188 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



and Necturus skin was similarly affected by frog plasma. But skin of 

 Necturus, as well as of Triturus, grew well in plasma and serum of Necturus. 

 It may then be concluded that the differences in the reactions against other 

 than autogenous tissues are much less when the tissues are grown in homoi- 

 ogenous or heterogenous serum or plasma, than when they are placed in 

 homoiogenous or heterogenous living hosts. However, it must not be con- 

 cluded from these and other similar experiments that no differences exist, 

 as far as tissues growing in vitro are concerned, between the effect of 

 homoiogenous and heterogenous media; results obtained by Hitchcock al- 

 ready suggest that such differences do exist. Likewise, experiments with 

 mammalian tissues indicate that homoiogenous plasmas and sera are pre- 

 ferable to heterogenous ones, although the admixture of heterogenous tissue 

 extracts to such media seems not to interfere seriously with the life and 

 growth of tissues under these conditions. Thus it has been possible for fibro- 

 blasts from the subcutaneous tissue of the adult mouse to grow actively for 

 many successive generations in a culture medium of chickenplasma, chick 

 embryo extract and horseserum without serious interference with the pro- 

 liferation, motility and structural potentialities of these cells. 



There exist various differences between tissues living in their normal 

 environment, tissues transplanted into other living organisms, and tissues 

 cultivated in vitro. In tissue culture, the aggressive action of host cells which 

 attack the grafts is eliminated; in vitro the tissues are exposed merely to 

 the action of homoiogenous or heterogenous organismal differentials, con- 

 tained in the bodyfluids, and the toxic effect produced on them by the latter 

 is less than when they are transplanted into living hosts. 



As to the conditions which render these bodyfluids less injurious in tissue 

 culture: (1) One factor is probably the small amount of blood plasma or 

 serum present in the culture media, which contains the toxins, as compared 

 to the continuous current of fluid carrying fresh supplies of homoio- and 

 heterotoxins to the transplant in the living body. Such a condition may be 

 active also when the homoiogenous plasma of an animal which had proved 

 to be immune to the growth of a certain tumor, is used as a culture medium 

 for a piece from the same tumor growing in vitro; it does not prevent the 

 growth of the tumor; under these circumstances, the amount of homoio- 

 toxins present at a certain time is presumably insufficient. (2) A second factor 

 concerns the growth momentum of cells in vitro. Cells growing in vitro are not 

 components of an ordinary, relatively resting tissue; they are very actively 

 growing and are either of embryonal origin or are derived from adult cells, 

 or, they may be cancer cells. Both embryonal cells and cancer cells are under 

 the influence of factors which stimulate them to grow continuously, while 

 cells derived from adult cells, being separated from their normal environ- 

 ment, are continuously regenerating. In all these types of cells the growth 

 momentum is increased, and furthermore, it is possible that in the case of 

 the embryonal cells the individuality differentials may not yet be completely 

 developed. Such an increase in growth momentum makes it possible for 

 these cells to overcome difficulties to which other cells might succumb; in 



