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



13 Complex terriiccd discs from right ventricle. Groups 1 and 2 come suc- 

 cessively into view as the focus is lowered, and are continuous around the left 

 margin. The lower group separates a contracted from a relaxed region; the 

 upper group lies in a relaxed area. The discs are peripherally placed, and form 

 portions of a spiral, possibly the combined result of a fusion of adjacent trabeculae 

 and a subsequent spiral twisting of the new fiber. A spiral twisting is indicated 

 also by the fusion of the telophragmata above. 



14 Portion of a longitudinal section of a myocardial trabecula of the right 

 ventricle showing an extensive group of band-like discs. The discs are periph- 

 erally located and a number can be seen to be continuous across the lateral 

 border as the level of focus is changed, thus revealing an annular or spiral form. 

 The numerals indicate successive levels of focus at which the discs appear. 

 The great number and considerable variety of discs in such a small area would 

 seem to exclude interpretation in terms of intercellular cement, tendinous 

 structures, or growth regions. 



15 Transverse and longitudinal sections of trabeculae of right ventricle of 

 four-month fetal heart (compare with figures 8 and 10). In the fetal heart of this 

 stage the trabeculae have a relatively lesser diameter; more vesicular and more 

 regular nuclei; fewer myofibrils, peripherally disposed; more widely-spaced, less 

 robust, and less regularly arranged telophragmata. 



16 Two adjacent fibers from the atrium, with typical band-discs bounded 

 on both sides by telophragmata. The structural units are clearly modified 

 portions of the myofibrils. 



17 Disc from atrium, composed of three portions interconnected by a 

 deeply-staining membrane, probably the result of an upward dislocation of a 

 central portion of the original band disc. 



18 Two apposed discs within the same sarcomere. They apparently repre- 

 sent apposed halves of successive contraction-bands, which failed to relax. 



19 Semidiagrammatic sketch illusttating the possible origin of the type 

 of terraced disc lacking coarser 'risers,' from an original band-disc by process 

 of division and dislocation of the resulting segments to successively lower levels 

 in a lateral direction; and a subsequent rearrangement of .the telophragmata at 

 regular intervals. 



20. Group of discs from atrial trabecula. The discs represent various 

 degrees and combinations of myofibril modification. 



21 Atrial disc bifurcating on the right to pass into the two successive telo- 

 phragmata. This is the only t^pe of disc which gives plausible basis for inter- 

 pretation as a region from which a sarcomere develops (Heidenhain), but it may 

 equally well be interpreted as a partial failure of reversion of apposed halves of 

 two successive contraction Ijands. 



