THE LIFE-HISTORIES OF FISHES i6i 



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 &^^ 



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 &%^. If a 



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



be seen that the yolk comes to rest with a whitish 



II 



