107 



tioii the muscle cells, ameboid cells, and so on dis- 

 appeared and the endothelial cells or l"ibrol)lasts remain- 

 ed. This method for obtaining pure cultures of fibro- 

 blasts, the physiological elective method, is good and safe 

 because all the other cell types do not grow so rapidly 

 or as well as the connective tissue cells which sooner or 

 later ■ overgrow any of the other types of cells. Cultivation 

 of this tissue has now been enlarged upon so thoroughly 

 that there is no need to go more into details about it. The 

 strains of connective tissue cells have l:)een used as a reagent 

 for the detection of substances in the humors of the body, 

 which have the power of increasing or decreasing the rate 

 of growth and many biological problems have been studied 

 with the fibroblasts as reagent and references are already 

 made to the various papers on these subjects. 



EPITHELIUM. 



The growth of epithelium in vitro has been described 

 very often. 



Several attempts had been made to obtain strains of 

 epithelium for permanent cultivation outside the body, as 

 has been carried out with the connective tissue cells. From 

 the very beginning of the experiments of cultivation of tis- 

 sue cells in vitro — the cultivation of epithelium has tempted 

 most investigators. It has been possible to cultivate epithe- 

 lium in vitro for short periods of time, long enough to ob- 

 serve and describe the characteristic growth of epithelium 



Most investigators, who have worked with tissue cultures, 

 have not been able to keep strains of tissue cells over long 

 periods of time, from months to years. Tissue cultures have 

 mostly been studied in the culture of first passage — and 

 kept under observation in the same culture medium for a 

 few days only or a couple of weeks. Should it therefore 

 be possible to cultivate a strain of epithelium for a long 

 period of time, and the cells still remaining characteristic 

 for epithelium all that time, first then one may conclude. 



