The Influence of Environment 301 



demand. 1 This constancy is so great that the rate 

 of heart-beat of these embryos could in fact be used 

 as a rough thermometer. The influence of tempera- 

 ture upon the rate of heart-beat is completely reversible 

 so that when we measure the rate for increasing as well 

 as for decreasing temperatures we get approximately 

 the same values as the following table shows. 



TABLE XIV 



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 Fun- 

 dulus the heart-beat is a function almost if not exclu- 



• Loeb, J., and Ewald, W. F., Biochem. Ztsckr., 1913, lviii., 179. 



