STRIPED MUSCLE OF WASP 1h 
Rollet’s description and illustration of the contraction band 
in contracted muscle. Such contraction band and the simplest 
‘band’ type of intercalated discs are essentially identical, except 
that the latter generally only represents a portion of the former. 
We need only postulate that under certain conditions a con- 
traction band becomes incapable of reversal (disappearance) 
in whole or in part to secure the beginnings of the several 
varieties of the simple band type of intercalated disc. 
The contraction band is a composite structure, consisting of 
two (or four, where accessory discs occur) deeply staining 
portions of separate origin fused about a definitively bisecting 
telophragma. If the entire contraction band should fail to 
reverse, then there would result the variety of intercalated 
dise bisected by the telophragma. Subsequently this primary 
condition becomes modified by increase of tissue fluid within 
the disc, as indicated by its reaction to silver-nitrate treatment. 
This variety is ontogenetically and phylogenetically the prim- 
itive form of disc. It may represent the complete contraction 
band, in which case it becomes a genuine ‘disc,’ or more generally 
only a portion of the contraction band. ‘Entire’ as used above 
only refers to the longitudinal length, that is, to the double 
constitution of the disc, not to its radial extension. If only 
one of the halves of the contraction band fails to reverse, there 
results the variety of disc only bounded on one side by a telo- 
phragma. The variety of intercalated disc bounded on both 
sides by a telophragma may be conceived to arise by a subse- 
quent modification, due mainly to the accumulation of tissue 
fluid, resulting essentially in a fusion, of two adjacent irrever- 
sible halves of successive contraction bands. 
From these three varieties of the primitive band type, classi- 
fied according to relation to telophragma, the several varieties 
of the more complex step-type are readily derived through the 
operation of secondary mechanical factors, inherent in the 
oblique stresses of the functioning heart due to the profusely 
branched condition of the definitive myocardial trabeculae. 
The still more complex serrated type, especially abundant in 
hypertrophying hearts, is the result of factors inherent in the 
