The Microscopic Structure of Striped Muscle of Limulus. 287 



The original myoblasts are lost in a syncytium, in the skeletal muscle 

 appearing as multi-nucleated distinct fibers, in the cardiac muscle as 

 a loose-meshed intricate network of larger and smaller trabeculse. The 

 muscle-substance here consists of a finely granular sarcoplasm (with 

 large lipoid spherules in the vicinity of the nucleus), throughout which 

 are scattered the myofibrils and the nuclei. The relationship of ground 

 membrane to nuclear wall and to sarcolemma, as also the result of the 

 abundant nuclear amitosis in cardiac muscle, would hardly be intelli- 

 gible on any other ground. In view of the evidence derived from the 

 study of the muscle from the adult mouse (Jordan), and the data 

 recorded by Asai (3) on the histogenesis of this same heart-muscle, the 

 cellular hypothesis of muscle structure must be definitely abandoned. 



If additional evidence were required in refutation of the interpreta- 

 tion of interfibrillar muscle-cells in striped muscle, it may be drawn from 

 the structure of the striped muscles of tunicates (e. g., tail-muscles of 

 Amaro&cium) ; here the "fiber" is a mononucleated elongate hexagonal 

 cell, with its nucleus centrally and the myofibrillse peripherally disposed. 



The observations here recorded with respect to the fibrillar structure 

 of Limulus muscle, namely, the resolution of the muscular substance 

 into finer and finer complexes to the limit of visibility, strongly sup- 

 port the protomere hypothesis of Heidenhain (5), which maintains that 

 all living matter is divisible into progressively smaller specific units, 

 the ultimate vital molecular units being the "histomeres," "proto- 

 meres," or "metafibrillse." 



SUMMARY. 



1. Both the skeletal and the cardiac muscles of Limulus consist of 

 trabeculse of finely granular sarcoplasm, holding regularly aggregated 

 collections of myofibrillse, and confined by a cell-membrane or sarco- 

 lemma; throughout the trabeculse are scattered irregularly the numer- 

 ous nuclei. 



2. In cardiac muscle the main trabeculse and their branches form a 

 loose-meshed syncytium. 



3. Neither type of muscle contains mesophragmata. 



4. Very rarely an intercalated disk of the simple-comb type appears 

 in the cardiac muscle. 



5. Both types are very similar in respect of the presence and arrange- 

 ment, in the same phase of contraction, of Q and J disks, and the 

 telophragmata. 



6. The telophragmata are continuous membranes closely attached 

 centrally to the nuclear wall, which is frequently drawn out into pro- 

 jections at the points of attachment, and peripherally to the sarco- 

 lemma. 



7. The sarcolemma is a very delicate membrane, closely associated 

 with the enveloping endomysium, but reacting differently to specific 

 connective-tissue stains. 



