Leo Loeb and E. M. Strong 277 



crease in the number of mitoses could be seen. We have made the same 

 observation in the case of wounds in frog skin. The movement of 

 epithelium is not limited to one layer of cells but involves several. The 

 lower portions of the cells in the deepest layer of the epidermis move 

 first toward the wound, and the cell-axes are occasionally rotated almost 

 as much as 90° in the process, so that they come to have a horizontal 

 position in place of the former vertical direction. This movement of the 

 epithelium begins during the first hours after the operation and is com- 

 pleted within one to three days. There is no distinct increase in the 

 number of mitoses, however, until after the second day. 



The rapidity with which the epithelium moves over the wound after 

 operation, together with the absence of any increase in the number of 

 cell divisions normally occurring, indicates that the movement is not 

 due to cell-proliferations " but to other causes. 



Some observations made by L. Loeb on regenerating tadpole epithe- 

 liimi seem to indicate considerable tension in the regenerating epithelium 

 which may be more or less responsible for the movements. He found 

 papillae which had ben formed apparently through the folding of the 

 upper layers of regenerating epidermis, though they may have been the 

 result in some cases of a degeneration of epithelial cells. These papillae 

 may occur within a few hours after the operation or a few days later. 



The changes determining the movements aft'ect the cells nearest the 

 wound first, but are later extended to the epithelium farther away. The 

 epithelium moves only in contact with solid bodies. 



Mitoses occur not only in the deepest layers of the epidermis but also 

 as high as the fifth or sixth layers, whereas in regenerating guinea-pig's 

 skin, they were found almost exclusively in the two lowest layers. 



Within forty-eight hours after the wound is made hypertrophy is seen. 

 Individual cells enlarge and become more and more numerous. 



We almost invariably find degeneration of epidermal cells connected 

 with this hypertrophy. Such degeneration is also seen in the tissue 

 which has advanced farthest over the wound, and is also found in the 

 deepest layer of the epithelium in cases where many leucocytes penetrate 

 this tissue. The nucleus of degenerating cells usually becomes kayor- 

 rhectic and the cytoplasm homogeneous, staining well with eosin. The 

 degenerative changes accompanying the hypertrophy may be found even 



^ The proliferation of cells in the regenerating epithelium takes place both 

 by mitosis and amitosis. Amitosis was first described for regenerating mam- 

 malian epithelium by L. Loeb, 98; later by Marchand, 01; and Werner, 02; 

 and Nussbaum, 82, has observed amitosis in regenerating epithelium in the 

 cornea of amphibia. 



