THE MUSCULAR SYSTEM. 557 



rate to the episkeletal muscles of Huxley, is practically certain, but 

 the details of the process have not been made out. 



In the Perennibranchiata the fish-like arrangement of muscles is re- 

 tained through life in the tail and in the dorso-lateral parts of the trunk. 

 In the tail of the Amniotic Vertebrata the primitive arrangement is also 

 more or less retained, and the same holds good for the dorso-lateral trunk 

 muscles of the Lacertilia. In the other Amniota and the Anura the 

 dorso-lateral muscles have become divided up into a series of separate 

 muscles, which are arranged in two main layers. It is probable that the 

 intercostal muscles belong to the same group as the dorso-lateral muscles. 



The abdominal muscles of the trunk, even in the lowest Amjthibia, 

 exhibit a division into sevei-al layers. The recti abdominis are the least 

 altered part of this system, and usually retain indications of the primitive 

 inter-muscular septa, which in many Amphibia and Lacertilia are also 

 to some extent pi-eserved in the other abdominal muscles. 



In the Amniotic Vertebrates there is formed underneath the vertebral 

 column and the transverse processes a system of muscles, forming part 

 of the hyposkeletal system of Huxley, and called by Gegenbaur the 

 subvertebral muscles. The development of this system has not been 

 worked out, but on the whole I am inclined to believe that it is derived 

 from the muscle-plates. Kcilliker, Huxley and other embryologists believe 

 however that these muscles are independent of the muscle-plates in their 

 origin. 



Whether the muscle of the diaphragm is to be placed in the same 

 category as the hyposkeletal muscles has not been made out. 



It is probable that the cutaneous muscles of the trunk are derived 

 from the cells given off from the muscle-plates. Kblliker however believes 

 that they have an independent origin. 



The limb-muscles, both extrinsic and intrinsic, as may be concluded 

 from their development in Elasmobrancliii, are derived from the muscle- 

 plates. Kleinenberg found in Lacertilia a growth of the muscle-plates 

 into the limbs, and in Amphibia Gotte finds that the outer layer of the 

 muscle-plates gives rise to the muscles of the limbs. 



In the higher Vertebrata on the other hand the entrance of the muscle- 

 plates into the limbs has not been made out (Kolliker). It seems therefore 

 probable that by an embryological modification, of which instances are so 

 frequent, the cells which give rise to the muscles of the limbs in the higher 

 Vertebrata can no longer be traced into a direct connection with the muscle- 

 plates. 



The Somites and muscular system of the head. 



The extension of the somites to the anterior end of the body in Am- 

 phioxus clearly proves that somites, similar to those of the trunk, were 

 originally present in a region, which in the higher Vertebrata has 

 become ditferentiated into the head. In the adult condition no true 

 Vertebrate exhibits indications of such somites, but in the embryos 

 of several of the lower Vertebrata structures have been found, which 

 are probably equivalent to the somites of the trunk : they have been 

 frequently alluded to in the previous chapters of this volume. These 

 structures have been most fully worked out in Elasmobranchii. 



