270 THE PHYSIOLOGY OF REPRODUCTION 



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

 and best-known representative. Like all fats, it is an ether 

 compound of glycerine and fatty acids, such as stearic, palmitic, 

 and oleic acid, and is, like all fats, 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 nitro- 

 genous 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 1 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 amount of iron in organic com- 

 bination, and it is this substance which is responsible for the 

 presence of iron in the yolk of the egg. According to Bunge, 1 

 this substance plays an important part in the formation of 

 haemoglobin in the chick. It is the precursor of haemoglobin, 

 and has, therefore, been called by him haematogen. It contains 

 5- 19 per cent. P., and 0'29 per cent. Fe. Recenth* Plimmer 2 

 has isolated from egg-yolk another protein, livetin, soluble 

 in water and containing (H per cent, phosphorus. 



Two different fats have been isolated from the yolk the 

 one solid, rich in palmitic acid ; the other fluid, containing equal 

 parts of palmitic and oleic acids. A small amount of stearic 

 acid is also present in both fats. The composition of the fat is 

 influenced by the food, the fat of the food passing into the yolk 

 in the same kind of way as it passes into the fat deposits of the 

 adult organism. 3 



The food has also an influence upon the colour of the yolk, 



1 Bunge, "Uber die Assimilation des Eisens," Zeitschr. f. physiolog 

 Chemie, 1884, vol. ix. 



* Aders Plimmer, "The Proteins of Egg- Yolk," Journal Chemical Soc., 

 1908. 



3 Henriques and Hansen, " Uber den Ubergang des Nahrungsfettes in 

 das Huhnerei," Skandin. Arch. f. Phyinologie, vol. xiv., 1903. 



