GROWTH AND SENESCENCE 59 



ing because of the inhibiting effect of the lactic acid, or ulti- 

 mately, because of the finite nature of the milk container 

 (which is the universe for these bacteria). If the receptacle 

 holding the milk were indefinitely large the concentration of 

 the acid would always be insignificant, and the bacteria could 

 go on growing at a constant percentage rate indefinitely. The 

 same general reasoning probably applies to the limited period 

 of growth in the tissue in the living chicken as contrasted to the 

 indefinite period of growth of the same tissue when grown in 

 vitro by the method of continued irrigation developed by Carrel 

 and associates. 



3. Why should the body grow at a constant percentage rate in 

 the earlier periods of its life history? Because of the tendency 

 of cells to reproduce at a constant percentage rate, that is, at 

 equal intervals of time, when all other conditions remain the 

 same. 



4. Why should there be breaks in the growth curves of higher 

 animals corresponding to metamorphosis in lower animals (cf. 

 Figures 37 and 39)? Because certain growth-inhibiting influ- 

 ences exceed certain threshold values. Thus the milk-souring 

 organisms, B. lactis acidi, grow at a constant percentage rate until 

 the acidity designated by pH 4.7 is reached 5 pH 4.7 is, there- 

 fore, the threshold value for this type of organism in milk. In 

 multicellular animals the nature of the threshold mechanisms 

 is less understood, but mechanisms of the nature of hormonic 

 action may be concerned in the process. Thus there is a species 

 of salamander in Mexico which does not normally metamor- 

 phose. Recently specimens of this salamander were fed with 

 thyroid substance and thus brought about metamorphosis. Ac- 

 cording to Gudernatsch, metamorphoses in certain tadpoles 

 which normally occur during the third or fourth month of life 

 can be brought about at will almost at any age by feeding thy- 

 roid substance. 



5. Following puberty, the percentage rate of growth declines 

 constantly. All the buffer mechanisms neutralizing the effects 



