INTERCALATED DISCS OF THE HEART OF BEEF 331 



the assumption of a fusion along the line of the terrace of originally separate 

 portions of the fetal syncj'-tium. The disc portions of the 'terraces' appear at 

 angles to the obUque line of fusion, in close association with the telophragma, 

 as the expression of a modification of the myofibrils by reason of new stresses 

 imposed incidental to a functional recoordination of myofibrils in the altered 

 trabeculae. The modification may be of the nature of an irreversible contrac- 

 tion-band, subsequently modified both mechanically and chemically, possibly 

 also by the accumulation of tissue fluid. 



6 Portion of a longitudinal section of a fiber of the right ventricle, showing 

 a disc overlying the end of the nucleus. The telophragmata are attached cen- 

 trally to the serrated nuclear membrane and peripherally to the festooned 

 sarcolemma. 



7 and 8 Transverse sections of ventricular trabeculae at the level of the 

 nucleus, showing the peripherally disposed intercalated discs. The myofibrils 

 are aggregated into peripheral lamellae and central more or less irregular cylin- 

 ders. Some of the lamellae show a peripheral longitudinal split, probably a 

 growth phenomenon. 



9 Portion of a longitudinal section of two adjacent trabeculae from the 

 right ventricle. Both fibers show a single branch demarked by an intercalated 

 disc. In the upper portion occur four discs of the simple band type. The upper 

 one on the left extends from the branch across the nucleus, into the main trabe- 

 cula, and into the lateral portion of the adjacent fiber. This disc can best be 

 interpreted according to the hypothesis which relates it to a modified contrac- 

 tion band. In the lower portion of the field on the fiber at the left occur three 

 discs at successive telophragmata levels. In the center where the two fibers 

 have come into close apposition, involving a fusion of the sarcolemmae in such 

 a manner as to produce an asymmetrical arrangement of the adjacent sarcomeres, 

 a long step-disc appears. The 'risers' here consist of the fused sarcolemmae. 



10 Two adjacent fibers with sarcolemmae fused medially. Several small 

 band-like discs appear at the upper and lower levels of the central fusion at 

 approximately right-angles to the line of fusion. The lower group of two discs 

 lies within a contracted area. At the upper and lower terminals of the central 

 fused area, the myofibrils have become modified to form simple discs, presum- 

 ably a consequence of the change in the direction of function of the involved 

 myofibrils. 



11 Semidiagrammatic drawing of two adjacent fibers whose sarcolemmae 

 have fused in such a manner as to produce an alternation of apposed telophrag- 

 mata, showing the arrangement of two band-discs with relation to the common 

 sarcolemmae an9 the telophragmata. The formation of the discs in such cases 

 probably proceeded the spiral twisting to which the fusion of the sarcolemmae 

 is due. 



12 Irregular type of band-disc. Two trabeculae, or branches, have appar- 

 ently fused end to end at an obtuse angle. The new functional requirements 

 on the part of the myofibrils in this region of fusion effected a modification which 

 resulted in this peculiar type of disc. The disc is readily interpretable in terms 

 of modified irreversible contraction-bands. 



