422 



ON INCREASE IN SIZE 



[PT. Ill 



Finally, B is the difference between the mature weight of the 

 animal and the weight the animal would have had at concep- 

 tion (a minus quantity) if the whole of growth was represent- 

 able by the curve for the autostatic or self-inhibiting phase. This 

 genetic growth-constant also was found for many animals by 

 Brody. From Fig. 50 it can be seen that, the higher the value of k, 

 the more rapidly the mature value is approached (pigeon > mouse 

 > rat > guinea-pig > sheep > pig > cow > man), and that the fact 

 can be equally well accounted for on the assumption that a substance 



Yrs.cS^ 



Age (from blrbh) man 

 7 8 9 10 11 12 



Mos.a' 



60 64 68 72 76 



12 16 20 24 28 32 36 40 44 48 52 66 

 Age (from conception) of animals 



OoJ 



Fig. 50. 



is used up during growth, or on the assumption — perhaps more 

 likely in view of the work of Carrel and his collaborators on tissue 

 culture (reviewed by Pearl) — that during growth a growth-retarding 

 substance is produced according to the law of monomolecular change. 

 The paper of Brody, Sparrow & Kibler was concerned with age 

 equivalence. They showed that, with the aid of the formula pre- 

 viously established by Brody, 



W=^ A- Be-""^ 



(where W is the weight at age t, A a. genetic growth-constant, the 

 mature weight, B another genetic growth-constant which increases 

 in value with increase in length of the processes preceding the point 



