles of distinct muscles is carried 

 further. Also in the frog, a marked migration of separate bundles of muscle 

 fibers occurs, while the fusion of parts of separate myotomes is indicated in 

 the development of the M. longissimus dorsi which superficially appears to 

 be segmented (fig. 327D). There is a pronounced tendency, therefore, in 

 the development of the frog musculature for the primitive myotomic plan 

 to be distorted and myotomes fuse, split, degenerate or migrate to serve the 

 required functional purpose of the various muscles. 



c) Differentiation of the Truncal Myotomes in Higher Verte- 

 BRATA and Particularly in the Human Embryo. The principles of myo- 

 tomic modification by fusion, splitting into separate components, migration 

 of parts of myotomes away from the primitive position, and degeneration 

 of myotomic structure as exemplified in the developing musculature of the 

 frog, are utilized to great advantage in reptiles, birds and mammals. The 

 end to be served in all instances is the adaptation of a particular muscle or 

 muscles to a definite function. 



In the development of the adult form of the musculature in the human 

 embryo, the basic division of the primitive myotomes into dorsal (epaxial) 

 and ventral (hypaxial) regions occurs (fig. 328A). The dorsal region of the 

 myotomes is located alongside the developing vertebrae, dorsal to the trans- 

 verse processes. The ventral portions of the myotomes pass ventrally external 

 to and between the ribs, enclosing the developing viscera. 



In a slightly older embryo, the dorsal or epaxial musculature begins to lose 

 its primitive segmentation, and the myotomes fuse into an elongated myo- 

 tomic column, extending caudally from the occipital area (fig. 328B). The 

 deeper portions of the myotomes, associated with the developing vertebrae, 

 appear to retain their original segmentation, and the Mm. levatores costarum, 

 interspinales, intertransversarii, and rotatores persist as segmental derivatives 

 of the myotomes. The outer layer of the dorsal or epaxial musculature splits 

 lengthwise into an outer muscle group, the dorsally placed Mm. longissimus 

 dorsi and spinalis dorsi, and a latero-ventral Mm. iliocostalis group (fig. 



