292 S. J. HOLMES 



The behavior of epidermal epithehum in vitro serves to throw 

 hght on several features of the activities and functions of epi- 

 dermis in its normal situation on the body. It is well known that 

 abraded or cut surfaces are quickly covered by an epithelial 

 layer. The chief factor in this process, as Oppel and Osowski 

 have contended, is epithelial migration and not the growth or 

 the mere passive movement of epithelial cells. Surfaces may 

 be covered with little or no cell multiplication, but simply by 

 the wandering of epithelium. The cells of the epidermis have an 

 inordinate tendency to lateral spreading. They have a strong 

 tendency to cling to their own kind of cells, and under normal 

 conditions seldom become isolated from their neighbors. The 

 adhesiveness of the newly formed hyaline pseudopods serves 

 to give a point of attachment, and as the hyaline border advances 

 the epidermal cells are pulled out more and more. Various 

 observations have shown that the epithelial membranes formed 

 were drawn out despite a certain contractilit}^ of their substance. 

 The fact that as soon as the attachment of the pseudopods is 

 broken the membrane draws back and begins to shrivel up indi- 

 cates that the real seat of activity responsible for pulling out the 

 sheets or strands of cells is the amoeboid movement of the ad- 

 vancing margin. The extending of this margin often causes 

 the cells to become exceedingly thinned out. The membrane 

 tends to creep out until the advancing margin comes into contact 

 with other epithelial cells, thereby forming a continuous covering 

 of the exposed surface. 



The strong tendency of the epidermal cells to lateral spread- 

 ing insures the formation of a continuous membrane; this is an 

 important property in relation to several functions, and especially 

 those related to osmosis. Studies made on the peritoneal cells of 

 amphibians showed that the behavior of these cells has much in 

 common with the behavior of the cells of the epidermis. There 

 can be little doubt that the peculiarities of arrangement of the 

 cells lining the body cavity, the endothelial cells of the blood 

 vessels, and the cells lining various ducts and other passages 

 depend upon certain features of cell behavior. It is quite 

 feasible to study the characteristic features of the behavior of 

 these cells by keeping them isolated in vitro. Considerable light 



