SECT. 6] 



OF THE EMBRYO 



935 



Plastic efficiency 

 coefficient 

 O Gray : Cumulative 

 © Needham : Incremental 



and naturally shows the relative cost in grams of soHd of building 

 the embryo. The higher the efficiency coefficient, the smaller the 

 amount of burnt substance in relation to stored substance. 



Gray, in his memoir on the chemical embryology of the trout, 

 already referred to, found that its average plastic efficiency coefficient 

 (P.E.C.) was 0-63. He worked it out for the chick from Murray's 

 data in a cumulative way, but a more instantaneous picture is given 

 when it is calculated on a daily basis, as I showed in 1927. How 

 expensive is it on each day of development to build what is built on 

 that day? It is evident from Fig. 254 that both curves fall and then 

 rise, and the lag in the cumula- 

 tive one is not significant, for p.^.c. 

 each day's point bears, as it 

 were, in itself the effects of the 

 previous days. The incremental 

 P.E.C. shows the instantaneous 

 change. 



There must be some signifi- 

 cance in the deep trough through 

 which the curve passes between 

 the 7th and 1 2th days. Evidently 

 at that period development is 

 most expensive; the amount of 

 burnt substance is greater re- 

 latively to the amount of stored 

 substance then than at any other 

 time. This suggests a correlation 

 combustion, which is indeed very 

 as may be seen from the vertical line 

 that we have here to deal with 

 action is difficult to resist 

 responsible. 



The average P.E.C. for the whole of the chick's develop- 

 ment is 0-68. It is interesting to enquire which of the food-stuffs 

 contribute principally to this degree of efficiency. Knowing that fat 

 is the chief food-stuff combusted, and that protein is the chief archi- 

 tectural material, it would be natural to predict that the most 

 efficiently stored substance would be protein. The exact figures 

 follow. 



Days-* 5 



Fig. 254. 



with the intensity of protein 

 exact — in fact, strikingly so, 

 in Fig. 254. The inference 

 an effect of specific dynamic 

 but probably more than one factor is 



