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



trabeculae branch. Terraced discs arise in at least two different 

 ways: (1) As dislocations of original band-forms following func- 

 tional or developmental stresses; (2) as concomitants of a fusion 

 along an oblique surface of the two originally discrete portions 

 of the myocardial plexus. The methods of the original formation 

 of the several types of terraced discs will be further described 

 and discussed below. 



Figure 3 illustrates the opposite surfaces of the same fiber. 

 At the upper level of focus (a) the disc appears of the usual 

 simple band-form, composed of modified portions of the in- 

 volved myofibrils, in series with the telophragmata. There is 

 no evidence that this disc is. bounded on either side by a telo- 

 phragma. In passing to the opposite surface the disc appears 

 distorted, as if by opposed stresses, in such a manner as to form a 

 two-step disc. A common telophragma bounds the lower border 

 of the left segment and the upper border of the right segment. 



In figure 4 is shown a similarly dislocated disc, the two seg- 

 ments having been moved somewhat farther apart, and having 

 remained connected by a deeply-staining membrane, probably 

 portion of a telophragma. 



Another complex type of disc is illustrated in figure 5. The 

 band elements shade into the telophragmata. The steps are in- 

 terconnected by membranes. The different levels of location 

 of the several portions are indicated by numerals. The disc as a 

 whole has an interrupted spiral form, and bounds a wedge-shaped 

 lighter-staining area at the left. 



The occasional super-nuclear position of the disc is illustratred 

 in figures 6, 7, 8 and 31. Figure 6 shows also the close union of 

 the telophragmata with the sarcolemma and the nuclear wall. 



Figure 9 illustrates clearly one manner of the formation of ter- 

 raced discs. Here two originally discrete muscular trabeculae 

 have fused. The 'risers' or connecting membranes of this com- 

 plex disc have resulted from the fusion of the apposed sarco- 

 lemmae. An irregularly terraced disc resulted in consequence, 

 the several segments having been contributed in part by one, in 

 part by the other fiber. Since the fusion was such as to produce 

 disaccordauce of the apposed sarcomeres, the discs became ar- 



