290 THE PHYSIOLOGY OF REPRODUCTION 



membrane has been found to consist of pure mucin.^ In Teleostean 

 fishes it has been investigated in the case of the perch,^ and found 

 to be of the nature of a mucin. 



It would be interesting to find out by systematic investigations, 

 such as those of Pregl * and Buchtala,* whether the chemical nature 

 of the substances protecting the egg varies with th'e different 

 zoological classes, or whether there is a chemical adaptation to 

 external environment. 



The investigations of Hammarsten brought to light the interesting 

 fact that a chemical change takes place in the cover of the eggs 

 during ripening. The immature eggs swell with water, and a 

 mucilaginous solution of mucus is formed, from which the mucin 

 may be precipitated by the addition of acetic acid. If mature eggs 

 are treated with water they do not swell. The water dissolves out 

 the contents of the egg and the empty covers of the eggs remain, 

 and can be transformed into mucin by weak alkali. During the 

 ripening of the eggs there is therefore a change from mucin to 

 mucinogen. 



The composition of the eggs of fishes is essentially the same as 

 that of birds' eggs. From the eggs of salted herrings Hugounenq has 

 obtained the following figures calculated in percentage of the dried 

 material : — 



Per Cent. 

 Pure fats - 10-35 



Phosphorised fats 6'53 



Keratin 2-27 



Protein (" clupeovin ") 81 -47 



The organic constituents consist chiefly of protein, fats, and 

 phosphorised fats, with some cholesterin. 



The following analysis of the ash of caviar ^ gives an idea of the 

 composition of the ash of the eggs of fishes : — 



In the egg the protein is present in the form of a phosphoprotein. 

 Valenciennes and Fr^my, who were the first to isolate this substance, 

 called it Ichthulin. Later Walther showed that this substance very 

 closely resembles the vitellin present in birds' eggs. On peptic 

 digestion it yields an iron-containing pseudonuclein. A similar 



'■ Giacosa, "Etudes sur la Composition cMmique de TCEuf et de ses 

 Enveloppes chez la Grenouille commune," ZeiUch. f. physiol. Chem., vol. vii., 1883. 



2 Hammarsten, " Chemie des Fischeies," Skandin. Arch. f. Phydol., vol. xvii., 

 1905. 



^ Pregl, " tjber die Eihaute von ScylUum stellare und ihre Abbauprodukte,'' 

 ZeiUch. f. phydol. Chem., vol. Ivi., 1908. 



* Buchtala, ibid. 



^ Albu and Neuberg, Min&'alstoftoeclnel, p. 241. 



