---------------+-------------------------------------------- _Temperature_ | _Time Required for Nineteen Heart-beats in | the Embryo of Fundulus_ ---------------+-------------------------------------------- °C. | _Seconds_ | 30 | 6.25 25 | 8.5 20 | 11.5 15 | 19.0 10 | 32.5 5 | 61.0 10 | 33.5 15 | 18.8 20 | 12.0 25 | 10.0 30 | 6.0 ---------------+-------------------------------------------- Why does each embryo have the same rate of heart-beat at the same temperature in contradistinction to the enormous variability of the same rate in man? The answer is, on account of the elimination of all secondary disturbing factors. In the embryo of _Fundulus_ the heart-beat is a function almost if not exclusively of two variables, the mass of enzymes for the chemical reactions underlying the heart-beat and the temperature. By inheritance the mass of enzymes is approximately the same and in this way all the embryos beat at the same rate (within the limits of the fluctuating variation) at the same temperature. This identity exists, however, only as long as the embryo is relatively quiet in the egg. As soon as the embryo begins to move this equality disappears since the motion influences the heart-beat and the motility of different embryos differs. In man the number of disturbing factors is so great that no equality of the rate for the same temperature can be expected. Differences in emotions or the internal secretions following the emotions, differences in previous diseases and their after-effects, differences in metabolism, differences in the use of narcotics or drugs, and differences in activity are only some of the number of variables which enter.