194 The Fibrils of the Heart Muscle Cell of the Chick 
Krause’s membrane being continuous with the line of demarcation be- 
tween two adjacent sarcoplasmic discs. If the latter were the case, it 
would seem that the fibril bundles should show a tendency to break and 
separate along the line of Krause’s membrane. This did not happen. 
Hence it may be that what appears to be Krause’s membrane may only be 
the line of demarcation between two successive sarcoplasmic discs seen 
through the lightly staining parts of the fibril bundles. 
Teased tissue showed the fibril bundles each to be made up of smaller 
longitudinally disposed parts, the fibrils. The fibril bundles were in all 
cases surrounded by sarcoplasmic discs. 
In the adult tissue the fibril bundles are more abundant in the periphery 
of the cell than toward the center, which is composed for the most part 
of a network of undifferentiated protoplasm (Fig. 16). This figure 
represents a longitudinal section through the center of the cell. Part 
of the sarcoplasmic discs of a fibril bundle is marked sa in this figure. 
Cross sections show cells of widely varying diameter. Fig. 17 repre- 
sents three cells of a medium type. Cells of three and four times the ° 
diameter of these are also found. Fig. 17 is exceptional in that it shows 
the cell boundaries very distinctly. In most cases the cells are so closely 
applied that it is difficult to distinguish cell walls. These sections show 
the fibril bundles as dark, deeply staining patches a, surrounded by the 
sarcoplasmic discs, (sb) (Fig. 17). Some of these discs are seen to 
be further subdivided (sd), into what MacCallum, 97, has described as 
“small sarcoplasmic discs.” 
Between the cells as represented in Fig. 17, and connecting them, may 
be noticed a number of threads ¢, which resemble very much the strands 
of the reticular structure of the interior of the cell. Just what the origin 
and nature of these structures are, could not be determined. A study of 
these structures would doubtless throw some light on the question of 
whether the heart muscle fiber is a syncytium or not. No structure 
analogous to the “ protoplasmic bridges” of mammalian heart tissue or 
the “stratum granulosum terminale” (Prezwoski, as quoted by Mac- 
Callum, 97, p. 611), of human heart muscle, was found connecting the 
ends of the cells. 
Such, in brief, are the structures met with in the heart muscle cell 
of the adult chick. 
EMBRYONIC DEVELOPMENT. 
To study the different stages through which the cell passes in its 
development, sections from embryos varying in length from 8 mm. (15 
somites), to 22 mm. (8 days), were examined. Cells characterized by 
