STRIATED OR VOLUNTARY MUSCLE. 



465 



of both varieties of terminations in striped muscle will be found under nerve-endings 

 (page 1014;. 



Development of Striped Muscle. — The early appearance of a series of 

 quadrilateral segmental areas, the somites, within the tract of the paraxial mesoblast 

 on each side of the neural tube has been described (page 29). Likewise the sub- 

 sequent breaking up of each somite into the centrally situated sclerotome and the 

 peripheral myotome (Fig. 34). The latter soon becomes a compressed C-shaped 

 mass, in which the more compact lateral part is usually described as the cidis-plate 

 and the medial portion as the muscle-plate. The histological characters of these parts 

 of the myotome differ, the cutis-plate consisting of several layers of closely packed 

 cells resembling epithelial elements, while the muscle-plate is composed of more 

 loosely disposed spindle-cells, between which lie irregularly round cells, many of 



Fig. 4S7. 



Neural canal 



Lateral plate 



Medial 

 plate 



Transition 

 zone 



Myotome 



Interseg- 

 mental ves- 

 sel 



Ectoblast 



Wall of neural 

 tube 



Neural 



Fig. 488. 



Frontal section of rabbit embryo, showing 

 myotomes, x loo. 



Intersegmental 

 blood-vessel 



Ectoblast 



Lateral plate 



Developing 

 muscle-fibres 



Intersegmental 

 septum 



Wall of neural 

 tube 



Developing 

 muscle- fibres 



Intersegmental 

 blood-vessel 



Frontal section of two myotonies of labbit embryo, 

 showing developing muscle. X 130. 



which are actively engaged in division. The less differentiated round cells, or 

 myoblasts, become elongated and transformed into the spindle-cells, the elements 

 which are directly converted into the young muscle-fibres. The spindle-cells, at first 

 mononuclear, rapidly increase in length, the round or oval nucleus at the same time 

 undergoing division. In consequence the elongated muscle-cells become multinuclear. 

 The cytoplasm of the cells early exhibits differentiation into a peripheral and a 

 central zone. During the second foetal month the former manifests a disposition to 

 become fibrillar, while the central zone for a time remains undifferentiated and contains 

 the muscle-nuclei. 



On cross-section the young muscle-fibres at this stage appear as stippled rings 

 enclosing an indifferent core surrounding the nuclei, the stippling being due to the 

 partially differentiated fibrillae. The latter appear first as marginal groups, but 

 later form a continuous peripheral zone. This gradually widens and, by the close 

 of the sixth foetal month, the fibres composing the muscles of the upper extremity 



30 



