SECT. 7] 



OF EMBRYONIC DEVELOPMENT 



949 



also, of course, osmotic work and secretion work to be considered. 

 I shall return later to this point; meanwhile, it is necessary to pene- 

 trate further into the facts concerning "Entwicklungsarbeit". The 

 terminology presents difficulties, but the method adopted will be to 

 speak of the "Entwicklungsarbeit" in Tangl's sense as Ea. and of 

 the true "Entwicklungsarbeit", if there is any such thing, as O.E. 

 or organisation energy. Thus Ea. will be defined as the amount of 

 energy not stored in the em- 

 bryonic tissues and not left be- 

 hind in the unused raw materials 

 at the end of development, while 

 O.E. will be defined as the 

 amount of energy, if any, laid 

 up in the embryo, which, though 

 appearing as calorific value of 

 combusted wet tissues, would 

 not result from the combustion 

 of an unorganised mixture of its 

 constituents^. If all the con- 

 stituents of the finished em- 

 bryo could be mixed together 

 mechanically and the mixture 

 then combusted, the calories 

 contained in it might be slightly 

 fewer than those contained in 

 the same substances when or- 

 ganised into an embryo. This difference is the O.E. These concep- 

 tions are illustrated by the diagram in Fig. 259, to which reference 

 should be made. 



Tangl's first experiments, then, showed that the Ea. of the starling's 

 egg was 755 caL, i.e. 24-6 per cent, of the original amount of energy 

 present, corresponding to a loss of dry weight during development 

 of 15-7 per cent. 



His next experiments were done on hen's eggs of three races, 

 all of which had dry weights when unincubated varying between 

 24-3 and 24-9 per cent., and calorific values of between 6906 and 

 7078 cal. per gram, dry substance, and between 1692 and 1734 cal. 

 per gram wet substance. The whole egg contained 88-9 Cal. What 



1 Definitions of this and other terms will be found summarised together on p. 999. 



Days -^ 5 



Fig- 257- 



