GROWTH AND SENESCENCE 



53 



the self-accelerating phase of growth (the phase preceding pu- 

 berty in animals and flowering in plants). 



Robertson was probably the first to attempt a rational quanti- 

 tative analysis of the growth curve, including the self-acceler- 

 ating phase of growth. Space does not permit to give a detailed 

 exposition of Robertson's work on growth and of the critical 

 literature that it has brought forth. I shall, therefore, take the 

 liberty of explaining the situation as I see it in as brief a manner 

 as I can, referring the reader for details to Robertson's mono- 

 graph on The Chemical Basts of Growth and Senescence, and 

 to a paper in the /. Gen. Physiol. , 1926, VIII, 463 j also to two 

 papers by Davenport, one on "Human Metamorphosis" in Am. 

 J. Physical Anthropology, 1926, IX, 205, and another on "The 

 Human Growth Curve" in /. Gen. Physiol., 1926, X, 205. 



It is well known that certain kinds of animals (for example, 

 insects and amphibians) pass through several distinct stages of 

 growth separated from each 

 other by changes known as 

 metamorphoses. Everyone has 

 heard, for example, of the 

 metamorphosis of a maggot 

 into a fly or of a tadpole into 

 a frog. It is legitimate to ask 

 the question whether warm- 

 blooded animals likewise pass 

 through stages of growth 

 more or less distinct, analogous 

 to metamorphosis in cold- 

 blooded animals. In 1908 

 Robertson published a paper 

 in which he showed that warm-blooded animals do, in fact, 

 pass through several stages of growth, which he termed growth 

 cycles. He analyzed these cycles mathematically from the point 

 of view of the physical chemist, and he came to the conclusion 

 that each of these cycles is limited by an autocatalytic mono- 



Figure 35. The theoretical curve of Robert- 

 son's cycle. The axis of ordinates represents 

 time rates of growth, that is, gains in 

 weight per unit time. The axis of abscissae 

 represents ages. According to Robertson, 

 there are at least three cycles (infantile, 

 juvenile, and adolescent), of similar shape 

 in the time rate curves of higher animals. 

 Each cycle is, according to Robertson, 

 (theoretically) symmetrical about its center. 



