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



but scattered; in the myocardial trabeculae they are relatively 

 larger and elongated elements (fig. 49). 



Figm*e 51 illustrates conditions at the level of transition be- 

 tween the Pur kin je fibers and the myocardium of the moderator 

 band. The three cells shown are histologically of the Purkinje 

 type, and are in process of fusion to form a fiber. Various short 

 discs occur all along the surfaces of fusion. The connecting 

 membrane represents the fused sarcolcmmae of the adjacent 

 fibers. The discs appear to have arisen peripherally in con- 

 nection wdth these areas of fusion. These are, however, not in the 

 line of fusion, but at various angles to it, and in connection with 

 the teloplii'agmata. The appearance is such as to suggest a pene- 

 tration of intercellular fluid along some of the telophragmata 

 peripherally; the teloplii-agmata may furnish more favorable 

 channels for the capillary imbibition of such fluids; the fluids 

 might conceivably alter the myofibrils in the close vicinity of 

 these telophragmata into the disc-structure. 



The above interpretation of the discs in terms of a local chemi- 

 cal modification of the myofibrils by tissue fluid, which at first 

 consideration seems plausible, is stated simply for purposes of 

 sharper contrast "with the interpretation which seems to us, in 

 the light of more inclusive e\adence, to be the correct one; namely, 

 that the intercalated discs, many of which undoubtedly arise in 

 connection Tvith surfaces of fusion, are the products of modifica- 

 tions, of the natm-e of irreversible contraction bands on the 

 peripheral myofibrils, resulting from unusual strains upon the 

 fiber at the points of fusion incident to a rearrangement and new 

 coordination of the peripheral fibrils in accord with the new 

 stresses imposed by the fusion of distinct cells into a unit fiber. 

 This point ^\dll be further discussed below. 



/. The fetal myocardium 



This order of description follows the actual order of the in- 

 vestigation. It might at first seem a more logical procedure to 

 have begun the description with the younger fetal material and 

 then to have passed from that through later fetal and early 



