286 S. J. HOLMES 



Cultures ma}^ be kept alive for a long time at room temperature, 

 but they require more frequent washing and change of medium. 

 Extensions of cells occur more rapidly at room temperature than 

 when the cultures are kept in an ice box. 



While it is clear that extensions of ectoderm are mainly 

 due to the outwandering of cells, a certain amount of cell di- 

 vision is also found to occur. Division figures were not infre- 

 quently observed several days after the preparations were made. 

 In two cases a division figure first seen in the prophases was 

 watched continuously through the entire process of mitosis. 

 The individual chromosomes could be distinctly seen and as 

 many as fifteen could be readily counted, although the actual 

 number is greater. The arrangement of the chromosomes in 

 the equatorial plate, their pulling asunder, and the formation 

 of the two daughter nuclei could be easily followed. The 

 chromosomes appeared as V-shaped rods, and during the ana- 

 phase the open end of the V's were directed towards the middle 

 of the division figure. Cleavage of the cytoplasm was completed 

 soon after the telophase of the nuclear division. The whole 

 process of mitosis was completed in less than three hours. 



Mitoses become rarer the longer the tissue is kept in a given 

 supply of culture medium. In one preparation from the larva 

 of Diemyctylus mitotic figures were common four days after the 

 tissue was isolated, and some were found on the seventh day, but 

 none later. Changing the tissue to a fresh supply of culture 

 medium, however, may cause cell division to be resumed. One 

 preparation which had been subcultured several times showed 

 numerous division figures fifty days after it had been removed 

 from the body of the animal. Two days after its last transfer 

 into fresh culture medium it showed over twenty-five mitotic 

 figures. One of these was watched through into the telophase 

 when the cell body could be distinctly seen to constrict into two 

 separate cells. I have never seen mitotic figures so abundant 

 as in this piece of epithelium which had been kept for fifty days 

 in an artificial medium. The washing in Ringer's solution 

 and its transfer to a fresh supply of fluid had apparently given 

 the tissue a new lease of life. 



