INTERCALATED DISCS OF THE HEART OF BEEF 305 



fuscly for a short distance the peripheral ends of the telophrag- 

 niata (figs. 36 and 37). Between adjacent cells are larger and 

 smaller intercellular spaces (figs. 36 and 50) ; through the inter- 

 vening 'intercellular bridges' pass the myofibrils. Figure 43 

 shows the various shapes of the cells in cross-section. That the 

 above interpretation of the serrations of the cell-borders is cor- 

 rect is further demonstrated by the appearance of the cells in 

 macerated preparations. Here the cells have a sharp contour 

 (fig. 44) . The nuclei appear homogeneous, the cytoplasm finely 

 granular; the myofibrils are indistinctly visible and very irregu- 

 larly distributed. 



In figure 42 is illustrated a peculiar condition where an arteri- 

 ole appears to lie within the cell. This definitive condition is 

 probably the result of a secondary adaptation of the cell to the 

 growing blood vessel. 



The various histologic conditions above described for the 

 cells of the atrioventricular bundle indicate a relatively slight 

 differentiation, or an embryonic condition. Such interpretation 

 has frequently been given to the cells and their slightly modified 

 forms, the Purkinje fibers. But that they actually represent 

 embryonic forms of myocardial fibers, that is, that they are simi- 

 lar to the elements from which the myocardium develops has 

 been disputed by certain investigators, e.g., Moenckeberg^ who 

 points to the fact that in the human embryo they are already 

 clearly differentiated froin the myocardium at the fifth fetal 

 month. In the beef heart they can be readily distinguished 

 already at the end of the second month. But the very close 

 structural correspondence between the cells of the atrioventricu- 

 lar bundle and the myocardium in the two-month fetal heart of 

 the beef very strongly suggests the interpretation of the three 

 elements in terms essentially of a difference in degree of pro- 

 gressive differentiation. This point will be further discussed 

 below. 



The presence of myofibrils and especially of telophragmata in 

 the cells of the atrioventricular bundle characterizes them as 



- Cited from Lange (16). 



