I was not entirely comfortable with emphasizing that C and especially D were constants at growth rates between 1 and 3 doublings/h at37◦C (although that turned out to be a reasonable and useful concept), but I was convinced of the “ICD” idea. I was also gratified that the model explained the previously mentioned “rate maintenance” phenomenon, since it stipulated that the rate of cell division would be expected to remain unchanged for C + D min after a shift-up (Helmstetter et al., 1968). Plus, being a novice physicist, I saw the possibility of using this simple idea to develop quantitative expressions for the chromosomal DNA contents of cells in exponentially growing cultures. I was not particularly skilled in calculus but I loved the challenge.