340 LUCILE WITTE 
At five months a new feature in the discs appears. Instead of 
the discs’ extending across the fiber in a compact formation, they 
seem to be made up of coarser granules on each separate fibril, 
as is shown in figure 13. Figure 14 shows three fibers which are 
parallel and are separated by wide, clear spaces. The dises are 
at the same level in the three fibers, but instead of their being 
straight bands, they are zigzag, as though the separate fibrils 
had been pulled back and forth till the regions of coarse granules 
were out of line with each other. This type of disc occurs quite 
frequently throughout this stage. 
The year-old tissue shows almost entirely the zigzag bands 
running across several fibers. They cannot be described as exact 
step formations, for they do not lie parallel to the telophragma 
(figs. 15 and 16). Those dises which are straight are short. The 
type of dises and the arrangement of the nuclei in the tissue are 
just as described by Miss Werner. However, I cannot verify her 
statement or her drawing showing that the nuclei are bounded at 
either end of the series by discs. If the discs were visible at one 
end of the series of nuclei, they were not at the other end. 
The year-old tissue, then, represents the limit reached in this 
study, where the discs extend over several fibers at nearly the 
same level, or go up and down in a series of ‘steps’ and ‘risers.’ 
These last represent the most complex of the discs found. 
DISCUSSION 
As regards the early embryonic structure of the heart muscle, I 
have been able to verify J. B. MacCallum’s statements. The early 
tissue exists as spindle-shaped cells, and it is very evident how the 
individual cells gradually anastomose and form the fibers of the 
adult heart tissue. MacCallum contends that these spindle- 
shaped cells gradually lengthen and branch and unite with each 
other at the ends of the branches by definite lines ef demarcation. 
He adds that these lines of demarcation correspond to the proto- 
plasmic bridges found in tissues of other animals described by 
some investigators. 
Thus MacCallum places himself on the side with Zimmermann 
and his students and contends that adult heart muscle does not 
