346 THE UNFERTILISED EGG AS A [pt. in 



I '14. Fats, Lipoids and Sterols 



Studies on the fatty substances of the undeveloped eggs of different 

 animals have resulted in much interesting information. There has 

 been, of course, a great body of histological work, and the yolks 

 of all kinds of eggs have been repeatedly subjected to microscopic 

 examination (for example, Kaneko's study on the silkworm); but, 

 in spite of many attempts, I have not succeeded in finding more than 

 a few hints in this literature which are of value to the chemical worker. 

 This subject has been dealt with in a general way by Ransom and 

 by Dubuisson, to whose papers those interested in the histological 

 aspects of yolk must be referred. Of the way in which the fat and 

 the protein are intermingled in the yolk we know practically nothing, 

 and it would be most desirable to investigate the yolk with the 

 methods which modern colloidal chemistry has developed. But that 

 the association between fat and protein indicated by the histological 

 evidence is not very close is shown by the interesting centrifugation 

 experiments of McClendon on the amphibian egg. If the egg of the 

 frog is centrifuged for five minutes under the right conditions, it 

 separates into three perfectly distinct layers, the upper one being 

 oily and yellow, the middle one translucent, colourless and proto- 

 plasmic, and the lowest one black, containing practically all the yolk. 

 By using a considerable number of eggs, McClendon was enabled 

 to obtain suflticient material for the chemical analysis of each layer. 

 The figures he obtained are shown in Table 43. It is evident from 

 a slight inspection of his results that the upper layer is composed 

 mainly of neutral fats and a little lecithin, and the middle layer of 

 water, salts and protein, with no fats or lipoids. The lowest and much 

 the largest layer is made up of the vitellin (ranovin or batrachiolin) 

 together with the major part of the lecithin. It is interesting that the 

 association between the phosphoprotein and the lipoid was the only 

 one that centrifuging could not break, for, as we have already 

 seen, the observation of a loose lecitho-vitellin combination in 

 the hen's egg is very old. McClendon found that mitotic figures 

 were all present in the middle layer, and that this centrifuging 

 produced a variety of monstrous embryos. He was led to regard 

 the protoplasm of the egg as constant in composition throughout, 

 but "anisotropic as regards its axes, in other words crystalline 

 in structure". 



