THE MUSCULAR SYSTEM. 673 



soon extend to the summit of the neural canal, and their lower 

 ones nearly meet in the median ventral line. The original band 

 of muscles, whose growth at first is very slow, now increases 

 with great rapidity, and forms the nucleus of the whole volun- 

 tary muscular system (fig. 380, 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 converted into muscles very much sooner than the somatic 1 . 



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 complicated outlines of the inter-muscular 

 septa become gradually established 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. The 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 (m.pl) 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 dis- 



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

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

 strate 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. 



B. in. 43 



