INTERCALATED DISCS OF THE HEART OF BEEF 321 
first to function; hence they support all the strains of contrac- 
tion and recoordination at the very time when rearrangement of 
fibers (trabeculae), fusions and twistings are most active, and 
before they are reinforced by more central fibrils. These struc- 
tural peculiarities of the trabeculae explain in a measure both the 
formation of the discs as possible irreversible contraction bands, 
and their peripheral location. As more centrally placed myo- 
fibrils develop in later cardiac histogenesis, they would be more 
likely to be modified in a similar manner at the levels of the 
earlier-formed more peripheral discs, and thus the discs would 
tend to grow coarser in a radial direction and wider in a trans- 
verse direction. But growth in radial width is probably more 
largely a matter of multiplication through fission of the disc- 
units of dividing myofibrils. The initial simple discs apparently 
arise both in relation to surfaces of fusion and independently of 
fusions. The common factor in the production of these original 
discs is presumably a strain effect upon localized portions of 
myofibrils causing an irreversible condition of a contraction 
band. 
Finally, .when one turns for evidence in support of this hypo- 
thesis to the Purkinje fibers one finds here a combination of the 
distinctive differential characteristics of the musculature of the 
atrioventricular bundle and the ventricular myocardium, namely 
both serrated cell-margins (in histologic preparations) and a few 
simple hand-discs. In other words, the Purkinje fibers at the 
level of transition from the cells of the atrioventricular bundle 
are still largely distinct cells, but they are drawn out into fibers 
towards the ventricular myocardium where they contain also a 
few discs, and where they are undergoing fusion. The Purkinje 
fibers of the adult heart are apparently at the histogenetic stage 
attained by the ventricular myocardium at about the beginning 
of the third fetal month. This structural condition is incompat- 
ible with an interpretation of the discs as intercellular cement 
substances or as tendons. An interpretation in terms of dif- 
ferentiating sarcomeres is likewise inadmissible on evidence 
already stated. 
