A CHEMICAL EXPLANATION OF THE VARIABILITY OF THE GROWTH RATE.

The general belief that uniform cells under uniform conditions will all multiply at the same moment implies that the smallest units of the chromosomes, i.e., either the genes or the molecules of which the genes are composed, all double at exactly the same moment in all cells. Since the doubling of chromosomes is a synthetic chemical process, it seems more probable that it would follow chemical laws. With the assumption that the corresponding molecules in a number of uniform cells obey the mass law in their process of doubling, a definite order in the multiplication of identical cells is established which can be formulated mathematically for the simplest case. This is the same assumption which the author has used to account for the differences in the order of death between bacteria and higher organisms. This theory demands a great variability of the growth rate of uniform cells, so great that it must be experimentally measurable even for cells with a million molecules to the chromosome. The theory demands further that the frequency curve of cell divisions plotted for successive time intervals, be skewed to the left, and that the relative range of variation become smaller as the number of genes or gene-type molecules increases. Experiments on the growth rate of Bacterium aerogenes and Saccharomyces ellipsoideus showed regularly a frequency curve skewed to the left. The yeast had a relatively narrower range of variability than the bacterium. Even with multicellular organisms, theoretical calculations show a range of variation of the growth rate from the egg cell which should still be measurable though it decreases relatively with the number of cells produced. An experiment on the size of bacteria colonies at different ages of development agreed with the theory.