The Behavior of Cells 185 



recent work of Leo Loeb, Oppel and Harrison tends 

 strongly to confirm this view. 



A particularly favorable method of studying the 

 problem is presented by the cultivation of epithelial 

 cells in some nutrient medium outside the body. The 

 writer has employed this method in the study of the 

 epithelium of larval and adult amphibians. It was 

 found that a piece of epithelial tissue kept in a hang- 



FIG. 5. Strands of epithelial cells extending along and between 

 fibers of cotton, tt and t' t'. The dark part represents the 

 outgrowth from a piece of a frog tadpole cultivated in lymph. 



ing drop culture soon showed a fringe of flattened 

 cells extending into the culture medium. In some 

 cases the area of the extending tissue was over 

 twenty times that of the implanted piece. Exam- 

 ining with a high power of the microscope the mar- 

 gin of the epithelial extension, numerous very fine 

 processes or pseudopodia were seen extending in the 

 direction of movement. These processes were seen 

 to be in constant change. They are the active agents 

 in causing the sheet of cells to be pulled out, for they 

 are both adhesive and contractile. The effect of 



