V. 



PURE STRAINS OF TISSUE CELLS. 



Soon after a technique had heen developed for cultivating 

 tissue in a way somewhat similar to the technique for cul- 

 tivating bacteria, it was desirable to develop a method by 

 which it would be possible to obtain cultures of tissue cells 

 belonging to a definite type. 



Carrel ^■^) found that when a fragment of tissue had 

 undergone several passages, in vitro, the outgrowth of new 

 cells could easily be distinguished and a large group of cells 

 belonging morphologically to the same type of cells could 

 be separated from the other cells by cutting out the pai't 

 of the fibrin film containing the cells desired and transfer- 

 ring it to a new medium. 



There has been much discussion about dedifferentiation of 

 cells in vitro, i. e., that the various tissue cells, whatever 

 they may be, mesenchyme, or ectoderm, were said to be 

 transformed into an embryonic indifferent type of cells. 

 It was therefore necessary to find methods by which it would 

 be possible to work with absolutely pure cells from the 

 very beginning, in order to see if such transformations occur. 

 Much has been written about the identification and clas- 

 sification of the different cells cultivated in vitro ^^^ The 

 problem is brought out in connection with an assumption 

 by C h a m p y 123)^ t^.^t ^ dedifferentiation of the epithelial 

 cells to the type of fibroblasts takes place in vitro. This does 

 not seem to be correct and is rather to be regarded as an 

 apparent phenomenon. As long ago as in 1898, Leo L o e b 

 372) came to the same conclusion as C h a m p y by observing 

 the changes which epithelial cells undergo when cultivated 



