INTERCALATED DISCS OF THE HEART OF BEEF 313 



very difficult to understand their originally sharp segregation 

 along the telophragmata. 



Study of the two-month fetal heart throws further light also 

 on the nature of the cells of the atrioventricular bundle and of 

 the Purkinje fibers. In this heart the resemblance between 

 these elements and the fusiform cells of the myocardium is much 

 closer than in later fetal hearts. The element in each case is a 

 fusiform cell. The cell of the atrioventricular bundle is short and 

 very stout, the cells of the Purkinje fibers are longer and less 

 stout, that of the myocardium is still longer and relatively slender. 

 Moreover each type multiplies its nuclei by amitotic division. 

 The atrioventricular bundle cell more generally has only two 

 nuclei; and an occasional nucleus may be seen in amitotic division. 



It is quite true that even at the two-month stage the atrio- 

 ventricular bundle can be clearly recognized. But this fact is no 

 proof that these cells and the Purkinje fibers are not actually 

 less differentiated myocardial fibers. It seems probable, in 

 view of the evidence from the two-month fetal heart, that origi- 

 nally the three types came from a similar tissue or syncytium. 

 The atrioventricular bundle cells differentiate only to a certain 

 early stage. This stage is characterized by a stout fusiform 

 shape, much sarcoplasm, few fibrils slightly differentiated and 

 peripherally arranged. The cells, moreover, frequently have 

 only a single large central nucleus. The nucleus occasionally 

 divides by mitosis, but more generally at this stage by amitosis, 

 to produce a binucleated cell. The cells remain distinct, but 

 are closely united by intercellular bridges and continuous myo- 

 fibrils. The Purkinje fibers progress to a somewhat later stage 

 characterized by an elongated fusiform shape, amitotic division 

 of nucleus, and a fusion to form fibers, the fusion involving the 

 formation of discs. The myocardium passes through very simi- 

 lar earlier stages, but progresses along the same lines to a 

 higher degi*ee of differentiation. From this viewpoint it is quite 

 correct to speak of the Purkinje fibers and the atrioventricular 

 bundle cells as less highly differentiated myocardial elements. 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 22, NO. 2 



