282 THE PHYSIOLOGY OF REPRODUCTION 



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 present in the fresh egg about one-third disappears during 

 incubation. The remainder reappears in the embryo. 



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 sugars (pentoses or hexoses) (see p. 307). 

 Neither nucleoprotein nor sugars 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, not only of nucleoprotein, but 

 also of some of its constituent groups from the reserve material 

 of the egg (proteins and phosphorised fats), takes place during 

 development. 2 The purine bases found in the embryo are essentially 

 the same as those found in the adult organism. 



Of the phosphorised fats of the yolk, lecithin is the simplest and 

 best-known representative. Like all fats, it is an ester, a compotmd 

 of glycerine and fatty acids, some saturated, as stearic and palmitic 

 acids, others unsaturated, as oleic acid. Like all fats, it is soluble in 

 alcohol and ether. With water it swells up and forms a colloidal 

 solution. It is distinguished by the presence in its molecule of 

 one molecule of phosphoric acid to which one molecule of an 

 organic nitrogenous base, choline, is attached. If boiled with 

 baryta water it is decomposed into glycerophosphoric acid, fatty 

 acids, and choline. Lecithin forms loose compounds with proteins, 

 the so-called lecithalbumins, of which vitellin is probably one. 



Vitellin is an ill-defined compound between lecithin and a protein 

 substance which itself contains about one per cent, phosphorus. It 

 is insoluble in water, but soluble in dilute solutions of neutral salts. 

 behaving in that respect like a globulin. On peptic digestion a 

 pseudonuclein, rich in phosphorus, is formed from the protein part 

 of vitellin. This pseudonuclein contains also a relatively large 



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

 K.i-tus of the Pregnant Rabbit," Proc. Rot/. Soc., Series B, vol. Ixxx., 1908. 



2 Kossel, " Weitere Beitriige zur Cheraie des Zellkernes," Zcitsch. f. I'lii/wit. 

 < '/,f>,n., vol. x., 1886. Mendel and Leavenworth, "Chemical Studies on Growth : 

 VI. Changes in the Purine- Pentose- and Cholesterol-Content of the Developing 

 E gg>" '-lmer. Jour, of J'hysioL, vol. xxi., 1898. Fridericia, " Untersuchungen 

 iiber die Harnsaureproduktion und die Nucleoproteid neubildung beim Hiihner- 

 embryo," StatuK*, .1 /-/,. f. l>hysiol., vol. xxvi., 1912. 



