156 University of California Publications in Zoology [Vol. 11 



layered arrangrement, through the various foldings, invaginations 

 and outpushings that play so prominent a part in the formation 

 of organs in more advanced stages of development. While the 

 direction of cell division, and His's principle of unequal growth 

 are factors which may be influential in the production of these 

 layered structures, it is evident that the appropriate behavior 

 of the component cells is a factor of essential importance ; and 

 in certain cases at least it can be shown that, in the absence of 

 cell division and any notable amount of growth, the behavior 

 of the cells alone may lend to the formation of a membrane-like 

 layer. The formation of epithelial layers is a factor of such 

 general occurrence in embryonic processes that any study of the 

 behavior of epithelial cells that will throw light upon the mech- 

 anism of this formation cannot fail to be of importance from the 

 standpoint of causal morphology. 



While the extension of epithelial layers over abraded surfaces 

 or other solid substances has often been spoken of as if it were 

 the result of cell multiplication or the passive shoving along of 

 cells as a consequence of pressure, many studies have made it 

 evident that the epithelial cells play an active rather than a 

 passive role in this process. Active wandering of epithelial cells 

 has been described in different animals as due to a sort of amoe- 

 boid movement more or less like that which occurs in leucocytes 

 or certain cells of the mesenchyme. In a recent extended paper 

 on the subject, however, Oppel has come to the conclusion that 

 while the movement of epithelium depends upon the activities 

 of the cells themselves it is not an amoeboid motion. 



The experiments of Oppel (1912b) were performed on various 

 kinds of epitheliiun from the dog, cat and rabbit. Small pieces of 

 different organs were freed from epithelium over a part of their 

 surface by scraping off the cells with a sharp scalpel. The pieces 

 of tissue were then placed in blood plasma, incubated in a warm 

 oven, and the progress of overgrowth of the denuded surface 

 was followed partly by direct observation of the living tissue, 

 but mainly by means of sections cut at different intervals of time 

 after the preparations were made. Soon after the tissues were 

 placed in plasma the epithelial cells near the cut surface began 

 to extend upon the denuded connective tissue substratum, often 



