THE MUSCULAR SYSTEM. 551 



The voluntary muscular system of the Chordata. 



The muscular fibres, The muscular elements of the Chordata 

 undoubtedly belong to the myo-epithelial type. The embryonic 



FIG. 376. MUSCLE-CELLS OF LIZZTA KOLLIKERI. (From Lankester; after 



O. and B. Hertwig.) 



A. Muscle-cell from the circular fibres of the subumbrella. 



B. Myo-epithelial cells from the base of a tentacle. 



muscle cells are at first simple epithelial cells, but soon become 

 spindle-shaped : part of their protoplasm becomes differentiated into 

 longitudinally placed striated muscular fibrils, while part, enclosing 

 the nucleus, remains indifferent, and constitutes the epithelial ele- 

 ment of the cells. The muscular fibrils are either placed at one side 

 of the epithelial part of the cell, or in other instances (the Lamprey, 

 the Newt, the Sturgeon, the Rabbit) surround it. The latter ar- 

 rangement is shewn for the Sturgeon in fig. 57. 



The number of the fibrils of each cell gradually increases, and 

 the protoplasm diminishes, so that eventually only the nucleus, or 

 nuclei resulting from its division, are left. The products of each cell 

 probably give rise, in conjunction with a further division of the 

 nucleus, to a primitive bundle, which, except in Amphioxus, Petro- 

 myzon, etc., is surrounded by a special investment of sarcolemma. 



The voluntary muscular system. For the purposes of descrip- 

 tion the muscular system of the Vertebrata may conveniently be 

 divided into two sections, viz. that of the head and that of the trunk. 

 The main part, if not the whole, of the muscular system of the trunk 

 is derived from certain structures, known as the muscle-plates, 

 which take their origin from part of the primitive mesoblastic 

 somites. 



It has already been stated (pp. 243 246) that the mesoblastic 

 somites are derived from the dorsal segmented part of the primitive 

 mesoblastic plates. Since the history of these bodies is presented 

 in its simplest form in Elasmobranchii it will be convenient to com- 

 mence with this group. Each somite is composed of two layers a 

 somatic and a splanchnic both formed of a single row of columnar 

 cells. Between these two layers is a cavity, which is at first directly 

 continuous with the general body cavity, of which indeed it merely 

 forms a specialized part (fig. 377). Before long the cavity becomes 

 however completely constricted off from the permanent body cavity. 



