INTERCALATED DISCS OF THE HEART OF BEEF 315 



comeres of fetal muscle. Moreover, this disc may equally 

 plausibly be interpreted as a partial relaxation of adjacent halves 

 of two successive contraction bands. But the strongest counter- 

 vailing evidence to any interpretation in terms of sareomeric 

 differentiation are the facts above stated that trasitions between 

 alleged differentiating discs and definitive sarcomeres are lack- 

 ing, and that the discs are not abundantly present, nor at all in 

 definitive form, when the heart grows most actively during fetal 

 life, and that they do not disappear at the close of physiologic 

 maturity. Moreover, the discs show a progressive increase and 

 development from the sparse, delicate, simple types of early 

 fetal life to the abundant, more robust, and complex types and 

 modifications of the adult heart. 



When one wishes to argue for the tendinous nature of the 

 discs, he may point to the fact of the bulging of certain con- 

 tracted areas which are bounded at both ends by discs. But 

 again such appearances are relatively rare. Moreover, the 

 alleged tendons (discs) do not react to specific stains for collagen 

 fibrils, the discs frequently lie within contracted or relaxed 

 areas, and the irregular varieties have no close structural re- 

 semblance to tendons. The discs only fortuitously bound such 

 contracted bulging regions. 



Similarly with respect to Dietrich's coordination-mechanism 

 theory. The discs in general occupy the proximal regions of 

 trabecular branches, and might perhaps serve well to coordinate 

 the functional activity of the included myofibrils; but no evi- 

 dence accrues that such is actually the case. Our evidence 

 indicates rather that an attempt at coordination (or recoordina- 

 tion) is the cause of disc formation; not that the discs effect the 

 coordination as Dietrich (3) believes. 



In attempting an interpretation of the intercalated discs all 

 the available evidence must of course be included. The correct 

 interpretation must be able to comprehend in logical form all 

 modifications of type and relationship of the discs in fetal, nor- 

 mal adult and pathologic hearts. We incline to believe that the 

 interpretation of the discs as secondary modifications of the 

 myofibrils at certain areas characterized by unusual functional 



