INTERCALATED Discs OF THE HEART OF BEEF 327 
the cells of the atrioventricular bundle. Occasional band and 
short step-like discs occur on these fibers. The transition from 
the cells of the atrioventricular bundle to the Purkinje 'cells' 
is characterized by an elongation and fusion of the cells to form 
true fibers (trabeculae), with intercalated discs. The interca- 
lated discs of the Purkinje fibers occur (arise) along the surfaces 
of oblique fusion of original cells (figs. 48 and 51). Such definite 
evidence of the origin of the discs in the Purkinje fibers gives the 
clue to their origin, in part at least, also in the general myo- 
cardium, namely in relation to surfaces of fusion of originally 
distinct elements (cells; trabeculae'). This can actually be dem- 
onstrated in the early fetal heart. The origin of the discs in 
regions of fusion between cells, approximately at right angles to 
the surfaces of fusion and in relation to telophragmata, is inter- 
preted in terms of a strain effect resulting in a local modifica- 
tion of adjacent myofibrillae (essentially an irreversible con- 
traction band in the initial condition) and incidental to a re- 
coordination of the peripheral myofibrils of the fusing cells or 
trabeculae. 
10. Intercalated discs occur in the fetal heart already towards 
the end of the second month as deeply-staining granular modifi- 
cations of certain telophragmata in their lateral extensions 
among the peripheral myofibrils. The myocardial elements are 
long, slender, fusiform cells in process of lateral and terminal 
fusion to form the trabeculae and branches of the later syncytial 
musculature. The discs are located at angles to surfaces of 
fusion. The resemblance between early fetal myocardial ele- 
ments and the Purkinje fibers of the adult heart is striking. The 
Purkinje fibers, as also the cells of the atrioventricular bundle 
of the fetal heart, are fusiform elements Whose myofibrillar con- 
stituents are associated with telophragmata and extend from 
cell to cell via intercellular bridges. The relation of the very 
similar intercalated discs of the Purkinje fibers of the adult 
heart and those of the fusiform elements of the early fetal heart 
to surfaces of fusion is the same in both cases. 
11. The new data disclosed in this investigation, namely, the 
origin of the intercalated discs in relation to surfaces of fusion of 
