12 H. E. JORDAN 
processes of a longitudinal splitting of sarcostyles and the 
irregular tensions prevailing in abnormal hearts. Still another 
but rare type of disc, in appearance little more than a 
thickened telophragma, may be explained as the telophragma 
constituent of a contraction band from which a remnant of the 
deeply staining substance failed of removal at relaxation, the 
whole subsequently becoming modified by the accumulation of 
tissue fluid. This last type will be further discussed below. 
In order more firmly to establish my hypothesis that inter- 
calated discs represent essentially modified irreversible con- 
traction bands, as above briefly outlined, I undertook a thorough 
histologic study of the contraction phenomena in the leg-muscle 
of the sea-spider.? I convinced myself, in the study of this 
material, of the essential accuracy of Rollet’s description of 
the formation of the contraction band. Figure 2, taken from 
an earlier paper, is here reproduced to further emphasize my 
agreement with Rollet in regard to the formation of the con- 
traction band, and to illustrate the details in our disagreement 
regarding the specific morphologic characteristics of the relaxed 
condition of the sarcomere. Portions of four fibers (A to D) 
are shown, arranged in horizontal series from left to right 
according to the degree of contraction. Fiber A is in a con- 
dition of repose. Arrows, Z, mark the limits of a single sar- 
comere. The dark disc (Q) stains very deeply throughout 
and exists as a compact undivided disc. The accessory dise 
(N) is faintly visible on either side of Z in the light dise (J). 
Fiber B is just passing into an early phase of contraction. The 
dark disc has become longer while the sarcomere as a whole 
has become shorter. Moreover, the dark disc is becoming 
lighter medially, the constituent myofibrils having become 
distinctly knobbed, and, if possible, still more deeply staining 
terminally. This condition clearly demonstrates a movement 
of the deeply staining substance toward Z. In fiber C contraction 
is approximately at midphase. The dark dise is now bisected 
by the secondarily formed median disc (H). The resulting 
halves of Q have fused in part with the accessory discs encount- 
ered in their passage toward Z. The dark disc, including H, 
