PLATE 2 
EXPLANATION OF FIGURES 
9 Another example of the ‘step’ and ‘riser’ forms of discs found in 238-mm. 
heart muscle. 
10 Dises from the 251-mm. stage. They are thicker at this stage than in 
the earlier stages. 
11 Fibers from the 277-mm. stage, showing a typical grouping of discs. 
12 Fibers from the 303-mm. stage, showing a more developed ‘step’ and ‘riser. y 
13 Fibers from five-month tissue in which the discs appear as granules laid 
down on the separate fibrils. 
14 Three parallel fibers from five-month tissue in which a peculiar type of 
zigzag dise occurs. 
15 and 16 Zigzag discs as found in year-old tissue. These are very charac- 
teristic of this tissue. 
17 and 18 These figures were drawn from 277-mm. tissue at a magnification 
of 2000 diameters, to show a larger area and thus show the comparison between 
the number of discs in a contracted area (17) and a relaxed area (18). 
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