The Microscopic Structure of Striped Muscle of Limulus. 279 



Its continuity with the Z-membranes marks it as cellular in origin, 

 corresponding to the cell-membrane of the original myoblasts. In 

 picric acid-fuchsin stained material the telophragmata and the sarco- 

 lemma remain unstained (appearing dark hi such preparations), while 

 the surrounding connective tissue (endomysium) stains a light red. 

 When the staining process is unduly prolonged the sarcolemma also 

 stains red, but then the Z-membrane and nuclear membrane and the 

 peripheral sarcoplasm also take on the same red color. 



THE NUCLEUS. 



The nuclei lie scattered throughout the fiber, at any point on the 

 radii, and are surrounded by a spindle-shaped mass of extremely atten- 

 uated sarcoplasm. These sarcoplasmic areas contain a small amount 

 of granular material and several large lipoid spherules. This peri- 

 nuclear sarcoplasm is bounded by myofibrils which converge apically. 

 There is no suggestion of a cell-membrane. Indeed, that there can 

 be no thought of cell structure is demonstrated by the relation of the 

 telophragmata to the nuclear wall. These two structures are contin- 

 uous; and the nuclear wall in many instances is drawn out into spines 

 (ridges) at the point of connection with the membrane. In a contracted 

 fiber such projections are less widely spaced, corresponding to the 

 closer apposition of the membranes; in relaxed fibers they are spaced 

 more widely and the projections are less sharp. 



CARDIAC MUSCLE. 



GENERAL STRUCTURE. 



A cross-section of the tubular heart of Limulus shows the vast 

 majority of the fibers cut longitudinally. They are collected into 

 larger and smaller bundles, arranged approximately radially, tangen- 

 tially, and circularly, crossing each other at angles of all degrees; a 

 very few scattered cross-cut fibers may appear peripherally and cen- 

 trally. The vast majority of the fibers are thus transversely disposed, 

 only an insignificant moiety longitudinally. A median longitudinal 

 section, on the contrary, shows a majority of the fibers cut transversely. 

 Between these, and separating them into wider layers, are narrower 

 bundles of longitudinally cut fibers, the radial fibers. One may search 

 many sections cut near the medial longitudinal plane without seeing a 

 single longitudinal fiber. The latter are certainly extremely rare, but 

 a very few are apparently present, scattered peripherally and next the 

 lumen. Both transverse and longitudinal sections immediately dis- 

 close an intricate loose-meshed muscular syncytium. 



Figure 6 A illustrates a cross-section under low magnification (1,000 

 diameters) of a large fiber at the level where it has become resolved into 

 several main and a number of secondary branches. Each branch is 

 enveloped by a delicate sarcolemma, the peripheral portion fusing 



