BIOCHEMISTRY OF THE SEXUAL ORGANS 273 



term "work of development" by the" term "energy of 

 development. 



The nature of the substances which by their oxidation 

 furnish the " energy of development " is different in the 

 different classes of animals. In birds it is furnished, as we 

 have seen, by the oxidation of fats, and possibly also of the 

 fat group of phosphorised fats. In Mammals in which de- 

 velopment proceeds in utero, and there is a constant exchange 

 of material between the mother and the foetus, the investigation 

 of these problems is more difficult, owing to the complexity of 

 the conditions. Investigations on the respiratory quotient of 

 the embryo in pregnant guinea-pigs and rabbits ^ indicate that 

 there is an oxidation of carbohydrate material, and systematic 

 chemical investigations of the placenta and foetus of pregnant 

 rabbits^ have shown that there is a constant and regular dis- 

 appearance of glycogen from the placenta, which reappears 

 only partly as such in the embryonic tissues. It can there- 

 fore 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 appKed to all the 

 Mammals, since, in the case of the cow and of the sheep, for 

 instance, very little glycogen is found in the placenta. 



In reptiles ^ also the chemical energy used up during de- 

 velopment is furnished mainly by carbohydrates. 



Similar observations have been made on the eggs of fishes,* 

 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 that purpose. 



' Bohr, " Der respiratorische Stoffwechsel des Saugethierembryos,'' Skan- 

 dinar. Arch. f. Phyaiologie, vol. x., 1900. 



' 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, " Uber den respiratorischen StofEwechsel beim Embryo kaltblutiger 

 Tiere," loc. cit. 



* Tangl and Farkas, " Beitrage zur Kenntniss der Outogenese : IV. Uber 

 den Stoff u. Bnergieumsatz im bebriiteten Forellenei," Pfluger's Archiv, vol. 

 civ., 1904. 



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