SECT, i] PHYSICO-CHEMICAL SYSTEM 281 



through the keratin membrane. It would be very interesting to make 

 a more extended study of the osmotic properties of the vitelline 

 membrane (see in this connection Section 5*6). 



The yolk of the egg was investigated earlier in the modern period 

 than the white. We may pass directly, excluding the curious analysis 

 of the eggs of Struthio casuarius by Holger in 1822, to the papers of 

 Gobley, which appeared from 1846 to 1850, and which, with those 

 of Valenciennes & Fremy from 1854 to 1856, still remain models 

 of embryo-chemical work. "John, a German chemist," said Gobley, 

 "appears to have been the first to occupy himself with serious 

 researches on the yolk of the egg. The chemists who preceded him 

 considered it as made up only of water, albumen, oil, gelatine, and 

 colouring matter; such was the opinion of Macquer, Fourcroy, and 

 Thomson. John concluded from his experiments, which he published 

 in 181 1, that the yolk was composed of water, a sweet yellow oil, 

 traces of a free acid which he thought was phosphoric acid, and a 

 small amount of a brownish red substance, soluble in ether and 

 alcohol. Besides these he found gelatine, sulphur, and a great deal 

 of a modified albuminous substance." Gobley referred also to the 

 work of Prout, of Chevreul, of Berzelius and of Lecanu, who dis- 

 covered the presence of cholesterol in yolk in 1829. 



Gobley himself found in the yolk nearly all the substances which 

 we now know to be there. His own list of them ran as follows : 



1 . Water. 



2. An albuminous matter, "vitellin", 



3. Olein. 



4. Margarin. 



5. Cholesterin. 



6. Margaric acid. 



7. Oleic acid. 



8. Phosphoglycerilic acid. 



9. Lactic acid. 



10. Salts such as chloride of sodium, chloride of potassium, chlorhydrate 



of ammonium, sulphate of potash, phosphate of lime, and phos- 

 phate of magnesia. 



11. A yellow colouring matter and a red colouring matter. 



12. An organic substance containing nitrogen, but which is not al- 



buminous. 



Most of the constituents of egg-yolk may be recognised under this 



