THE MICROSCOPIC STRUCTURE OF STRIPED MUSCLE OF 



LIMULUS. 



BY EL E. JORDAN. 



INTRODUCTION. 



The study of the skeletal muscle of Limulus was undertaken with 

 two chief objects in view: 



(1) To attempt further to test my conclusion suggested by a series 

 of earlier studies on the intercalated disks of vertebrate cardiac muscle, 

 namely, that these disks are properly interpreted as "irreversible con- 

 traction bands." If this conception is correct it seems probable that 

 something very similar to intercalated disks may be found in power- 

 fully acting skeletal muscle. Such structures have been repeatedly 

 reported in vertebrate skeletal muscle, but the similarity to interca- 

 lated disks is still disputed. It must be emphasized, however, that 

 even certain evidence of the presence of true intercalated disks in 

 skeletal muscle is not crucial for the theory, but only confirmatory. 

 Lack of such evidence can not invalidate the theory as applied to 

 heart-muscle, nor does its possession necessarily prove it correct. 



(2) To seek additional evidence in further refutation of the recently 

 revived hypothesis that striped muscle can be interpreted in terms of 

 "muscle-cells" and intercellular myofibrillse. 



A priori, the abdominal muscles which control the caudal spine of 

 Limulus seemed to offer very favorable material for the search of the 

 chief data desired. This surmise proved true beyond expectation as 

 respects the second point. Moreover, the material yields very clear- 

 cut data also as to a number of other disputed details concerning 

 muscle structure, chiefly the relation of the ground membrane to the 

 sarcolemma, to the nuclear wail, and to the myofibrillae. 



Concerning the first point no unequivocal evidence accrues. How- 

 ever, this study led to a reinvestigation of Limulus cardiac muscle with 

 a different staining technic, and I can now report the presence of a very 

 few, structurally of the simpler type, of intercalated disks. This obser- 

 vation would seem to warrant the generalization that cardiac muscle 

 is characterized, at least as low as Limulus, by intercalated disks. 

 The evidence concerning selachii and cyclostomes only remain uncer- 

 tain. On the other hand, the evidence may perhaps be regarded as 

 confirming the belief of some zoologists that Limulus is closely related 

 to the more direct vertebrate ancestors. But similarity of histologic 

 structure need not necessarily have phylogenetic significance; it may 



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