INTERCALATED DISCS OF THE HEART OF BEEF 323 



bisected by a telophragma) is comparable, structurally, tinc- 

 torially, and in respect of relation to telophragma, to a con- 

 tracted portion of a myofibril (see e.g., figure 6, illustrating 

 contracted leg muscle of sea-spider; Jordan (12) ), an explana- 

 tion of intercalated discs is suggested by the microscopic evi- 

 dence in terms of a modified contraction band, possibly an 

 irreversible band. 



But this conclusion must be brought into harmony with the 

 fact of the formation of discs in the Pur kin je fibers and in the 

 early fetal hearts, in relation to surfaces of fusion among adja- 

 cent cells and trabeculae. The hypothesis that the interca- 

 lated discs of heart muscle are of the nature of irreversible con- 

 traction bands must be able to include and harmonize the evi- 

 dence that in the PurkiAje fibers and the fusiform elements of 

 the fetal heart the discs arise in relation to fusion-areas between 

 elongating cells like those of the atrioventricular bundle. If it 

 cannot do this it must be abandoned. It will be observed that 

 the discs do not generally arise in the areas of fusion but at 

 right angles (approximately) to such fusion areas. Where two 

 fibers fuse along oblique surfaces, the peripheral myofibrils at 

 least, must be brought into coordinated functional relationship. 

 This conceivably involves special stresses and strains at the 

 point or levels of recoordination. Since at these earlier stages 

 when lusions are most extensively made, the myofibrils are rela- 

 tively less abundant while peripherally arranged and in union 

 with the telophragmata, and since only the most peripheral 

 fibrils are probably involved in the new coordination, the periph- 

 eral location of the discs and their spatial relationship to the 

 telophragmata is accounted for. 



The above discussion would seem to bring into harmony with 

 the newer hypothesis here accepted of the significance of the 

 intercalated discs, two other hypotheses, namely the inter- 

 cellular and the coordination-mechanism hypotheses. The in- 

 tercalated discs originate along original intercellular surfaces 

 but not generally in such surfaces; they accordingly in part 

 outline more or less accurately original intercellular or inter-fiber 

 region^. The discs may be conceived as the result of attempts 



