INTERCALATED DISCS OF THE HEART OF BEEF 299 



fibrils here to be altered on either one or both sides, or (perhaps 

 only secondarily) between successive telophragmata. 



The peculiar disc illustrated in figure 17 is of the same nature 

 as those in figure 16. Here, however, a central segment appears 

 dislocated, the three segments remaining interconnected by a 

 deeply-staining membrane, probably the telophragma about 

 which the original band disc originated. 



It is significant that in the common form of terraced disc, the 

 lateral surfaces of successive segments are in myofibril series, that 

 is, the several segments do not overlap. This phenomenon is 

 illustrated in the semidiagrammatic illustration, figure 19. The 

 relatively wider space between successive disc-bundles further 

 strongly indicates that the terraced condition arose from a dis- 

 location of an original band-disc at the higher level. Such in- 

 terpretation involves the assumption of a realinement and fusion 

 of the disconnected telophragmata, for which appearances illus- 

 trated in figure 13 give some basis of fact. Moreover, in the 

 less highly differentiated condition of the trabeculae in the 

 younger heart, when such dislocations more probably occur, the 

 telophragmata are relatively less rigid and less firmly attached 

 to sarcolemma and nuclei (fig. 15). 



Figure 20 includes discs at various stages of development, pre- 

 sumably from contraction bands. The uppermost one shades 

 laterally into a telophi-agma; this disc is practically a contrac- 

 tion band, and in so far as contraction involves a segregation 

 of Q-substances about a telophragma the disc is in part of 

 anisotropic nature (Jordan, (5) ). Similarly the two discs next 

 below are bands, in width more like the unmodified regular con- 

 traction bands of the muscle (figs. 2 and 30), The diminution 

 in width laterally to the character of the telophragma, illus- 

 trated also in figures 5, 13 and 14, corresponds with the condition 

 of the discs as seen in transverse section (figs. 7, 8 and 25). The 

 succeeding three series of discs represent modifications of con- 

 traction bands in that they are placed to one side of their re- 

 spective telophragmata, perhaps irreversible halves of contrac- 

 tion bands. The wide, short disc to the left is again of the modi- 



