MUSCLE. 



95 



from the external cells. These fibres join to form a limiting 

 capsule. The middle layer is composed of spindle-shaped 

 cells ; the main layer, of columnar cells (Fig. 63). The round 

 cells of the median muscle lamella are known as myoblasts. 

 From these the spindle-shaped cells are derived by an elonga- 

 tion of the body of the cell (Fig. 64). Karyokinetic figures 



FIG. 64. 



Cells from a horizontal section of muscle lamella of a pig'sembryo 8 mm. long. (Bardeen.) 

 a, myoblast; b, young spindle cell ; c, <l, older spindle cells. 



are found abundantly among these cells. In the protoplasm 

 there is to be seen only an irregular network. No fibril bun- 

 dles are present. The cells of the epithelial lamella become 

 converted constantly into myoblasts. For a detailed descrip- 

 tion of the development of the myotome and its various cell 



FIG. 65. 



Fio. 66. 











** 



Cross-section of voluntary muscle from 

 the thigh of an embryo pig 25 mm. long. 

 A, cell showing the nucleus ; B, cell show- 

 ing sarcoplasmic disks. (MacCallum.) 



Cross section of voluntary muscle from 

 the thigh of an embryo pig 45 mm. long, 

 showing fibril bundles at the periphery of 

 the cells. (MacCallum.) 



layers, the reader is referred to Bardeen's original article. 

 The later stages in the development of the muscle cell are 

 as follows (J. B. MacCallum): In Fig. 65 is represented a 

 cross-section of the muscle fibres of a pig's embryo 25 mm. 



