280 Papers from the Department of Marine Biology. 



intimately with the endomysium which incloses the entire bundle and 

 separates the main branches. For purposes of minuter description a 

 smaller trabecula, or branch, may be used. The myofibrillse are 

 grouped into large masses, in which they are arranged in lamellae 

 (fig. 6b) undergoing a peripheral radial and a central vertical splitting. 

 The nuclei are located both centrally and peripherally. 



Figure 7 illustrates the typical appearance of a small fiber. The 

 myofibrillae are segregated centrally and are enveloped by a consider- 

 able area of granular cytoplasm confined by a delicate sarcolemma. 

 The nucleus here lies wholly in the extrafibrillar sarcoplasm. Upon 

 close inspection the lamellae are seen to consist of fibrillae radially 

 arranged. The apparent lamellae illustrated in figure 6b are therefore 

 smaller groups of fibrillae. 



As in the skeletal muscle, the cardiac fiber consists of peripheral 

 lamellar and central cylindrical groups of fibrillae. The close similarity 

 between the cardiac and the skeletal muscle extends to the structure 

 and distribution of the nuclei, the finer resolution of the apparent 

 elementary fibrillae (fig. 8), and the presence of a true cell-membrane 

 or sarcolemma. 



In longitudinal sections the appearance of the fiber again, as in the 

 skeletal muscle, and almost identically, varies according to the degree 

 of contraction. Certain fibers (fig. 9) show a wide darker-staining 

 Q-disk and an alternating slightly wider /-disk bisected by a deep- 

 staining granular Z-membrane. In certain other fibers, also uncon- 

 tracted, only the deep-staining granular Z-membrane is conspicuous 

 (fig. 11). The contracted fibers (fig 13c) show only a succession of 

 contraction bands, alternating with lighter-staining disks. The con- 

 traction bands are very similar to those of the skeletal muscle and 

 probably likewise represent a thickened Z-membrane, due to a segre- 

 gation of Q-substance about them. 



THE TELOPHRAGMA. 



The telophragma is also a continuous membrane in cardiac muscle. 

 It spans considerable intervals between widely separated adjacent 

 fibrils (sarcostyles) (figs. 10 and 11); at its level the fibrils swell at the 

 point of attachment ; it is continuous peripherally with the sarcolemma 

 (figs. 8, 10, 11, and 13) and centrally with the nuclear wall. That the 

 granular appearance is actually due to swelling of the fibrils at the 

 level of attachment is proved by the fact that that portion of the 

 membrane which spans the space between the outermost fibrillae and 

 the sarcolemma, as also that which spans the perinuclear sarcoplasm, 

 is non-granular (fig. 8). There is no suggestion of a mesophragma. 



THE SARCOLEMMA. 



That a true sarcolemma actually exists can not be doubted. This 

 conclusion is contrary to that of Meek (1909). He holds that a sarco- 



