The Microscopic Structure of Striped Muscle of Limulus. 277 



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-lamellse in Limulus 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. €., 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 / 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 



