EPIDERMIS WHEN CULTIVATED OUTSIDE BODY 285 



In several cases a part of the epithelial outgrowth of a piece of 

 tissue was cut off and transferred to another drop of the culture 

 medium. The act of cutting causes a violent contraction of the 

 epithelial outgrowths, and the operation usually has to be done 

 quickly to be successful. A broad and very thin sheet of cells 

 may shrivel up, after it is cut off, into a small rounded mass 

 that has very little resemblance to its previous condition. Sub- 

 sequently these isolated pieces may spread out as widely as before. 



The application of an unfavorable degree of heat causes a 

 contraction of the epithelium quite aside from any stimulating 

 influence of a change of temperature. Often the areas of epi- 

 thelium may be broken up in this way into isolated masses of 

 cells or even into individual cells. The influence of heat on 

 epithelium is very similar therefore to its effect on the blasto- 

 meres of a dividing egg. The cells or cell masses tend to become 

 rounded up and inert. Light has very little effect on the epithe- 

 lial cells. If the heat rays are filtered out, epithelium may be 

 exposed for hours to the most intense light without manifesting 

 any evident reaction. 



In general one may say that epithelial cells, like so many free 

 organisms, respond to various unfavorable influences by con- 

 traction. When kept for several days in the same culture medium 

 the epithelial cells show a tendency toward rounding up. This 

 often results in the rupture of strands of cells, or the breaking 

 up of sheets of cells into isolated masses. Under favorable 

 conditions epithelial cells rarely isolate themselves from the 

 general mass. It is probably the accumulation of products of 

 excretion that causes the contraction of cell masses which are 

 kept too long in the same medium. The breaking up occurs 

 much more quickly in cultures kept at room temperature than 

 in those kept in a cooler place, owing probably to the more rapid 

 metabolism and the greater accumulation of toxic products. 

 That it is not the injurious influence of room temperature alone 

 that causes the contraction of the cells is shown by the fact that 

 by washing the contracted masses in Ringer's solution and giving 

 them a fresh supply of the culture medium, the epithelial cells 

 may again form extensions at ordinary room temperature. 



