Albert C. Evcleshymer 295 



is based first upon the fact that in the rabbit embryo there are cytoplasmic 

 bridges between the ends of the myoblasts of the adjoining myomeres, 

 and secondly upon the observation on the continuity of the fibrilla? 

 through these bridges. 



In the stages of Necturus immediately preceding the differentiation 

 of myol)lasts there are numerous anastomosing cytoplasmic processes 

 among the mesothelial cells, indicative of a widely extending syncytium. 

 When the myoblasts can be distinguished by their cylindrical outlines, 

 they are heavily laden with yolk granules, yet it is not ditficult to find, 

 in carefully teased preparations, cytoplasmic strands connecting the ends 

 of these myoblasts. As soon as the yolk is absorbed in the ends of these 

 myoblasts (6-8 mm. embryo) the connecting strands are more clearly 

 defined in both teased preparations and longitudinal sections. From 

 this stage up to the 10 mm. embryo, when the mesenchyme has grown in 

 to form the septa, cytoplasmic continuity between the ends of the 

 myoblasts is frequently observed. After the septa are formed and the 

 myoblasts fibrillated (17-26 mm.), I have been unable to trace their 

 continuit}'. 



While there is a more or less complete syncytium of the myoblasts in 

 the early stages, there is no evidence whatever in Necturus to support 

 Godlewski's view that the muscle fibre is formed through an end-to-end 

 union of the myoblasts in adjoining myotomes. Indeed, strong evi- 

 dence against this view is found in the fact that in each of the closely 

 connected stages, from the formation of the myoblasts up to and includ- 

 ing the 26 mm. larva, the myoblasts may be easily isolated. Exact 

 measurements show that the length of each corresponds precisely to the 

 length of the myotome from which it was taken. There is not the 

 slightest evidence that the fibrilla3 of one myoblast are continued into 

 another myoblast. 



As to the mode of increase in the number of fibres, I think it can safely be 

 asserted that the majority of investigators believe that it is by the differen- 

 tiation of new myoblasts around the margin of the myotome. It was 

 claimed by Kiihne, 71, Goette, 75, Bremer, 83, Kolliker, 88, and others, 

 that the increase takes place not only in this way, but also by the post- 

 embrA'onic formation of inter-muscular spindles. The later investiga- 

 tions, however, indicate that these are the endings of sensory nerves. 

 The sarcoplasts, which were considered by Margo, 59, and Paneth, 85, as 

 embryonic muscle cells, have later been interpreted by Felix, 88, S. 

 Meyer, 86, Bardeen, 00, Godlewski, 02, and others, as degeneration pro- 

 ducts. Others have held that the fibres increase in numl^er through 



