276 Papers from the Department of Marine Biology. 



mean nothing more than the result of physiologic similarity. Ob- 

 viously much comparative microscopic investigation of cardiac muscles 

 is desirable below teleosts. 



Other points of considerable interest touch the very close funda- 

 mental structural similarity between the spine-muscle and the heart- 

 muscle of Limulus, and between the spine-muscle and an early stage 

 of the development of striped muscle in teleosts. 



MATERIAL AND METHODS. 



As stated above, the material consists of spine and cardiac muscle. 

 The former was collected during the summer of 1914, while engaged 

 in the study of another problem at the Marine Biological Laboratory 

 of the Carnegie Institution of Washington at the Dry Tortugas; the 

 latter was collected at the Cold Spring Harbor Laboratory on Long 

 Island Sound. 



The cardiac muscle was fixed in Zimmermann's absolute alcohol- 

 nitric acid solution, and stained with iron-hematoxylin and picric acid- 

 fuchsin. The abdominal (spine) muscle was fixed both in Helly's fluid 

 and in Flemming's strong solution, and also counterstained with the 

 picric acid-fuchsin mixture. 



SKELETAL MUSCLE. 

 GENERAL STRUCTURE. 



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 

 lamellae appear split peripherally, producing a deep V- or Y-shaped 

 structure. This represents a radial longitudinal splitting of the lam- 

 ellar myofibril bundle. Centrally these lamellae appear to split off 

 smaller lamellae and cylindric bundles of myofibrils. Between these 

 lamellse lies the delicate finely granular sarcoplasm (fig. IB). 



Of special interest is the similarity between this adult condition in 

 Limulus and an early ontogenetic stage in striped muscle of teleosts. 



