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THE AMERICAN NATURALIST [Vol. XLIX 



of certain ganglia cells when other tissues would survive. 

 But neither of these facts prove that under favorable con- 

 ditions very much differentiated cells, like ganglia cells, 

 might not have the power to live indefinitely, although they 

 have lost the power to multiply. Sensitized connective 

 tissue cells of the uterus may after transplantation into 

 the same or another individual in which corpus luteum 

 substance is circulating grow very energetically and pro- 

 duce placentomata, while after transplantation of fully 

 developed deciducomata no further growth can be ob- 

 tained and all or almost all the cells die. Here also the 

 fully "differentiated" cells have lost the power to multi- 

 ply and at the same time they have apparently become 

 more sensitive to the effect of injurious influences than 

 young and yet undifferentiated predecidual cells of the 

 uterine mucosa. 



But here we can observe that some strands of fully 

 differentiated placentoma tissue may survive even under 

 those unfavorable conditions — without, however, resuming 

 growth or returning to the undifferentiated condition — 

 namely, such strands of tissue as are situated under the 

 best environmental conditions, in close proximity to the 

 host tissue, at places most accessible to the foodstuffs or 

 oxygen supplied by the circulating blood or by the peri- 

 toneal fluid. This suggests that even much differentiated 

 cells which have lost their power to propagate may still 

 have the power to live, when kept under favorable condi- 

 tions, and that their death is the result of unfavorable 

 environmental influences. Thus we must at least admit at 

 the present time the possibility that also the ganglia cells, 

 while they do no longer multiply, may still possess a 

 potential immortality; that cellular differentiation pre- 

 cludes the latter possibility has as yet not been demon- 

 strated. We must therefore sharply distinguish between 

 the power of cells to grow and their power to live ; while 

 the former seems to be destroyed through differentiation 

 — at least in some cases — the latter may still exist. In 

 the case of other tissue cells we have it to a certain 

 extent in our power through experimental conditions 



