CROSS STRIATED MUSCLE IN TISSUE CULTURES 187 



this holds even among the nuclei that lie side by side in the same 

 group. We have often noticed similar differences among the 

 nuclei of mesenchyme cells when active mitotic division is taking 

 place. In fact everyone who has studied embryonic material 

 has probably noted such differences in the staining reactions 

 of nuclei. It is especially well marked, for example, in the cells 

 of the neural tubes of young amphibian embryos where active 

 mitotic division is taking place. We have been able to demon- 

 strate in our cultures that the nuclei of the young daughter 

 cells of the mesenchyme always stain deeper than the nuclei 

 of the resting cell. This ability of the daughter nuclei to stain 

 more deeply lasts for an hour or two after the mitotic division. 

 If mitosis were taking place to any great extent in the muscle 

 buds we should probably have observed it especially iii the ex- 

 panded end of the bud. Yet here as well as elsewhere in the 

 muscle bud the stainable differences in the nuclei are found in 

 abundance. Of course it may be that the nuclei undergo mitotic 

 division in the old piece out of range of direct observation in the 

 living. On the other hand, there is, of course, the possibiUty 

 of direct division. Direct division seems to be extremely rare 

 in our cultures and Macklin, after an extensive series of obser- 

 vations, was able to observe but one case of direct division of 

 the nucleus in the mesenchyme cells, and that without division 

 of the cytoplasm. We have not observed direct division of 

 muscle nuclei and have no data on the staining reaction of nuclei 

 after direct division. 



The observations on the nuclei of muscle buds in the living 

 are much more difficult than are those upon the nuclei of the 

 mesenchymal cells and for the present at least many questions 

 in regard to the origin of these nuclei must be left unsettled. It 

 is often stated that direct as well as indirect division of the nuclei 

 takes place in the regeneration of muscle in amphibia and mam- 

 mals. Such statements are based not on direct observation of 

 the living but on fixed preparations. It is evident from our 

 studies on the living cells in tissue cultures that such observa- 

 tions on fixed and stained material in regard to direct division 

 are no indication of what actually occurs in the living. Many 



