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



lemma may become inturned (Heidenhain (4) ) and form a con- 

 necting membrane between the inturned portions of- peripheral 

 discs. These discs are not formed as the result of fusions in 

 the manner of those previously described, but are simply morpho- 

 logical modifications of discs already present by reason of a 

 secondary t\visting and fusion. 



From the above it seems clear that all varieties of discs can be 

 explained in terms of a band-disc connected with a telophragma, 

 as secondary modifications incident to the various tractions and 

 tensions acting upon adjacent groups of myofibrils, or even adja- 

 cent single fibrils. The latter condition would result in the 

 more delicate serrated types, which in the case of growing or 

 hypertrophying fibers would involve also the telophragmata 

 included among the splitting fibrils (figs. 24 to 27). The pres- 

 ence or absence of a delicate connecting membrane between the 

 segments of a step-disc in the former condition might depend 

 upon whether the elasticity of the telophragmata was sufficient 

 in any given instance to withstand the strain- ol extension to 

 the distance of one or several sarcomeres. 



We may now return again to a consideration of the initial 

 stage in the formation of discs. We appreciate the fact that 

 the weakest link in the chain of argument in support of the inter- 

 pretation of the original discs as modifications of contraction 

 bands is the explanation of their inception. But the histo- 

 genetic data also seem to point to such an interpretation. 

 When once formed in their simplest condition, all the various 

 types of more complex discs can be readily explained by our 

 hypothesis. An explanation is not equally easy on the basis of 

 any other hypothesis previously proposed. The facts that the 

 discs make their first appearance while the heart is actively 

 growing (about the end of the second month in the beef) and 

 persist thereafter in coarsened and modified forms, and that they 

 are at first invariably peripheral in position, have also a special 

 bearing in this connection. In the growth of the fiber, myo- 

 fibrils are being constantly added centrally by process of splitting 

 from the more peripheral older fibrils of the radial lamellae. The 

 more peripheral fibrils are first formed and are consequently the 



