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



The only other hypothesis that has any appearance of plausi- 

 bihty as suggested by certain conditions of the cells of the 

 atrioventricular bundle, the appearance of the early fetal myo- 

 cardium, and by adult myocardium treated with silver nitrate — 

 is that at certain levels, for some unknown cause, intercellular 

 tissue-fluids may penetrate via the telophragmata and modify 

 the myofibrils in these regions. Such an interpretation has not 

 to our knowledge been previously proposed, but it may at least 

 be stated. Once formed in this manner, the discs could again 

 be altered by the mechanical factors incident to development 

 and function as above explained. But a complete interpretation 

 on this basis would still demand an explanation of the original 

 causal factor upon which the locally increased penetration of 

 tissue-fluid depended. 



The cause of such localized (selected) relatively more per- 

 vious regions is obscure, unless on some basis requiring a previous 

 modification of the telophragmata concerned and as a concomi- 

 tant result of a modification of the attached portions of the in- 

 volved myofibrils. Such modification might again conceivably 

 be a result of a local unusual functional requirement, possibly 

 producing an excessive strain effect. The peculiar diffuse stain- 

 ing-reaction of the intercalated discs in general may be the 

 result of a relatively more profuse collection of intercellular 

 tissue fluid in the already modified portions of the myofibrils 

 represented by the discs. This is indicated more especially by 

 the appearance of cardiac tissue treated with silver nitrate: the 

 intercalated discs are not sharply outlined, but their margins 

 are vague and irregular, and the myofibrils appear masked by 

 the granular precipitate and show no resemblance to the definite 

 comb-discs of the hemahim-stained tissue. The relative in- 

 crease of tissue-fluid in the discs is more probably the result 

 than the fundamental cause of disc formation. 



The unit of the original disc is a modified focus of a myofibril 

 at the level of a telophragma. By transverse linear combina- 

 tions of such units, and subsequent mechanical modifications 

 all the types of discs may be readily conceived to be derived. 

 Since this initial unit (a bacillary portion of the myofibrils, 



