SECT. II] FAT METABOLISM 1187 



this may be regarded as a consequence of their special needs. Energy 

 is no doubt the same from whatever source it comes, but one source 

 may be more convenient than another. In the hen's egg, according 

 to Murray's data, 6-32 gm. of water are lost from the system during 

 its development, and 2-01 gm. of solid. As 91 per cent, of the solid 

 lost is fat, about 2-1 gm. of "metabolic water" are added to the 

 egg, so that 8-4 gm. of water would have been lost if fat had not 

 been burnt, or 40 per cent, more than what actually is lost. The 

 chick in its closed terrestrial box cannot afford to despise this extra 

 two grams of metabolic water. 



Table 174. 



Fatty acids combusted during 



development in % of the total 

 Aquatic or fatty acids present in the egg 

 Terrestrial at the beginning. (All figures 



It is difficult to ascertain exactly how much fat is combusted by 

 different animals; for as it burns away, leaving no measurable in- 

 combustible residues, its combustion-rate cannot be calculated from 

 them, and no one can say a priori what proportion of the carbon 

 dioxide eliminated comes from fatty acids. Then in some animals, 

 as we have seen, there is a veritable synthesis of fatty acids during 

 embryonic development, obscuring any combustion of them. One 

 could therefore predict that the difference between terrestrial and 

 aquatic embryos would not be so clear-cut in the case of fat com- 

 bustion as it is in the case of protein combustion. However, Table 1 74, 

 constructed in the same way as Table 162 in Section 9-15, shows 

 that embryos do divide into the two classes according to their en- 

 vironment in this case also. Terrestrial eggs burn between 60 and 

 70 per cent, of their initial fat stores, and aquatic ones only about 

 20 per cent. The difference is really even more striking than it seems 



