THE LIFE-HISTORIES OF FISHES 161 



The Eggs of Sea-fishes 



Sea-fish ova conform very generally to one 

 definite type of structure, though, as in the case 

 of the breeding habits, some notable departures 

 from such a type are known. Such an egg as 

 that of the cod is a good example of all the rest. 

 It is a small, perfectly spherical body a little over 

 a sixteenth of an inch in diameter. When ripe, 

 it is clear and transparent, and is not easily seen 

 when floating in sea-water. It is bounded by a thin 

 though fairly strong capsule or shell, within which 

 is a mass of transparent " food-yolk " (see the 

 diagrams at the end of this chapter), and at one 

 pole of this spherical mass of yolk is the " germinal 

 disc," a little cap of protoplasmic material which 

 is destined to form the body of the embryo fish. 

 Immediately over this germinal disc is a minute 

 aperture in the shell the " micropyle." The yolk 

 is somewhat lighter than the germinal disc, so 

 that, when floating in the water, the latter, and the 

 micropyle, are directed downwards. Such an egg 

 can be compared with that of the fowl. In the 

 latter case there is a shell (which, however, is limy 

 and opaque), and within the latter is the albumen 

 or " white," and within the latter again the yolk, 

 which corresponds closely, except in colour and 

 opacity, with the yolk of the cod's egg. If a 

 hen's egg be carefully broken into a saucer, it will 

 be seen that the yolk comes to rest with a whitish 



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