186 WARREN H. LEWIS AND MARGARET R. LEWIS 



Aside from the mitochondrial inclusions the cytoplasm con- 

 tains varying numbers of neutral red granules. These are 

 minute and not very abundant, and are usually situated in the 

 neighborhood of the nuclei. 



THE NUCLEI 



The nuclei appear in the young muscle buds soon after the 

 protoplasmic ends begin to project from the explanted piece. 

 They gradually increase in number as the bud increases in length 

 and size. There is usually a large group of nuclei in the ex- 

 panded end. They occupy the more proximal part of this 

 expansion while the more distal part is usually free from nuclei. 

 The narrow part of the muscle bud connecting the protoplasmic 

 end with the explanted piece has a varying number of nuclei 

 scattered along it. The isolated myoblasts and fibers contain 

 varying numbers of nuclei from one to many. We have exam- 

 ined repeatedly both living and fixed cultures for indications 

 of nuclear division but only in a few instances have we seen 

 mitotic divisions and those occurred in the mononuclear myo- 

 blasts that were free in the culture. When the nuclei of the 

 muscle buds were studied the condition of the mesenchyme in re- 

 gard to the frequency of cell division was usually noted and it was 

 not uncommon to see three or four mitotic figures in the mesen- 

 chjone cells in the neighborhood of the muscle buds in one field 

 of the microscope. In spite of the fact that we have very little 

 direct evidence of nuclear division in the muscle buds it seems 

 probable that nuclear division does take place. Some muscle 

 buds have thirty or forty or more nuclei and they must either 

 have arisen by division from a few or more that came out from 

 the old piece or have all migrated out from the old fiber as the 

 muscle bud grew out from it on to the cover slip. The indirect 

 evidence in favor of nuclear division is revealed through the 

 staining of fixed specimens. In such specimens, stained either 

 with iron hematoxylin or with Ehrlich's hematoxylin and eosin, 

 it is seen that the nuclei vary considerably in their staining 

 reaction. Some are darkly stained, others rather lightly, and 



