i86 



Studies in Animal Behavior 



their activity is to cause epithelium to spread more 

 and more widely, and in several preparations that 

 were made, practically all of the available solid sur- 

 face was covered by an investment of epithelium. If 

 individual cells are isolated they frequently spread 

 in every direction until they become flattened to 

 an extreme thinness, and I have observed scattered 

 individual cells flatten out until the adjacent cells 



FIG. 6. Successive stages in the advance of a strand of epithelial 

 cells from a frog tadpole. This represents on a more enlarged 

 scale the end of the strand lying between the two fibers of 

 cotton t and t' shown in Fig. 5. The dotted line indicates a 

 fixed position. 



met, forming a continuous membrane, scarcely dis- 

 tinguishable from certain epithelial membranes found 

 in the body. The marked thigmotaxis of these cells 

 leading them to spread over surfaces and to keep 

 in close contact with one another is a very im- 

 portant factor in bringing about the layered arrange- 

 ment characteristic of epithelial tissue, as well as in 

 causing this tissue to form a coating over the ex- 

 posed surfaces of nearly all organs of the body. 

 Epithelial membranes usually form surfaces of se- 

 cretion and absorption which separate regions of 



