DESCRIPTION OF PLATES 
The illustrations were made with the aid of the Bausch & Lomb camera lucida. 
All figures are of tissue fixed in the alcohol nitric-acid mixture and stained 
in hemalum according to Zimmermann’s technic, and are magnified 3500 diam- 
eters, except 17 and 18, which are 2000. The original magnification is reduced 
one-fifth in reproduction. 
PLATE 1 
EXPLANATION OF FIGURES 
* 
1 Fibers from ventricular tissue of 76-mm. pig embryo, in which the dises 
first make their appearance. The bands here are short or merely granular dots 
in line with the anisotropic bands. 
2 Portion of fiber from 76-mm. pig embryo, showing a completely developed 
disc. 
3 Type of disc commonly found in an embryo of 89-mm. 
4 A dise taking the step formation, only rarely found in the 89-mm. stage. 
5 Fibers from heart muscle of the 115-mm. pig, showing the variable width 
and the larger number of complete discs. 
6 Fibers showing terminal anastomosing and a disc running across all four 
fibers (182-mm. embryo). 
7 Fibers showing the definite unstained cytoplasm which surrounds the 
nuclei in older heart tissue almost invariably. Also developing and completed 
discs are shown which explains how discs become more numerous (201 mm.). 
8 A definite example’of the step and riser found in 238-mm. heart muscle. 
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