SECT. I] PHYSICO-CHEMICAL SYSTEM 295 



& Thierfelder; Frankel & Bolaffio (whose egg-yolk neottin was only 

 a mixture of sphingomyelin and cerebrosides), McLean; Serono & 

 Palozzi; Eppler; Riedel; Wilson, and Trier, who prepared amino- 

 ethylalcohol from it, all comes under this heading. In McLean's 

 book will be found a review of it. Certain aspects of it, however, are 

 important here ; for instance, the question of the presence of very 

 unsaturated acids in ovolecithin. McLean in 1909 found stearic and 

 oleic acids in it, but Cousin was able to isolate linolenic and palmitic 

 as well, and Riedel; Hatakeyama; and Levene & Rolf obtained 

 linolic and arachidonic acids. In another paper Levene & Rolf 

 showed that the lecithin, carefully freed from kephalin, contained 

 only palmitic, stearic, and oleic acids : saturated and unsaturated 

 molecules being present in equal proportions. Again, Stephenson in 

 1 9 1 2 found an acid in the phosphatide fraction from egg-yolk, which 

 had 20 carbon atoms and 6 or 8 unsaturated linkages. Although 

 the proportion of unsaturated acids in egg-yolk is generally agreed 

 to be small, yet it may be of importance for the young embryo if 

 it passes through a period in the early developmental stages before 

 it has the power of desaturating the ordinary fatty acids. Evidence 

 which suggests this will be presented later (Section 1 1 • i ) . 



The nitrogenous radicle in ovolecithin is largely choline, but 

 difficulty was at first experienced in obtaining a theoretical yield 

 on hydrolysis; thus Moruzzi got only 77 per cent, in 1908 and 

 McLean only 65 per cent, in 1909. This was accounted for, however, 

 when it was found that amino-ethyl alcohol was also present. The 

 two bases together make up all the nitrogen in the molecule. Erlandsen 

 was the first to question the view that lecithin alone accounted for 

 the phosphatide fraction, but he was not himself able to isolate any- 

 thing else. Later workers (Levene & West and Stern & Thierfelder), 

 however, found that kephalin is also present in yolk, and it would 

 probably be in the kephalin molecule that the unsaturated fatty acids 

 would be present. Analyses of kephalin from the yolks of fowls are 

 given in Table 10 ^. McLean in 1909 isolated from egg-yolk a third 

 phosphatide which resembled cuorin, but it is very doubtful whether 

 this was a true chemical individual. Sphingomyelin has also been 

 found in egg-yolk by Levene (191 6), and lignoceric as well as hydroxy- 

 stearic acid was present in it. 



All these substances exist in the yolk in close association with 

 the proteins. Hoppe-Seyler it was who first observed that, after 



