56 H. E. JORDAN 
on this subject I have presented evidence in support of my 
hypothesis that these discs are essentially modified irreversible 
contraction bands. The simplest type of intercalated disc is 
practically identical in structure and staining reaction with a 
contraction band. The essential double nature of the con- 
traction band supplies the explanation of the several varieties of 
the simplest type of intercalated disc with respect of their re- 
lation to the telophragma. Since the two halves of a contraction 
band have a bisarcomeric origin, and in consequence a relatively 
independent relationship, it would seem to follow that one half 
might pass into relaxation while the other half remained in- 
capable of leaving the telophragma, the latter half thus be- 
coming an intercalated disc of the variety bounded only on one 
side by a telophragma. Where the entire contraction band 
failed to reverse, the variety of intercalated disc which is bisected 
by a telophragma would take origin. The variety of dise bounded 
on both sides by a telophragma could arise by the subsequent 
fusion of the opposite halves of two adjacent contraction bands. 
This process of ‘fusion’ may actually consist essentially of a 
secondary modification of those portions of the sarcostyles in- 
tervening between the opposite irreversible halves of adjacent 
contraction bands. 
Besides the other more complex types of discs (step and 
serrated forms), still another type of intercalated dise must here 
be considered. This is a relatively rare type in cardiac muscle. 
It occurs somewhat more frequently in the specimen of human leg 
muscle, where it is scattered among the predominating simplest 
variety with bisecting telophragma. In a paper by Jordan and 
Banks" on the intercalated dises of the beef heart, this type 
was included among the illustrations (fig. 32), but its interpre- 
tation was at that time not clear, and no description was at- 
tempted. It may be described now as essentially a thickened 
telophragma, or portion of a thickened telophragma. It would 
seem that a contraction band may disappear or reverse, at the 
time the fiber passes into repose, either as a whole or only in 
half, or finally only in part. In the latter circumstance a mere 
remnant of the deeply staining substance of the composite con- 
