278 THE PHYSIOLOGY OF REPRODUCTION 



The investigations of Hammarsten brought to light the in- 

 teresting 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 bi 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 there- 

 fore a change from mucin to mucinogen. 



The composition of the eggs of fishes is essentially the same 

 as that of birds' eggs. 



The organic constituents consist chiefly of protein, fats, and 

 phosphorised fats, with some cholesterin. 



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

 of the composition of the ash of the eggs of fishes : 



Total Ash . . K 2 0. Na 2 0. CaO. Fe 2 O 3 . P 2 6 . 01. 

 7-70 per cent. . 3'33 3077 5'02 0'22 10'55 47'44 



In the egg the protein is present in the form of a phospho- 

 protein. Valenciennes and Fremy, 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 substance containing phosphorus and 

 iron was isolated from the eggs of the salmon by Noel Paton, 

 from cods' eggs by Levene, and from perches' eggs by Ham- 

 marsten. 2 The statement by Walther that ichthulin, on boiling 

 with mineral acids, splits off a reducing sugar and differs in this 

 respect from vitellin has not been confirmed by the later workers. 



Ichthulin is probably identical with the crystalline material 

 observed in the eggs of the tortoise, the frog, the shark, and 

 other fishes, which is known morphologically under the name 

 of yolk-spherules or " Dotterplattchen." The unripe eggs of 

 the perch are embedded in a fluid from which a protein of the 

 nature of a globulin has been isolated. This protein received 



1 Albu and Neuberg, Mineralslo/wecheel, p. 241. 



1 Hammarsten, "Chemie des Fischeies," Skandinav. Arch. f. Physiologic, 

 vol. xvii., 1905. This paper contains a detailed review of previous work 

 done on this subject. 



