342 LUCILE WITTE 
pear till the heart has attained definite form, some time after the 
beating of the heart begins. The discs do not appear till some 
time after the striations, so that any theory that discs were formed 
at contraction or with the beginning of contraction of the heart 
would be disproved. 
It may be that the dises develop physiologically for the purpose 
of strengthening the muscle fibers. They appear shortly after 
the striations and at about the time the muscle cells begin to 
anastomose and form fibers. The fact that the discs become more 
numerous and complex with the increasing growth and activity 
of the heart seems to strengthen the theory mentioned above. 
SUMMARY 
1. The early heart tissue of the pig is cellular in structure, 
composed of spindle-shaped cells which later anastomose termi- 
nally and laterally to form a network of fibers. 
2. The striations appear earlier than the discs, but only here 
and there throughout the tissue. 
3. The discs appear at the 76-mm. stage, much earlier than in 
any other animal studied, with the exception of the cat embryo 
of four days. 
4. The dises are at first dots or incomplete bands beginning at 
the periphery of the fiber and growing across. With the advance 
in development, they become straight discs across the entire 
fiber, then across two or more, and finally assume the more com- 
plex type of discs and ‘risers.’ 
5. The dises do not appear more numerous in contracted areas 
than in relaxed areas. 
6. The discs are not to be found at either end of a series of 
nuclei, thus forming a cell, for they are almost invariably in 
close proximity to each other, occurring in patches, and seldom 
is a nucleus to be found between them. 
7. The theory is put forth that the discs serve as strengthening 
bands in the muscle fibers, since they appear at about the time 
of the change from cells to fibers, and increase in number and 
complexity with the growth and activity of the heart. 
