200 The Fibrils of.the Heart Muscle Cell of the Chick 
of its substance, giving rise to longitudinally disposed masses. These 
become what in the adult are known as fibril bundles and the discs are 
the sarcoplasmic discs.” 
Now the question is, How do these theories apply to the conditions met 
with in the heart muscle of the chick? We will consider MacCallum’s 
theory first. 
The occurrence of the breaking up of the meshes of the cyto-reticulum 
into smaller parts was noticed in the case of the chick. That the fibril 
bundles arise from the center of the meshes at the nodal points of the 
network appears, however, to be untrue. Here, in fact, it seems that the 
nodes of the original network mark the positions of the fibril bundles; 
in other words, that the primitive longitudinal threads develop into these 
structures. ‘To illustrate, the accompanying figure represents a diagram- 
matic cross-section of the cyto-reticulum. The unbroken lines represent 
the threads of the original network. The circular masses at the inter- 
sections of this network (a, b, c, ete.) represent the heavily-stained 
deposits. The dotted lines divide the large meshes or discs into the 
“small sarcoplasmic discs” of MacCallum (boc. cod, etc.). Now ac- 
cording to the author just mentioned, a fibril bundle arises in the center 
of any large disc at the point of intersection of the dotted lines (0, 2, etc.). 
The full lines would then represent boundaries of sarcoplasmic discs. 
However, in the case of the chick the facts seem to indicate that the fibril 
bundles arise at the points of intersection of the lines of the original 
network (a, e, b, etc.). Then the sarcoplasmic disc surrounding any 
one particular fibril bundle, as, for example, that one the cross section 
of which is represented by a, would be the area bounded by yboearzk. 
