CYTOMORPHOSIS 37 



you many other examples of completed cytomorphosis of the 

 most various cells. It might, however, be better to pass 

 over to other considerations. 



Death and subsequent removal of cells play a great role 

 in our lives. Even in early developmental stages we find cells 

 dying and even whole organs which maintain themselves only 

 for a certain period and then disappear almost or completely. 

 Thus there is an embryonic kidney in which only small re- 

 mains can be found in the adult. It is therefore clear that 

 there must be some arrangement provided to make good the 

 loss of cells. Nature accomplishes this by not bringing all 

 cells to further development and by preserving a stock of less 

 differentiated cells in the body. Of these I have already 

 mentioned an example, the epidermis, the cells of the .under 

 layer of which preserve an embryonic character. Only by the 

 presence of these cells which keep the essential embryonic 

 type is the continual renewal of the epidermis made possible. 

 For every hair there remains a special group of embryonic cells 

 upon the hair papilla, which provide for the growth of the 

 hair. Since these cells are not differentiated, they can multi- 

 ply and thus furnish cells for the formation of the hair. The 

 cells of the hair complete their cytomorphosis, but their sister 

 cells remain on the papillae undifferentiated. We thus see 

 that while cytomorphosis can go on only in the one direction, 

 it remains true that the cytomorphosis can be arrested and 

 that it may go on in the different tissues with unequal rapidity. 

 Thus it comes that we encounter cells in the adult animal in 

 every possible stage of cytomorphosis. There arise in every 

 one of us every day cells which complete their cytomorphosis, 

 and there are others which have hardly begun it. We cannot 

 understand the relations in the adult animal if we do not 

 consider at once both the daily dying off of old cells and 



