EPIDERMIS WHEN CULTIVATED OUTSIDE BODY 283 



forty-eight hours it may be five or six times as great. Usually 

 the ectoderm extends in the form of broad sheets, but some- 

 times narrow strands are seen which often branch, and occa- 

 sionally some of the branches may meet and fuse with those of 

 other strands. In many cases most of the epithelium had 

 migrated away from the implanted piece, but the latter was in 

 all cases covered by a thin layer of this tissue. It is common 

 for outgrowths to appear both in contact with the cover slip 

 and with the lower surface of the hanging drop, and they are 

 about as frequent in the one situation as in the other (figs. 1 and 

 2). During the period of active extension the advancing epi- 

 thelium is always furnished with an amoeboid border of very 

 clear protoplasm as has been described by Harrison. ^ This 

 border varies much in width and is often so exceedingly thin and 

 transparent that it is very difficult to follow its course. The 

 very fine processes that are sent forth are mostly in close contact 

 with the substratum, and a number of observations make it 

 evident that they possess considerable adhesiveness. The con- 

 clusion reached in my previous paper that the epithelial exten- 

 sions are due to the amoeboid activity of their hyaline margin 

 is confirmed by the discovery of several additional facts. 



It was found that the epithelial membranes possess a re- 

 markable degree of contractility. With the application of a 

 stimulus a large epithelial extension may shrivel up to about 

 one- tenth its original area. Often a very broad extension sev- 

 eral times the area of the tissue from which it came may con- 

 tract to a very narrow fringe, giving one the impression that 

 to a large extent it went back to its original situation. The 

 contraction starting at any one point may be seen to spread to 

 surrounding areas until finally the whole mass may be involved. 

 At other times only a small part of the sheet of ectoderm may 

 draw back. In either case the margin of the retracted ectoderm 

 becomes much thicker and rounded, the amoeboid processes 

 disappearing. If the amoeboid margin is watched carefully in 

 an epithelial extension that is just beginning to contract the 



^ Harrison, R. G. The outgrowth of nerve fiber as a mode of protoplasmic 

 movement. Jour. Exp. Zool., vol. 9, 787-848, 1910. 



