DEVELOPMENT OF CONNECTIVE-TISSUE FIBERS. 55 



individual fibrils can be traced into a cell, the bundle, or fiber itself, appeared 

 quite independent of the cytoplasm of the cells (plate 2, figs. 2 and 3). 



No mitosis was observed in cells whose exoplasm was actively developing 

 into fibrils during the time in which the exoplasm contained the fibrils. Mitosis, 

 however, continued, and many cells were seen to undergo mitosis in regions where 

 other cells were forming fibers. So far as can be determined from these observa- 

 tions, the cell may again undergo mitosis after the bundle of fibers has become 

 independent of the cell cytoplasm. Whether in such cases the cell actually sepa- 

 rates itself from that part of its exoplasm which has been differentiated into 

 fibrils or whether it simply divides the undifferentiated cytoplasm and mean- 

 while remains attached to the differentiated exoplasm (or fibers) could not be 

 determined. However, the cells which contributed fibrils to a fiber bundle grad- 

 ually increased in number and extended over a wider territory, and the bundle 

 became differentiated into a more and more definite fiber (plate 2, figs. 1 and 3). 



The study of the living cell, as well as of the fixed preparation, led to the 

 idea that the fibers became more and more separated from the cells, although it 

 is quite possible that they may merely continue through the exoplasm and become 

 more definite, on account of the separation of the cells. Certainly in no case, in 

 these tissue cultures, did the fiber become so well developed that the ending of the 

 various fibrils which made up the fiber could not be traced into the exoplasm of a 

 cell (plate 2, figs. 2 and 3) . No completely differentiated fiber was observed through- 

 out its development, although in a few instances, where the cultures were kept 

 in a healthy condition for several weeks, fibers which resembled those of an 18-day 

 chick embryo were developed. It seems probable that the development of these 

 fibers was by a continuation of the process described above. 



Certain preparations which had been carefully studied during their growth 

 and development were fixed and stained, and from these preparations most of the 

 photographs and drawings have been made. A few of the living cells were drawn 

 on successive days, and although it was frequently impossible to determine the 

 exact cell drawn the day before, at least a cell in its near neighborhood was taken. 



