286 THE PHYSIOLOGY OF REPRODUCTION 



quotient of the embryo in pregnant guinea-pigs and rabbits l indicate 

 that there is an oxidation of carbohydrate material, and Systematic 

 chemical investigations of the placenta and fetus of pregnant 

 rabbits ' 2 have shown that there is a constant and regular disappear- 

 ance of glycogen from the placenta, which reappears only partly as 

 such in the embryonic tissues. It can, therefore, be concluded that in 

 these animals glycogen furnishes at least part of the "energy of 

 development." But it is doubtful whether this conclusion can be 

 applied to all the Mammals, since in the case of the cow and of the 

 sheep very little glycogen is found in the placenta. 



In reptiles 3 also the chemical energy used up during development 

 is furnished mainly by carbohydrates. 



Similar observations have been made on the eggs of fishes, 4 where 

 the energy of development was found to be very small. 



In all these cases the chemical energy used up in the process of 

 development has been found to be furnished either by fats or by 

 carbohydrates. No conclusive evidence has as yet been obtained 

 that the store of nitrogenous substances' is used for energetic 

 purposes. Nor is it at all clear what changes the protein substances 

 of the egg undergo during development. 6 



Liebermann records a loss of nitrogenous substances in his analysis 

 of hens' eggs at various stages of development ; but as Hasselbalch 6 

 pointed out, this loss is accounted for by the egg-membrane, which is 

 left behind when the chick is hatched, and which was not included in 

 Liebermann's analysis. 



Reference has already been made to the nitrogenous constituents 

 of the yolk ; the two phosphoproteins vitellin and livetin. The other 

 protein substances of the white of the egg can be distinguished 

 according to their reactions as albumens, globulins, and a substance 

 behaving like a peptone in so far as it is not coagulated by heat and 

 not precipitated by ammonium sulphate or by hydrochloric and acetic 

 acids. According to the investigations of Morner, 7 this substance is 



1 Bohr, " Der respiratorische Stoft wechsel des Saugethierembryos," Skandin. 

 Arch.f. Physiol, vol. x., 1900. 



2 Lochhead and Cramer, "The Glycogenic Changes in the Placenta and 

 Foetus of the Pregnant Rabbit," Proc. Roy. Soc., Series B., vol. Ixxx., 1908, 

 p. 263. 



3 Bohr, " liber den respiratorischen Stoffwechsel beim Embryo kaltbliitiger 

 Tiere," loc. cit. 



4 Tangl and Farkas, " Beitrage zur Kenntniss der Ontogenese : IV. Uber 

 den Stoff u. Energieumsatz ini bebriiteten Forellenei," Pflilgei ) s Arch., vol. civ., 

 1904. 



5 Emrys-Roberts, " A Further Note on the Nutrition of the Early Embryo 

 with special reference to the Chick," Proc. Roy. Soc., B., vol. Ixxx., 1908. 



6 Hasselbalch, "Uber den respiratorischen Stoffwechsel des Hiihnerembryos," 

 Skandin. Arch.f. Physiol., vol. x., 1900. 



7 Morner " Uber die im Huhnereiweiss in reichlicher Menge vorkonimende 

 Mucinsubstanz," Zeitsch. f. physioL Chem., vol. xviii. 



