The Microscopic Structure of Striped Muscle of Limulus. 283 



batS; from which he records observations which he interprets as indi- 

 cating the absence of both the meso- and the telo-phragmata, the so- 

 called Z-stripe being the only striation discernible and to be interpreted 

 as a contraction band; (5) Heidenhain's work on the histogenesis of 

 striped muscle in the trout, on the basis of which he further supports 

 and extends to striped-muscle tissue his general histologic principle of 

 protomeric analysis, namely, the conception that muscle- tissue is built 

 up by the association into successively larger combinations of ultimate 

 fission elements, the metafibrillse or protomeres; (e) Jordan and Steele's 

 (1912) comparative study of the intercalated disks in vertebrate heart- 

 muscle, from which it appears that intercalated disks are present in 

 progressively simpler forms in all the vertebrate groups to and includ- 

 ing teleost fishes; and (7) my description (1912) of the intercalated disks 

 of hypertrophied human heart-muscle. 



DISCUSSION. 



The value of the data derived from the study of the Limulus muscle 

 depends in degree upon the extent to which they may legitimately 

 serve as a basis for generalization with respect to vertebrate muscle. 

 In the relative simplicity of its striations the Limulus muscle seems 

 more like that of vertebrates than like that of arthropods. Moreover, 

 the fundamental close similarity between the cardiac and the skeletal 

 type in Limulus is significant, especially as indicating that a main 

 difference between cardiac and skeletal muscle generally is essentially 

 a degree of differentiation, minute morphologic differences following 

 functional differences probably largely inherent in the syncytial arrange- 

 ment of the fibers in the heart-musculature. In both cases, as in verte- 

 brate heart-muscle generally, the conspicuous stripe is the Z-membrane. 

 Accordingly the skeletal muscle of Liviulus is apparently less highly 

 differentiated than vertebrate skeletal muscle, where the Q-disk gives 

 the most conspicuous stripe, a conclusion further supported by the 

 manner in which the nuclei are distributed. In its syncytial character 

 and the presence of intercalated disks the Limulus cardiac muscle 

 resembles very closely vertebrate cardiac muscle, a point already 

 emphasized by Carlson and by Meek (s). The infrequency of the 

 intercalated disks precludes an interpretation of these structures in 

 terms of cell boundaries or intercellular substances, or as regions of 

 growth (Heidenhain (12)). The disks consist of rows of modified foci 

 in the fibrillse. The modification consists of a change characterized 

 tinctorially by an increase in staining intensity for a short distance on 

 one or both sides of the telophragma. Structurally the disk is com- 

 posed of a series of rod-like areas of the fibrillse in transverse alinement. 

 The most probable explanation of their formation is that of a change 

 of position of the Q (anisotropic) substance from its usual location in 

 relaxed fibers midway between two successive telophragmata, to a 



