THE MUSCULAR SYSTEM. 555 



rapidity, and forms the nucleus of the whole voluntary muscular 

 system (fig. 880, mp). It extends upwards and downwards by the 

 continuous conversion of fresh cells of the splanchnic layer into 

 muscle- cells. At the same time it grows rapidly in thickness by the 

 addition of fresh spindle-shaped muscle-cells from the somatic layer as 

 well as by the division of the already existing cells. 

 ■ Thus both layers of the muscle- plate are concerned in forming the 

 great longitudinal lateral muscles, though the splanchnic layer is con- 

 verted into muscles very much sooner than the somatic^. 



Each muscle-plate is at first a continuous structure, extending 

 from the dorsal to the ventral surface, but after a time it becomes 

 divided by a layer of connective tissue, which becomes developed nearly 

 on a level with the lateral line, into a dorso-lateral and a ventro- 

 lateral section. The ends of the muscle-plates continue for a long 

 time to be formed of undifferentiated columnar cells. The compli- 

 cated outlines of the inter-muscular septa become gradually esta- 

 blished during the later stages of development, causing the well-known 

 appearances of the muscles in transverse sections, which require no 

 special notice here. 



The muscles of the limbs. 'J he limb muscles are formed in 

 Elasmobranchii, coincidently with the cartilaginous skeleton, as two 

 bands of longitudinal fibres on the dorsal and ventral surfaces of the 

 limbs (fig. 346). The cells, from which these muscles originate, are 

 derived from the muscle-plates. When the ends of the muscle-plates 

 reach the level of the limbs they bend outwards and enter the tissue of 

 the limbs (fig. 380). Small portions of several muscle- plates {mpl) 

 come in this way to be situated within the limbs, and are very soon 

 segmented off from the remainder of the muscle-plates. The portions of 

 the muscle-plates thus introduced soon lose their original distinctness. 

 There can however be but little doubt that they supply the tissue for 

 the muscles of the limbs. The muscle-plates themselves, after giving 

 off buds to the limbs, grow downwards, and soon cease to shew any 

 trace of having given off thet^e 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 Elasmobranchii; 

 so that all the higher Vertebrata pass through, with reference to their 

 muscular system, a fi.sh-like stage. The middle portion of the inner 

 layers of their muscle-plates becomes, as in Elasmobranchii, con- 



1 The brothers Her twig have recently maintained that only the inner layer of the 

 muscle-plates is converted into muscles. In the Elasmobranchs it is easy to de- 

 monstrate the incorrectness of this view, and in Acipenser {vide fig. 57, mp) the two 

 layers of the muscle-plate retain their original relations after the cells of both of them 

 have become converted into muscles. 



