The mass of muscle in the midline of the abdomen by which the spine (postanal telson) is moved consists of muscle-fibers varying greatly in diameter (fig. 1) and held together by a small amount of delicate wide-meshed connective tissue. The nuclei are scattered apparently promiscuously through the diameter of the fiber; as many as five (fig. 1), or even more, may be seen at approximately the same level in a cross-section of a large fiber. A narrow peripheral sarco plasmic layer free of myofibrillar material envelops the muscle-fiber; occasionally nuclei occupy this location, even causing the confining sarcolemma to bulge slightly.
Under a magnification of 1,000 diameters, or even less, the fiber in cross-section is seen to have a radially striped appearance shading centrally into a mottled arrangement. Under higher magnification this appearance resolves into broad lamella, some apparently extend ing completely through the entire radius of the fiber. Many of the lamella appear split peripherally, producing a deep v- or v-shaped structure. This represents a radial longitudinal splitting of the lam ellar myofibril bundle. Centrally these lamella) appear to split off smaller lamella and cylindric bundles of myofibrils. Between these lamellte lies the delicate finely granular sarcoplasm (fig. 1n).
Of special interest is the similarity between this adult condition in and an early ontogenetic stage in striped muscle of teleosts.
The structure here described corresponds very closely to that de scribed in the histogenesis of the striped muscle of the trout. It is desirable to know whether these muscle-lamelhe in Limulue also trace their origin to a single extranuclear fibril, as is the case in the cypri noid fishes (Maurer, 1894) and in the trout (Heidenhain, 1913), a matter which is reserved for a future investigation.
Under closer examination these radial lamellae and central straps and cylinders resolve into fibrils. The unit of structure, then, is the delicate myofibril variously associated into cylinders and lamellae. These latter are apparently derived from the earlier larger lamellae by process of splitting in two directions: (1) a radial direction, the split ting being initiated peripherally; (2) a vertical (paratangential) direc tion, which is chiefly confined to the central area of the fiber.
The limuli from which this tissue was derived were of medium size, i. e., about half-grown. Probably the muscles enlarge by an increase in size of the smaller fibers by the process above described. In very thin longitudinal sections viewed under very high magnification this process of splitting appears to extend to the myofibrils themselves, the assumed units of structure. Indeed, this longitudinal splitting of myo
fibrils continues to the limit of visibility, and its variously consummated condition gives to the whole a syncytial structure even to its ultimate fibrils (fig. 2B). The impression becomes increasingly strong, as one improves the amplification and the acuity of observation, that the actual ultimate fibrils out of which the visible fibrillae are formed are ultramicroscopic units—a confirmation thus far of Heidenhain's "Teil korper theorie" ("histomere" or "protomere" theory), elaborated on the basis of his study of the development of trout skeletal muscle. .
Viewed in longitudinal section, different fibers have a very different appearance, depending upon the functional phase of contraction or relaxation. In the uncontracted condition (fig. 2B) the Q-disk is con spicuous. Separating two successive Q-disks is an area of slightly greater width, very much less deeply stained, the J-disk ; this is bisected by a deeper-staining but still relatively pale granular membrane, the telophragma (Krause's ground- or Z-membrane, 1869). Only the latter spans the intervals between adjacent fibrils; it is therefore a true mem brane, as first recognized by MacCallum (1887) for heart-muscle, and as consistently urged by Heidenhain since 1899. The Q and J disks are regions of differentiation confined to the fibrils. Passing obliquely across the spaces, between adjacent myofibrils, are still more delicate fibrillae, the ultimate visible units. At the point where the ground membrane and the myofibrils meet the latter appear to swell, giving to the membrane a deep-staining granular appearance; here the ulti mate visible fibrils are apparently more closely associated into bundles, the so-called muscle-unit or sarcostyle. There is not the slightest indi cation in any fiber at any phase of function of any structure suggesting a mesophragma (membrane of Merkel, 1872; membrane of Heiden hain ; M-membisne). Such a structure must be lacking in Limultut 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.