I06 DOMESTICATED TROUT. 



those of most fish, except the sahiion. They average 

 about three sixteenths of an inch in diameter, varying 

 very considerably in size, the very largest containing 

 probably twice the bulk of the very smallest. They 

 are sometimes colorless, sometimes orange-hued, and 

 sometimes have a rich red tint. 



The cause of the variation in the color of the eggs 

 is not positively known. It has been thought to be 

 hereditary.^ It has also been attributed to the color 

 of the flesh of its parent, and to the nature of the par- 

 ent's food.f 



A correspondent of Mr. Buckland says that the 

 tints cannot depend on the color of the parent's flesh, 

 because graylings' eggs have similar tints, and all gray- 

 lings are white-fleshed. 



The outer membrane of the egg is very elastic and 

 tough. The internal structure of the egg is as follows. 

 On the outside is the shell membrane, corresponding 

 to the hard shell of birds' eggs. Inside of this shell, 

 which is formed, as with birds' eggs, at quite a late 

 period of the development of the egg in the ovary, 

 is another membrane called the yolk membrane. This 

 is very different from the shell membrane, and is quite 

 delicate. This yolk envelope contains the yolk of 

 the egg, in which are several drops of oil, which form 

 the food that the young alevin absorbs in the yolk-sac 

 stage. In the yolk also floats the germinal vesicle, 

 which is a small cell, and which contains another set 

 of minute cells called the germinative spots or points. 



* Massachusetts Fisheries, Report, 1868, p. 31. 

 t Fish Hatching, Buckland, pp. 19, 20. 



