THE MUSCULAR SYSTEM. 



669 



t>r 



except in Amphioxus, Petromyzon, etc., is surrounded by a 

 special investment of sarcolemma. 



The voluntary muscular system. For the purposes of 

 description the muscular system of the Vertebrata may conve- 

 niently 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. 

 292 ^296) 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 Elas- 

 mobranchii it will be convenient to 

 commence 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 specialised 

 part (fig. 377). Before long the cavity 

 becomes however completely constrict- 

 ed off from the permanent body cavity. 



Very early (fig. 377) the inner or splanchnic wall of the 

 somites loses its simple constitution, owing to the middle part of 

 it undergoing peculiar changes. The meaning of the changes is 

 at once shewn by longitudinal horizontal sections, which prove 

 (% 378) that the cells in this situation (mp') have become 

 extended in a longitudinal direction, and, in fact, form typical 

 spindle-shaped embryonic muscle-cells, each with a large 

 nucleus. Every muscle-cell extends for the whole length of a 

 somite. The inner layer of each somite, immediately within 

 the muscle-band just described, begins to proliferate, and produce 



FIG. 377. TRANSVERSE 

 SECTION THROUGH THETRUNK 

 OF AN EMBRYO SLIGHTLY 

 OLDER THAN FIG. 28 E. 



nc. neural canal ; pr. pos- 

 terior root of spinal nerve ; x. 

 subnotochordal rod ; ao. aorta ; 

 sc. somatic mesoblast ; sf>. 

 splanchnic mesoblast ; mp. 

 muscle-plate ; mp', portion of 

 muscle-plate converted into 

 muscle ; Vr. portion of the 

 vertebral plate which will give 

 rise to the vertebral bodies ; al. 

 alimentary tract. 



