DEVELOPMENT OF DISCS, HEART MUSCLE OF PIG 339 
are numerous, straight, and occur in patches in areas devoid of 
nuclei. The general arrangement of the striations is different in 
that some of the bands present a greater density than others. 
These light and dark bands occur alternately and are considered 
by most observers as areas of contraction. I have been unable to 
find any difference in the distribution of discs in this region and 
in other relaxed areas. There is also no marked change in the 
165-mm. stage. 
At 182 mm. we have the first example of a disc extending over 
more than two fibers. In figure 6 the fibers anastomose ter- 
minally and the disc runs across the four fibers. 
The 201-mm. embryo heart shows the colorless cytoplasm 
which exists invariably as a streak through the center of the 
fiber in older tissue, and surrounds the nucleus. This has not 
been noticed in the earlier tissues nor is it constant at this stage. 
The dises run to this cytoplasm, but not across it (fig. 7). 
In the periphery of the heart muscle of the 238mm. stage, 
the polynuclear fibers are first noticed. The nuclei tend to 
arrange themselves in rows consisting of two or four, rarely 
_ three. The mononuclear state exists farther toward the center 
of the ventricle. The fibers are very compact and give the ap- 
pearance of broad fibers. The discs take the step formation 
more regularly than heretofore (figs. 8 and 9). 
Figure 10 represents the discs at the 251-mm. stage. They 
are quite frequent in occurrence and many of them, as here rep- 
resented, are thick. 
Figures 11 and 12 represent the condition of the discs in the 
277-mm. and 303-mm. stages, respectively. There is no marked 
change. The risers occur more frequently and the discs often 
extend over two or more fibers. 
In all of the tissue studied so far the discs have increased in 
number by means of new and incomplete discs developing and 
gradually growing across the fiber. Figure 7 is an example of 
such discs. 
There is a gap between the embryonic tissue and the postnatal, 
since no stages were studied between the 303-mm. pig, which is 
very close to the age for birth, and the four-month-old tissue. 
There are, however, no very marked changes between the two. 
