Harry Lewis Wieman 199 
Of the later workers, Godlewski’s, 02, description is interesting, because 
of his denial of cell. structures in the heart muscle of the rabbit. Ac- 
cording to him the heart muscle is a syncytium in which there is no 
cell demarcation. The contractile substance is the fibril which shows the 
various markings described by other authors. - 
Thus, in general, in addition to the above, the work of investigators 
goes to show that the contractile substance in the muscle tissue is the 
fibril. A number of these in turn compose the fibril bundle. Workers 
are not agreed as to the origin of the fibril bundles. The older hypothe- 
sis that they are extra-cellular structures is now refuted and the generally 
accepted opinion is that they are intracellular. 
Two current views exist as to the nature of the fibril bundles. The 
first is that these structures are coagulation products, and that the living 
cell contains neither cyto-reticulum nor fibrils (Englemann, 73-81). 
The second view is that the fibrils are differentiated structures which 
are formed in the living cell. The latter theory is the one which the 
results of many workers seem to verify. Of the latest workers Hycle- 
shymer, 04, reports having observed the fibrille (fibrils) in the living 
muscle cells of Jarval necturus. 
Concerning the origin of the fibrils, there are what is known as the 
network theory and the fibrillar theory. The upholders of the network 
theory maintain that the muscle cell contains a contractile reticulum, the 
longitudinal threads of which form the fibrils, the meshes being filled with 
a more fluid substance. Others consider that the fibrils are produced by 
the coagulation of the fluid substance, as a result of the action of various 
reagents. Later work apparently refutes the latter idea. The advo- 
eates of the fibrillar theory maintain that the fibrils are the contractile 
elements, and further, that they arise independently of the cyto-reticulum. 
Hycleshymer’s, 04, work on necturus, supports this idea, and Godlewski, 
o2, in his work on the striated muscle cell of the rabbit, has been able 
to find no trace of a cytoplasmic network. 
In the theory urged by MacCallum, 98, we have a combination of the 
network and the fibrillar theories. In this author’s words (Goby. aalke 
simplifies the conception of the structure of striated muscle fiber greatly, 
to consider the fibril bundles and the membranes bounding the compart- 
ments in the sarcoplasm as derived from the primitive network found in 
the muscle cells of very young embryos.” (p. 209) “This network 
tends to become more and more regular until the meshes are of the 
form of large discs. Some of these break up into smaller ones and in 
the nodal points of the network there is an accumulation or differentiation 
