278 Papers from the Department of Marine Biology. 



a mesophragma (membrane of Merkel, 1872; membrane of Heiden- 

 hain; M-membrane). Such a structure must be lacking in Limulus 

 muscle; and in this respect it resembles the wing-muscles of certain 

 insects, and the analogous muscles (pectoral muscles) of birds and 

 bats as described by Thulin. 



The fiber in the contracted condition shows only alternating darker 

 and lighter stripes, the latter 2 to 3 times the width of the former. 

 One can not properly speak here of Q and J disks. The fiber consists 

 of a succession of contraction bands, composed of the ground mem- 

 brane and Q-substance. The dark-staining stripes of contracted muscle 

 correspond topographically to the telophragmata of the non-contracted 

 fiber, but differ from it in that the fibril components are short rods 

 instead of spherical granules. 



THE TELOPHRAGMA. 



The telophragma, as described above, is a continuous membrane, 

 extending from sarcolemma peripherally to nuclear wall centrally, with 

 both of which structures it is closely united, apparently continuous. 

 Occasionally, where adjacent myofibrils are widely separated, a certain 

 number of the Z-membranes can still be seen spanning the gap (fig. 3) . 

 The true-membrane condition of the telophragma is further proved by 

 the appearance in certain fibers which have suffered distortion, appar- 

 ently under pressure applied at right angles to the long axis (fig. 4). 

 Such fibers exhibit a partial folding, the lines of folding (distortion) 

 corresponding always with the telophragmata. The appearance is 

 exactly such as would be expected to result if the fibrils were actually 

 connected with each other by a membrane. The fiber could under 

 the assumed condition, as it does, only fold hi the same direction at 

 any level along the lines of the membrane. Such fibers show beauti- 

 fully also the granular swelling of the fibrils at the level of the mem- 

 brane. 



Centrally the membrane is closely related to the nuclear wall (fig. 3), 

 peripherally to the sarcolemma (fig. 5). To the picric acid-fuchsin 

 mixture the nuclear wall, Z-membrane, and sarcolemma have an iden- 

 tical staining reaction that is, they either do not stain at all or, after 

 long application of the dye, stain uniformly red. 



THE SARCOLEMMA. 



The muscle-fiber is enveloped by an extremely delicate sarcolemma 

 (figs. IB, 2A, 4, 5). There can be no doubt regarding its actual 

 presence. Its demonstration is generally difficult in ordinary prepa- 

 rations, due to the very close investure of the endomysium, which 

 is condensed immediately about the fiber. It is only after the study 

 of differentially stained preparations that the distinction between sar- 

 colemma and endomysium becomes marked; after that it can be iden- 

 tified as the innermost layer of the enveloping connective tissue, occa- 

 sionally lifted away from both the fiber and the endomysium (fig. 4) . 



