Albert C. Eycleshymer 299 



the embryo first begin in the anterior of the mid-dorsal myotomes and 

 in these the myoblasts are first fibrillated. The above considerations 

 lead the T^'riter to support the theory that the fibrillffi are pre-existent 

 structures and represent the principal contractile elements. 



ilacCallum, 98, has found that in the myoblasts of pig and of man there 

 is a primitive cytoplasmic reticulum, the meshes of which later assume 

 a regular form ; the transverse membranes of this meshwork eventually 

 form the so-called Krause's membranes, while the longitudinal give rise 

 to the fibrillffi. I quote the author's words (p. 211) : "It simplifies 

 the conception of the structure of striated muscle fibre greatly, to con- 

 sider the fibril bundles and the membranes bounding the compartments 

 in the sarcoplasm as derived from the primitive network found in the 

 riuscle 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 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 sarco- 

 plasmic discs." 



While MacCallum's theory is exceedingly ingenious and strongly ap- 

 peals to those who would reconcile the network and fibrillar theories, I 

 cannot see, at present, how it wHl explain the facts observed in the 

 fibrillation of the muscle cell of ^STecturus. 



In the study of the muscle cell of jSTecturus I have been unable to find 

 any evidence of a definite or fixed relation between the cytoplasmic net- 

 work and the fibrillte. It seems highly improbable that the longitudinal 

 threads or membranes of such a meshwork should converge at the noto- 

 chordal side of the myoblasts, which would be necessary if the fibrillEe 

 (lifi^erentiated in its meshes. Further, in conformity with the subdivision 

 of the muscle columns, the meshes must become progressively smaller 

 towards the end of the myoblasts, as a result of their repeated subdivision. 

 Even were this true, a further difficulty is encountered in the fact that 

 the cytoplasmic reticulum varies widely in the difilerent myoblasts, and 

 in different portions of the same myoblast. Another serious objection 

 is the fact that the fibrillas are unstriated for some time after their first 

 appearance. My observations agree with the views already expressed 

 by Wagner, 6g, Eabl, 97, and Bardeen, 00, all of whom maintain that 

 the fibrillse are at first without transverse markings. Further, it should 

 be borne in mind that some of the most recent investigators (Godlewski, 

 02) have been unable to find any evidence of such a network in the mam- 



