THE MUSCULAR SYSTEM. 675 



tinctness. There can however be but little doubt that they 

 supply the tissue for the muscles of the limbs. The Tmtsclc- 

 plates themselves, after giving off buds to the limbs, grow 

 downwards, and soon cease to shew any trace of having given 

 off these buds. 



In addition to the longitudinal muscles of the trunk just described, 

 which are generally characteristic of Fishes, there is found in Amphioxus a 

 peculiar transverse abdominal muscle, extending from the mouth to the 

 abdominal pore, the origin of which has not been made out. 



It has already been shewn that in all the higher Vertebrata 

 muscle-plates appear, which closely resemble those in Elasmo- 

 branchii ; so that all the higher Vertebrata pass through, with 

 reference to their muscular system, a fish- like stage. The 

 middle portion of the inner layers of their muscle-plates be- 

 comes, as in Elasmobranchii, converted into muscles at a very 

 early period, and the outer layer for a long time remains formed 

 of indifferent cells. That these muscle-plates give rise to the 

 main muscular system of the trunk, at any 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 Amphibia, 

 exhibit a division into several 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 preserved 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 sub- 

 vertebral 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. Kolliker, Huxley and other embryologists believe 

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

 origin. 



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