617 
II. Cells with a more regular network, appearing as adjacent 
circles in cross section, and as a succession of discs in longitudinal 
section (Figs. 7 and 8). 
III. Cells like those last described, but with some of the meshes 
broken up further into smaller discs by a radial division (Fig. 9). 
IV. Cells with a single row of fibril bundles around the peri- 
phery, each fibril bundle being surrounded by small sarcoplasmic discs 
(Fig. 10). 
In tissue from pigs 13 mm and 15 mm long the structure is 
essentially the same. The fibril bundles in some of the cells, however, 
Fig. 6. Fig. 8. 
Fig. 6. Cross-section of two heart muscle cells of type I from a pig embryo 
10 mm in length, 
Fig. 7. Longitudinal section of a heart muscle cell of type II from a pig embryo 
10 mm long. N nucleus; L large sarcoplasmic disc, 
Fig. 8. Cross-section of two heart muscle cells of type II from a pig embryo 
10 mm long. N nucleus; L large sarcoplasmic disc, 
Fig. 9. Cross-section of a heart muscle cell of type III from a pig embryo 
20 mm long. S small sarcoplasmic disc. 
Fig. 10. Cross-section of two heart muscle cells of type IV from a pig embryo 
20 mm long. F fibril bundle; S small sarcoplasmic disc; Z large sarcoplasmic disc, 
