9(5 



HISTOLOGY. 



long. This shows the cells of the muscle mass which has 

 developed in the leg from the myotomes for that region. In 

 the cells there is a protoplasmic network and no fibril bundles. 

 The nucleus is large and vesicular. At about this stage the 

 fibril bundles begin to be formed. Bardeen has pictured muscle 

 cells from embryos of this age with distinct fibril bundles scat- 

 tered irregularly around the periphery. In a pig's embryo 35 

 mm. long the fibril bundles are found in nearly all the cells. 

 They are small and not regularly placed. In embryos 45 mm. 

 in length there is a regular row around the periphery of the 

 cell. The nucleus is placed centrally, and the central proto- 

 plasm contains a more or less regular network (Fig. 66). 

 From this on, the fibril bundles increase in number and gradu- 

 ally fill up the entire cell. In embryos about 75 mm. long 

 there are, in addition to the central nucleus, several peripheral 

 nuclei. The latter are not vesicular, like the former, but stain 

 deeply and uniformly, like adult muscle nuclei (Fig. 67). The 





Cross-section of voluntary muscle from the thigh of an embryo pig 75 mm. in length. 

 A, central vesicular nucleus ; /?, peripheral solid nucleus. (MacGallnm.) 



central nucleus subsequently disappears. The sarcoplasm is 

 more and more encroached upon by the growth of the fibril 

 bundles, and in adult muscle it occupies a very small space. 

 Striations are noticed in the fibril bundles at their first appear- 

 ance, and frequently the ultimate relation between Krause's 



