DEVELOPMENT OF DISCS, HEART MUSCLE OF PIG 341 
become truly syncytial, but remains cellular with the lines of de- 
marcation, or discs, as I shall call them, representing the walls 
between the cells. 
In my study of this change from cellular to fibrous tissue, I 
cannot show that the discs form cell walls. They are too irregu- 
lar in position and too infrequent in occurrence at such an early 
stage. 
Observations show that striations and dises do not appear si- 
multaneously in the heart tissue of the embryonic pig. ‘The stri- 
ations appear first. These facts need not contradict what Jordan 
and Steele said, for they studied the embryonic heart of the guinea 
pig and cat. Nor should they contradict Jordan and Banks’ 
statement that the discs appeared in the heart of beef simultane- 
ously with the striations. The other work done on fetal mate- 
tial with regard to the discs has been more or less superficial. 
However, it may be true that the time of appearance of the dises 
varies widely among the different animals. 
It cannot be said that the dises in the embryonic pig’s heart 
are cell walls or cement lines. In the first place, they are only 
peripheral, for they pass out of focus very readily. Nowhere in 
the tissue is it possible to keep the discs in focus through the en- 
tire thickness of the fiber. Also, they occur so many times in 
rather close proximity with no nucleus between them, as-is shown 
in figures 5,7, 9,10,and11. The discs do not present a different 
structure in the older material except that they are more complex 
and occur more frequently in step-like forms. They are still 
peripheral in the five-month tissue and they are very distinctly 
darkened, portions of each fibril arranged in the form of a band, 
rather than extending across the fiber as a connected disc (figs. 
13 and 14). These facts would disprove what Zimmermann, 
Palezewska, and Werner concluded concerning the intercellular 
nature of the discs. | 
/ Jordan presents the theory that the discs may be functional in 
connection with the contraction of the heart. From the facts 
observed in this study, it seems*improbable that the discs would 
be caused by the contractions of the heart muscle or would result 
from the rhythmic beat of the heart. The striations do not ap- 
