420 ON INCREASE IN SIZE [pt. m 



It may further be argued that weighing is a very simple process, 

 and it is, therefore, difficult to see why errors should arise which 

 should reflect themselves in these rhythmic curves. A greater degree 

 of scepticism would be justified if they were the results of a com- 

 plicated estimation method for a chemical substance. But, as it is, 

 these curves form perhaps the best evidence which at present exists 

 for the applicability of the Ostwald-Robertson view to embryonic 

 growth. In the absence of a really exhaustive statistical investigation 

 of the growth of the chick in the egg, these rhythmic curves must be 

 accepted for what they are worth. They would be more convincing 

 if all the workers had found peaks in the same places, but the 

 variation which exists is no argument against the reality of the 

 phenomenon in view of the fact that different breeds of hen were 

 used. Further work is greatly needed to clear up this question. If the 

 peaks on the daily increment curve do turn out to be real, it may be 

 possible to relate them to the peaks of normal mortality which Payne 

 and others have studied, and which will receive further consideration 

 later. (See Fig. 443, Section 18-2.) 



The autocatalytic curve has also been found by Robertson to fit 

 the data of Stotsenberg already referred to for the growth of albino 

 rat embryos, and a peaked curve is obtained when the daily incre- 

 ments are plotted against time. But, as will be seen, Brody's ex- 

 ponential formula also fits these data, and it is probably right to 

 conclude, as McDowell and his collaborators do, that the figures are 

 not sufficiently good to allow us to distinguish between the two 

 formulae. They cannot be regarded as supporting, therefore, any 

 particular theory of embryonic growth. 



2-8. Instantaneous Percentage Growth-rate 



Brody introduced still another way of representing the facts. He 

 defined the "genetic growth constants" of animals as being the 

 same within each genetically identical group of animals, and as 

 corresponding to specific velocity constants and equilibrium constants 

 in chemical actions in vitro. It may be seen from Fig. 49 taken 

 from Brody's paper that the mature weight of the animal in 

 its life-span. A, is approached by successively decreasing gains in 

 weight after the point of inflection of the sigmoid curve has been 

 passed. The velocity of growth, therefore, declines in a geometrical 

 progression with age. The normal animal reaches, as Brody puts it. 



