BIOCHEMISTRY OF THE SEXUAL ORGANS 269 



clear that the formation of nucleoproteins cannot account for 

 this enormous consumption of phosphorised fats. Some of these 

 substances reappear in the embryo. A proportion of them 

 contributes to the formation of bones, which contain a con- 

 siderable amount of inorganic phosphates. Part reappears in 

 the foetal tissues as phosphorised fats, especially in the nervous 

 tissue, which is very rich in these substances. That portion of 

 the phosphorised fats which is transformed into inorganic 

 phosphates, may at the same time fulfil another very im- 

 portant function by the oxidation of the fat group in their 

 molecule. It will be shown below that the development of 

 the embryo is intimately associated with, and perhaps de- 

 pendent upon, the transformation of chemical energy into heat. 

 This transformation is brought about by the oxidation of 

 certain organic substances, which are different in the different 

 classes of Vertebrates. It will be shown also that in birds the 

 chemical energy is furnished by fats, and it is very probable that 

 the phosphorised fats furnish at the same time material for 

 the formation of the tissues of the embryo and fat as a source of 

 chemical energy. 



It is interesting to note that a similar double function has 

 been assigned to glycogen in the case of the developing rabbit. 1 

 Of the cholesterin about one-third disappears during incubation. 

 The phosphorus which enters into the composition of nucleo- 

 protein is bound up therein in the form of phosphoric acid, 

 combined with purine bases and pentoses (see p. 294). Neither 

 nucleoprotein nor pentoses are present in the fresh egg, and 

 purine bases are present only in very small amounts. The fact 

 that during development these substances rapidly increase in 

 amount, indicates therefore that a synthesis of nucleoprotein 

 from the reserve material of the egg (proteins and phosphorised 

 fats) takes place during development-. The purine bases 

 found in the embryo are essentially the same as those found 

 in the adult organism. 2 



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

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



* Kossel, " Weitere Beitriige zur Chemie des Zellkernes," Zeitschrift fur 

 Physiologiache Chemie, vol. x., 1886. Mendel and Leavenworth, "Chemical 

 Studies on Growth: Vl.Changes in the Purine- Pentose- and Cholesterol-Content 

 of the Developing Egg," American Journal of Physiology, vol. xxi., 1898. 



