////; UNPRODUCTIVE GLANDS. 427 



The same holds good of the iahthin of shark eggs and the emydin 

 of tortoise eggs. 



As I have stated, the yolk of hens' eggs represents about 29 per 

 cent, of the entire weight. Its actual weight may thus vary between 

 8.7 and 2()..'> grammes. 'Flu 1 general composition of the yolk is seen 

 in the following analyses, which are taken from Gautier : 



Per cent. 



Water 47.19-51.49 



Solids 48.51-42.81 



Fats (olein, palmitin, and stearin) 21.30-22.84 



Vitellin and other albumins 15.63-15.76 



Lecithins 8.43-10.72 



Cholesterin 0.44- 1.75 



Cerebrin 0.30 



Mineral salts . 3.33- 0.36 



Coloring-matter) Q 553 



Glucose / 



Analysis of the mineral salts, calculated for 100 parts of ash, has 

 given the following results (Poleck and AVeber) : 



Sodium (Na 2 O) 5.12- 6.57 



Potassium (K 2 O) 8.05- 8.93 



Calcium (CaO) 12.21-13.28 



Magnesium (MgO) 2.07- 2.11 



Iron (Fe. 2 O 3 ) 1-19- 1.45 



Phosphoric acid, free (P 2 O 5 ) 5 -72 



Phosphoric acid, combined 63.81-66.70 



Silicic acid (SiO,) 0.55- 1.40 



Chlorine traces 



Of the mineral constituents, the large amount of calcium and 

 phosphoric acid is especially noteworthy. Soluble phosphates, 

 however, are not found as such in the yolk. The amount of potas- 

 sium and sodium, it will be observed, is much smaller than in the 

 albumen. 



The Albumins. Our knowledge of the individual albumins which 

 are found in the yolk is still very imperfect. But it appears from 

 recent researches that they are represented notably by nucleo- albu- 

 mins, which in turn may be combined with lecithins to form com- 

 plex lecithalbumins. This, however, is not proved, and it is as- 

 sumed by some that the lecithins which are obtained so commonly 

 together with the albumins do not exist in chemical combination, 

 but are to be regarded as contaminations. The best known repre- 

 sentative of the nucleo-albumins of the yolk is the so-called ovo- 

 vitellin. True nucleins do not occur in the yolk. 



Ovovitellin. Formerly this was regarded as a globulin, but it is 

 now known to be a nucleo-albumin in which an albuminous radicle 

 is combined with a paranuclcin /. e., a nuclei n which does not yield 

 nucleinic bases on decomposition with mineral acids. The substance 

 has thus far not been obtained free from lecithins, and it is for this 

 reason that the latter is thought bv some to be present in chemical 

 combination. 



