II34 PROTEIN METABOLISM [pt. in 



matter combusted by the chick embryo is protein, but the frog 

 embryo combusts as much as 71 per cent, during its embryonic Hfe. 

 Everything points to very deep-seated differences between eggs which 

 develop in the water and eggs which develop on land. Not only do 

 aquatic embryos burn much more protein in per cent, of the total 

 material burned, but also in per cent, of the initial store of protein. 



Table 161. Material burned as source of energy in per cent, of the total 

 material so burned. 



Carbo- 

 hydrate Protein Fat Investigators 

 Terrestrial. Chick [Callus 3-02 5-57 91-4 Murray; Needham; 



domesticus) Fiske & Boyden 



Aquatic. Frog {Ram tern- 6-84 70-70 22-4 Barthelemy & Bonnet; 



poraria) Faure-Fremiet & 



Dragoiu; Bialasce- 

 wicz & Mincovna; 

 Needham 



Aquatic. Trout {Savelinus — 63 37 Gortner 



fontinalis) 

 Aquatic. Plaice {Pleuronectes — 90 — Dakin & Dakin 



platessa) 

 Terrestrial. Silkworm {Bombyx — 10 64 Tichomirov; Farkas 



mori) 

 Aquatic(?) Turtle [Thalassochelys — 19 81 Tomita; Nakamura; 



corticata) Karashima 



The figures above the line are those most accurately known. Credit should be given 

 to Halban as the first to suggest that there might be a fundamental difference between 

 aquatic and terrestrial embryonic life. 



Table 1 62 shows this very clearly. The embryos of the chick and the 

 silkworm are the only terrestrial ones for which we have dependable 

 figures, and they agree in burning about 4 per cent, of their initial 

 store of protein. Among aquatic embryos, the frog, the trout and 

 the plaice agree in burning about 25 per cent. In the case of 

 embryos which hatch only half-way through their development, as 

 most of the aquatic ones do, it is interesting to find that up to 

 hatching their protein utilisation is not high, but that for the whole 

 embryonic period it much exceeds that of terrestrial embryos. Thus 

 there is reason for supposing that the terrestrial environment of the 

 embryo has two effects on its protein metabolism ; firstly, to suppress 

 the production of nitrogenous waste by removing the means of its 

 easy disposal, and, secondly, to elevate uric acid to the place of im- 

 portance as a means of excreting nitrogen. From this point of view, 

 the invention of viviparity was a " back-to-the-sea " movement on 



