326 H. E. JORDAN AND J. B. BANKS 



In the first type connecting membranes ('risers ') are generally 

 lacking; when present they are very delicate and probably rep- 

 resent portions of an involved telophragma. In the last two 

 types connecting membranes are more robust and stain more 

 deeply, and represent fused portions of the involved sarcolemmae. 

 The serrated types of disc result from unequal functional ten- 

 sions upon the units of an original band disc in a region where 

 the myofibrils are undergoing a longitudinal fission in the process 

 of growth of the fibers. 



7. Evidence is presented in contravention of the pre\dously 

 proposed hjqDotheses concerning the significance of the discs, 

 namely as intercellular cement-substances (Zimmerman, et al.), 

 as tendinous structures (Marceau), as developing sarcomeres 

 (Heidenhain), and as originally coordination mechanisms for the 

 myofibrils (Dietrich) . 



8. Histologic data are presented in further support of the 

 hypothesis proposed by Jordan and Steele that the simplest 

 types of discs are local modifications of adjacent myofibrils at 

 the level of a telophragma (possibly of the nature of strain effects 

 producing a condition of irreversibility ot contraction bands), 

 and that the more complex types are secondary mechanical 

 modifications of the simpler discs. Additional evidence indi- 

 cates further that the discs are incidentally modified through the 

 infiltration of intercellular tissue fluid along the telophragmata, 

 which accounts for the precipitation of silver nitrate in these 

 regions. A concomitant chemical modification would account 

 for the relatively greater ease with which the myocardium 

 fragments in macerating fluids in the regions of the discs. 



9. The atrioventricular bundle can be distinguished from the 

 myocardium already at the second month. It is composed of 

 short, stout, fusiform or polyhedral cells containing scattered 

 myofibrillar elements continuous from cell to cell. The cells 

 commonly contain two centrally located nuclei, closely apposed, 

 the amitotic division products of an originally single nucleus. 

 The bundle terminates distally in Pur kin je fibers, which connect 

 with the myocardium of the ventricles and of the moderator 

 band. The Purkinje fibers are elongated elements similar to 



