MORPHOGENESIS OF MUSCULAR SYSTEM 707 



aquatic forms the tail musculature is greatly reduced, and in some forms is 

 almost non-existent. 



A consideration of the effect that locomotive habits have upon musculature 

 development may be shown by a brief comparison of the musculature in a 

 water-living amphibian, such as Necturus, and in a land-going adventurer, 

 such as the frog. In Necturus, the dorsal (epaxial) musculature, the primitive 

 M. dorsalis trunci, is more like that of the fish, with the muscle fibers attached 

 to the myocommata (fig. 327C), although, contrary to the piscine condition, 

 the muscle fibers close to the vertebrae are attached directly to the vertebrae, 

 where they form short bundles. In the frog, the attachment of the epaxial 

 musculature to the vertebrae is more extensive. Bundles of muscle fibers, the 

 Mm. intertransversarii, pass between the vertebral transverse processes, while 

 Mm. intemeurales connect the transverse processes and spinous processes, 

 respectively, of the vertebrae. A separate muscle, the M. longissimus dorsi, 

 extending from the head to the urostyle, separates from the above-mentioned 

 dorsal muscles (fig. 327D). Although a slight suggestion of myocommata 

 may be present, there is little functional relationship of the myocommata to 

 the vertebrae. Laterally, Mm. coccygeo-sacralis and coccygeo-iliacus also are 

 present as differentiations of the dorsal musculature (fig. 327D). Therefore, 

 a definite formation of special and individual muscles occurs in the dorsal 

 or epaxial musculature of the frog, whereas in Necturus, the dorsal musculature 

 tends to resemble the segmental myotomic condition of the fish. It is to be 

 observed that the dorsal musculature of the frog is adapted to a land-going 

 existence, while the dorsal musculature of Necturus is suited to swimming 

 movements. 



A further land adaptation is shown in many salamanders, such as the 

 various species of Desniognathus, where the dorsal trunk musculature differ- 

 entiates in the neck region into several muscles which insert upon the skull. 

 The latter muscles permit lateral movements of the head. 



Turning to the hypaxial musculature, we find that this musculature in 

 Necturus also approaches the condition in fishes. Let us examine this mus- 

 culature in more detail. In the midventral abdominal area, the fibers assume 

 a primitive, strictly segmental, antero-posterior direction. These muscle bundles 

 form the M. rectus abdominis. Along the lateral side of the body wall, the 

 myosepta (myocommata) are retained between the segmented muscles. How- 

 ever, two layers of muscle fibers are present, an outer thick M. obliquus 

 externus, whose fibers' run postero-ventrally, and an inner thin layer, the 

 M. obliquus internus, with fibers coursing antero-ventrally. Turning now to 

 the frog, we find that a segmented rectus abdominis (M. rectus abdominis) 

 is present. In each lateral body wall, an outer external oblique muscle (M. 

 obliquus externus superficialis) runs postero-ventrally, while an internal 

 transverse muscle (M. transversus) courses antero-ventrally (fig. 327D). In 

 Necturus and the frog, therefore, the primitive myotomic condition of the 



