The Microscopic Structure of Striped Muscle of Limulus. 285 



the Z-membrane to the disk, as also the several occasional variations 

 when the membrane forms either one or both boundaries. The Z- 

 membrane forms also the mid-portion of the contraction band. As 

 an " irreversible contraction band," an intercalated disk also is there- 

 fore usually bisected by the Z-membrane. It may conceivably occa- 

 sionally happen that only half of the contraction band may be able to 

 relax, whereas the other half may become converted into an inter- 

 calated disk. Such a disk would then be bounded only on one side 

 by a ground membrane. If these same conditions should prevail in 

 two successive contraction bands, involving the adjacent halves of the 

 bands, then the resulting disk would be bounded on either side by a 

 Z-membrane. Such disks would be rare, the prevailing type being 

 bisected, or bounded on only one side, by the membrane. 



The presence of a sarcolemma distinct from the endomysium is 

 proved by the difference in staining reaction of the more delicate 

 inner and coarser outer layers of the closely enveloping connective 

 tissue. The outer reacts to specific stains for connective tissue; the 

 inner does not so react, at least not to the same degree, but always to 

 the same degree to which react also the telophragmata with which this 

 inner layer is in close structural connection. This inner layer is a true 

 sarcolemma. That it represents the original cell-membrane seems 

 proved by the fact that it is connected with the nuclear wall through 

 the telophragmata. The close association of sarcolemma and endomy- 

 sium must be emphasized; but this is exactly the same as obtains in 

 the case of the skeletal muscle, and exactly the same structural condi- 

 tions obtain. In neither case can there be any doubt about the presence 

 of a sarcolemma. The fact that one is present in Limulus cardiac muscle 

 should go far to remove further skepticism regarding its actual presence 

 in vertebrate heart-muscle. 



Neither in the cardiac nor the abdominal-spine muscle of Limulus 

 is there any indication of a mesophragma. Thulin records the absence 

 also of a mesophragma, and even of a telophragma, in the wing-muscles 

 of certain insects, birds, and bats. The meaning of this structural 

 peculiarity in these muscles is obscure. Heidenhain (12) claims that a 

 mesophragma is present even in human heart-muscle. The occurrence 

 of such a membrane in cardiac muscle is disputed by many histologists. 

 When apparently lacking, Heidenhain believes that it has a thickness 

 of less than 0.2 micron, the limit of microscopic resolution, hence 

 indiscernible. It seems obvious that the question as to whether heart- 

 muscle anywhere actually contains mesophragmata demands reinvesti- 

 gation. If it were actually present and of the same nature as the telo- 

 phragma, and if it had the same relation to the myofibrils and to the 

 sarcolemma, as Heidenhain believes, then it should cause festoons in 

 the sarcolemma like those caused by the telophragma, which is not the 

 case. Furthermore, if present, it should produce a similar folding to 



