SECT. 7] OF EMBRYONIC DEVELOPMENT 995 



combusted is free, not combined, carbohydrate^. There is already 

 evidence that the power of desaturation of fats only arises at a com- 

 paratively late stage of development, e.g. the loth to the 15th day in 

 the chick (see p. 1 1 7 1 ) . And we may look on the unsaturated fatty 

 acids which are notably present in the yolk (see p. 295) as a pre- 

 paration for these conditions. 



Or it may be that some conception of "ease of combustion " will 

 prove helpful. Quastel & Whetham, studying the action of B. coli 

 on various organic substances, found that carbohydrates were much 

 better hydrogen donators than substances of the protein or fatty 

 type. The following figures, taken from their paper, are striking. 



Reduction coefficient 

 (The reciprocal of the molar concentra- 

 tion required to reduce i c.c. of 1/5000 

 methylene blue in presence of a standard 

 Substance amount of organism in half an hour) 



"Carbohydrate". Glucose ... 5000 



Fructose . . . 5000 



Lactic acid ... 583 



"Protein". Alanine ... ... i 



Glycine ... ... o-8 



Glutaminic acid ... 25 



"Fat". Nonylic acid ... ... 0*4 



Heptylic acid ... ... 0-4 



The succession of energy sources might, then, be related in some way 

 to the changing rH of the embryonic cells or to other important 

 factors in the oxidation-reduction processes going on in them. A study 

 of the oxidation-reduction properties of embryonic cells throughout 

 development would be interesting. Aubel & Wurmser have made 

 some progress in this direction. 



The ontogenetic succession could be either "ovogenic" or "em- 

 bryogenic". On the former view, the energy sources would succeed 

 one another simply because the dynamic equilibria and the relative 

 concentrations of substances in the yolk and white necessitated it. 

 The embryo would play a passive part, combusting protein and fat 

 only since it could not get carbohydrate. On the latter view, the 

 succession of energy sources would be intimately connected with the 

 changing potentialities of the growing embryo. Energy must be the 

 same from whatever source it comes but the embryo — on this view 



1 And yet even here a preliminary combination with phosphoric acid may be 

 necessary. 



