FIG. 7. A longitudinal section through a small part of a muscle fiber found in the 

 myotome of a green frog (Rana clamitans) tadpole. The sarcoplasm between the 

 myofibrils (my) is occupied by profiles of membrane-limited structures representing 

 the ER of these cells. A face view of the system is shown at sarcomeres marked X, - 

 otherwise images are profiles. It is obviously differentiated with respect to the 

 adjacent sarcomeres of the myofibrils. At the level of the Z band its continuity is 

 interrupted by the interposition of small membrane-limited structures which ap- 

 pear to originate from the plasma membrane of the fiber (Smith, 1961; Porter, 

 1961c). The intermediate vesicle, plus the two lateral vesicles on opposite sides of 

 the Z band, is the triadic (tr) structure of earlier descriptions (Porter and Palade, 

 1957). A few particles of glycogen are scattered among the membranes. No ribo- 

 somes are attached to the membranes, hence they are properly classified as repre- 

 senting the smooth endoplasmic reticulum (ser) of the muscle cell. 



Though appearing very different from that of other cells, the cytoplasm of the 

 muscle fiber can be interpreted as one in which the fibrous elements of the cyto- 

 plasm are prominently developed and collected into large bundles. In spite of this 

 exaggerated development of matrix components, the endoplasmic reticulum is not 

 lost but is similarly crowded into the space between the myofibrils, presumably to 

 perform a function in contraction and relaxation. (Courtesy of Clara Franzini.) 

 Magnification: 50,000 X. 



136 



