EPIDERMIS WHEN CULTIVATED OUTSIDE BODY 287 



As stated in my previous paper, extensions of epithelium often 

 give strong evidence of amitotic nuclear division. Further 

 observation not only furnished additional evidence of the same 

 phenomenon, but revealed some of the conditions by which ami- 

 totic division is induced. Newly extended sheets of ectoderm 

 present little or no indication of amitotic division. The nuclei 

 of the cells are round or oval and rarely present any indenta- 

 tion of their outline. With epithelial outgrowths which have 

 been kept for a week or more in the same culture medium indi- 

 cations of amitosis are more frequent. Instances of two, three 

 or four nuclei in a cell are common (fig. 3), and in some cases 

 as many as eight nuclei were seen in a single cell. No clear 

 indications of cell division following the division of the nucleus 

 were observed. In many cases the amount of nuclear material 

 in relation to the cytoplasm was obviously increased to a very 

 considerable extent. In all cases where numerous nuclei were 

 seen in a cell there was a considerable amount of yolk present, 

 although cells with very little yolk remaining in them frequently 

 had two nuclei, but rarely more. 



While evidence of amitotic nuclear division occurred abundantly 

 in many preparations kept without a change of medium, other 

 preparations made of the same material but changed every few 

 days to a fresh medium showed no indication of amitosis, al- 

 though they were kept much longer than the preparations in 

 which the medium was unchanged. The appearance of indica- 

 tions of amitosis very frequently goes along with signs of dimin- 

 ished activity, such as the rounding up of cells, the disintegration 

 of certain cells in the culture, and the general inactivity of the 

 epithelial tissue. It is a conmionly received doctrine that ami- 

 tosis occurs most frequently in cells of declining vitality. Its 

 association in cultures of epithelium with life under unfavorable 

 conditions lends a certain support to this view. 



Most of the epithelial outgrowths observed showed cells of 

 somewhat different types. The ordinary cells of pavement 

 epithelium (fig. C), as soon as most of the yolk disappears, con- 

 sist mainly of a clear, rather homogeneous protoplasm, more or 

 less granular in the vicinity of the nucleus. A small amount of 



