144 MORPHOLOGIC VARIATION 



and cold and salt and phenol, than are resting cultures; that is, 

 there is with bacteria a physiologic youth that can be demonstrated 

 by the same means as physiologic youth in a growing animal or 

 plant. 



I have also shown previously that cultures which are lightly 

 seeded develop a higher growth rate and a greater degree of mor- 

 phologic variation in the embryonic cells than cultures which are 

 heavily seeded, and have compared this with the higher growth 

 rate in regenerating tadpoles' tails when only a small stump is left. 

 It would appear, therefore, that whether measured by degree of 

 morphologic variation in the cells, by the actual rate of cell division, 

 or by physiological processes, such as the susceptibility to poisons, 

 the growth rate both of free populations of cells, and of multicellular 

 organisms is dependent upon the relative volume of protoplasm which 

 is in the process of growing. 



While there have been made some quantitative studies of the 

 nucleo-cytoplasmic ratio of cells at different stages of growth in 

 developing animals, I am not aware of any quantitative investiga- 

 tions of cytomorphosis in multicellular organisms comparable with 

 those which I have presented for bacteria, by which the degree of 

 change in the cells can be directly correlated with the rate of growth. 

 There is, however, ample indirect evidence that such a correlation 

 exists. Minot noted that the "mitotic index," i.e., the percentage 

 of cells in mitosis (serving as a measure of rate of cell division), 

 steadily decreases with increasing differentiation, and that the rate 

 of differentiation becomes increasingly slower with age as does also 

 the growth rate. Child observed that the degree of differentiation 

 or dedifferentiation of the cells was correlated with the metabolic 

 rate as determined by susceptibility, which in turn was correlated 

 with the rate of growth. It would seem, therefore, not improbable 

 that the transition from embryonic to differentiated cells in multi- 

 cellular organisms is determined by a change from positive to nega- 

 tive acceleration in growth rate. A quantitative determination of 

 such a correlation, however, would present considerable technical 

 difficulties. 



Minot predicted that with further study, cell cycles in one- celled 

 organisms, protozoa, would be found to be of the same nature as 



