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



eahited discs, perhaps in essence irreversible contraction bands. 

 Such originally modified areas may become secondarily further 

 modified through the influence of, or by the addition of, rela- 

 tively more abundant tissue fluid upon which the reactions to 

 silver nitrate depend. It should be emphasized that the telo- 

 phragmata react similarly to silver nitrate, which indicates that 

 the telophragmata are the more direct paths for the penetration 

 of the tissue fluid, which fact further explains the presence of 

 tissue fluid in the discs because of the intimate relationship to 

 telophragmata. In adult ventricular tissue tested with silver 

 nitrate, the latter is precipitated in the spaces between adjacent 

 fibers, in the telophragma, and in the discs. The close union of 

 the telophragma with the sarcolemma gives the mechanical ex- 

 planation of the penetration of tissue fluid from exterior towards 

 center via telophragmata. 



In the two-month fetal heart we certainly come very close to the 

 beginning of the discs. The embryonic heart is a syncytium, 

 composed of anastomosing stellate and irregular cells. It is only 

 when these have become altered into fusiform elements, and the 

 latter begin to fuse to form the beginning of the secondary mesh- 

 work of myocardial syncytium (fig. 38) that the first discs appear. 

 This probably occurs somewhere early in the second month, and 

 the actual beginning is hardly very different from the one here 

 described for the two-month heart. 



The two-month heart shows, then, conclusively that the discs 

 arise in connection with cellular fusions, as modifications of the 

 myofibrils in lines corresponding to telophragmata, and ap- 

 proximately at right angles to surfaces of fusion. The evidence 

 is not incompatible with our general interpretation in terms of 

 contraction bands, more especially when the Limulus and 

 lower-vertebrate hearts are kept in mind, but it furnishes no 

 additional support to the hypothesis and it cannot be finally 

 denied that the myofibril modification might possibly be of the 

 nature of a splicing (for purposes of recoordination) of myo- 

 fibrils of fusing adjacent cells. If end to end sphcing of myo- 

 fibrils were the complete explanation of the discs, however, it is 



