190 WARREN H. LEWIS AND MARGARET R. LEWIS 



The foregoing conclusions of Champy and the less general 

 conclusions of Maximow in regard to the fate of endothelium 

 and mesothelium are certainly in need of further substantiation. 

 During the process of regeneration, in vertebrates at least, the 

 dedifferentiation never proceeds to an indifferent stage; muscle 

 is regenerated from muscle, nervous tissue from nervous system, 

 bone or cartilage from bone or cartilage, ectoderm from ecto- 

 derm, etc. 



In prolonged cultivation by means of frequent retransplanta- 

 tion of the culture such as was carried on first by Carrel, the fibro- 

 blasts seem to be the only cells which survive so that finally they 

 are obtained in pure cultures. It is probably that both Champy 

 and Maximow failed to realize that it is a question of the sur- 

 vival of the fittest and not complete dedifferentiation which is 

 responsible for the appearance in cultures that have been carried 

 on for many generations of but a single type of cell. Then too 

 we must bear in mind the fact that even in the early stages of 

 cultivation there is often great difficulty in distinguishing the 

 various types of cells. 



We are more especially concerned in this prehminary and es- 

 sential process of dedifferentiation. That it should take place 

 in a minute isolated piece of muscle outside the body, in an 

 artificial medium, is of great significance. It makes possible 

 an analysis of the process in a way that was not reahzable in the 

 living organism. Attempts to get growth and regeneration 

 from small pieces of muscle (one-half to one centimeter in diam- 

 eter) in vivo have failed. Such pieces even when transplanted 

 into muscle itself always degenerate (Volkman). It may be 

 that pieces as small as those used in tissue cultures would have 

 continued to live in vivo. 



The nature of the changes in the organization of the cells of 

 tissue cultures undoubtedly depends in part on the tissue ex- 

 planted, in part on the age of the embryos or animal employed 

 and in part on the culture medium and the peculiar conditions 

 to which the cultures are subjected. Tissues of late fetal stages 

 or of stages subsequent to birth in which differentiation is com- 

 plete could remain either stationary or dedifferentiate, while 



