SECT. 7] OF EMBRYONIC DEVELOPMENT 969 



per cent, for development up to the time of disappearance of the 

 external gills. Now it is evident that, in order to get a true efficiency 

 value for any embryo that has a prolonged post-natal yolk-sac or 

 "autophagic" period, the term "finished embryo" can only be 

 applied to the young organism at the end of this time. Since the 

 energy contained in envelopes, excreta, etc., may for the present 

 purpose be regarded as neghgible, and since at no time can embryo 

 be separated from yolk, the efficiency is given at any moment by the 

 amount of energy in the larva (i.e. the whole system) expressed in 

 per cent, of the amount of energy present in the whole egg at the 

 beginning. Absorption may be regarded as having taken place in- 

 stantaneously at fertilisation. Therefore an efficiency of 82 per cent, 

 by hatching simply means that 1 8 per cent, of the original energy 

 has been lost by combustion, and an efficiency of 75-5 per cent, by the 

 time of disappearance of the external gills means that 24-5 per cent, 

 has been lost by that time. As in each case the " embryo" is partly 

 embryo and partly yolk, these figures do not mean that the earlier 

 periods of development in the frog have higher efficiencies than the 

 later ones; on the contrary, they mean nothing. Happily, Faure- 

 Fremiet & Dragoiu followed the development of Rana temporaria right 

 through to the end of the autophagic period with the bomb calori- 

 meter, and their figure corresponds well enough with that found by 

 Tangl for the chick and with other work. 



7-4. Real Energetic Efficiency 



Terroine & Wurmser, in an important paper on the energy rela- 

 tions of growth, introduced certain new conceptions into the subject. 

 They defined the "rendement energetique" analogously to the 

 "plastic efficiency coefficient" as: 



Energy laid up in the organism 

 Energy in the raw _ Energy in the raw materials ' 



materials at zero hour at the end of development 



U' 



which is only another way of writing 



Energy stored Energy stored 



Energy absorbed Energy stored + Energy in soHd burnt ' 



62-2 



