956 ENERGETICS AND ENERGY-SOURCES [pt. m 



Tangl & Farkas made a comparison between the three kinds of 

 eggs they had studied, as follows : 



Loss in % of the original amounts from 

 fertilisation to hatching 



The slight loss of water from the trout's egg shown by Tangl & 

 Farkas does not contradict the findings of Kronfeld & Scheminzki, 

 for, as Fig. 238 shows, before hatching the water-content of the larva 

 as a whole is almost constant, although that of the yolk alone is 

 decreasing. The Ea. of 6-68 cal. was a remarkably small proportion 

 of the energy originally contained in the egg, only 3-5 per cent., and 

 contrasted with the 18 per cent, which is lost by the chick embryo 

 by the time of hatching, and the corresponding 24 per cent, of the 

 silkworm. But it must be remembered that hatching occurs relatively 

 early in the trout, and that for a long time afterwards the yolk is 

 the only source of food for the larva. The R.Ea., then, as Tangl 

 & Farkas pointed out, would have been distinctly lower than that 

 for the other embryos. Here we touch one of the fundamental 

 difficulties of Tangl's conceptions, for, when we define the S.Ea. as 

 the amount of energy used for combustion during the storage of, 

 i.e. the formation of, i gm. dry weight of the finished embryo, we 

 omit to define what a finished embryo is. Tangl assumed throughout 

 his work that the time of hatching was the natural index, but in 

 the case of organisms such as the trout, which have a prolonged 

 yolk-sac period, this is evidently wrong. It is probable that, if one 

 were to take the yolk-sac period into consideration, one would find an 

 R.Ea. very like those for the chick and the silkworm, but this has 

 not so far been done. 



7*2. Energy of Growth and Energy of Differentiation 



The sixth and seventh papers of the series were devoted by Tangl 

 to a study of the energetics of metamorphosis in the fly, Ophyra 

 cadaverina, and to a general discussion of the meaning of "Entwick- 

 lungsarbeit" in relation to embryonic growth and insect metamor- 



