620 
sarcoplasmic discs, both laterally and horizontally would consist of 
the same or very similar substances as the fibrils, since all would 
appear to be derived directly from the network found in the primitive 
cell. The fibril bundles could then be looked upon simply as regular 
accumulations of the substance of the network between small sarco- 
plasmic discs; and the Zwischenscheiben which are continuous with 
the partitions between the discs, mark the place at which the network 
of the sarcoplasm joins the fibril bundles. 
If the development consists as it does in the formation of a com- 
plex fibre having all the structure described above, from a cell with 
a simple irregular network; and if the development runs hand in hand 
with the acquirement by the muscle of a more perfect contractility 
and a greater capacity for work, then it seems that there must be 
an important relation between the differentiation of the network into 
a complex structure, and the development of a greater power to carry 
out the functions of the heart. It is only on the hypothesis that the 
whole network including the fibril bundles is contractile that this 
relation can be explained. If this be so, the contractile elements in 
the early embryonic heart would be represented by the irregular net- 
work seen in cells of type I, a view strongly supported by the beating 
heart of the young chick embryo, where no fibrils exist. In the adult 
muscle the contractile elements would be made up of the following 
structures: 
1) The network surrounding the small sarcoplasmic discs, zu 
forming all their boundaries. 
2) The Zwischenscheiben and the membranes continuous with 
these which form the horizontal partitions between the small sarco- 
plasmic discs. 
3) The fibril bundles themselves. 
In conclusion I wish to express my thanks to Dr. BARKER at 
whose suggestion this work was begun, and without whose guidance, 
the study could scarcely have been completed. 
