338 LUCILE WITTE 
In the 25-mm. embryo heart the cells are spindle-shaped in 
longitudinal section with a large central nucleus. The cytoplasm 
of the cells remains clear and here and there very faint striations 
appear. In cross-section the cells present the same condition 
that MacCallum describes. The cells seem broken up into 
smaller sarcoplasmic discs by walls which are continuous with 
the bounding membrane of the sarcoplasm. Some fibrils have 
appeared at the centers of these discs. 
At 38 mm. the cells have developed into a more fibrous struc- 
ture. There are still some of the spindle-shaped cells to be found 
near the center of the ventricle, but the cells of the auricle and 
peripheral part of the ventricle have elongated into fibers in 
which the cross-striations appear more distinctly, especially at 
the periphery of the heart. The fibers are finer and the nuclei 
are still large and centrally located. 
At 76 mm. the cells seem to have lost all eanilatice to spindle 
cells, practically all through the heart. They have elongated 
and anastomosed until there is a complete network or syncytium 
of fine fibers, all of which are definitely striated. At this stage, 
darker portions of the striations occur at intervals, beginning at 
the periphery of the fiber and proceeding across at right angles 
to the length of the fiber. The discs, as these dark portions 
very clearly seem to be, do not extend across the entire width of 
the fiber, but gradually shade into the striations or telophragma 
(fig. 1). In a few instances, there are faint discs extending 
entirely across a fiber (fig. 2). 
At 89 mm. the general structure of the tissue is the same as 
that of the preceding stage. The discs, however, are more dis- 
tinct and almost invariably extend across the fiber. They are 
no more numerous than in the 76-mm. stage (fig. 3). Figure 4 
represents a type of dise which occurs only very rarely at this 
stage. 
The 115-mm. embryo heart presents much more development 
in the discs. They are much more numerous and distinct and 
in every case extend entirely across the fiber. In this tissue 
some of the fibers are wide while others are still narrow (fig. 5). 
There is no change in the 120- and 140-mm. stages. The discs 
