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



complex types of discs originate from the simpler band-discs. 

 The first types in the order of simplicity are the terraced forms. 

 The same explanation that applies to a two-step form will apply 

 also to multiterraced forms. Moreover, the explanation must 

 hold as well for a terraced type in which the steps are only of one 

 order (descending or ascending) as also for those in which the 

 steps are of a double or compound order (descending combined 

 with ascending). But as we shall see the same explanation need 

 not apply also to the irregular terraced types. 



Obviously one of two explanations might apply to the terraced 

 types of regular order: (1) They might have resulted from a dis- 

 location of an original band form; or (2) they might have re- 

 sulted from the close allocation of originally disconnected short 

 bands. The fact that the interval between successive steps may 

 be one or several sarcomeric segments need not affect this con- 

 clusion. In the first case specified, connecting membranes or 

 'risers' would not be expected. Such step-forms appear abun- 

 dantly. It should be noted also that generally in the case of 

 step-forms the involved myofibrils are divided into bundles 

 corresponding in width with the width of the step-segments 

 (figs. 2, 13, 19 and 26). In the second case specified, the con- 

 necting membranes might conceivably be either a portion of the 

 sarcolemma or a portion of a telophragma. The possibility of 

 a contributory mesophragma need not be considered, since no 

 evidence appears that such an alleged membrane (Heidenhain 

 (4) ) actually occurs in the cardiac muscle of the beef. Where 

 a secondary spiral twisting of the fiber is superimposed, the con- 

 necting membrane may very likely be an inturned portion of the 

 sarcolemma. In the absence of a spiral twisting, in which case 

 the membranes ('risers') are relatively dehcate, the connections 

 may be formed by portions of an involved telophragma. But 

 as we saw from a study of the first origin of discs in the fetal 

 heart, step-discs may arise in relation to oblique surfaces of 

 fusion, and in such instances the connecting membranes are also 

 portions of the fused sarcolemmae of adjacent fibers. Whether 

 the fasciculation of the trabeculae above mentioned in connec- 

 tion with terraced discs lacking connecting membranes is sec- 



