Page:Popular Science Monthly Volume 4.djvu/38

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28
THE POPULAR SCIENCE MONTHLY.

nute opening through which a particle of the spawn—the soft roe—of the male fish enters, and the egg is fertilized. From this moment the young fish gradually develops, under the influence of the cold running water. At the end of about thirty-five days—more or less, according to the temperature, which should be about 40°—two little black specks can be seen, as at Fig. 2, which are .the eyes of the embryo fish; the vertebræ may be discerned in the form of a faint red line, and a small red globule, which shortly afterward appears, represents the vital organs of the embryo fish.

At the end of about 80 to 100 days from the deposition of the egg the fish has so increased in size that it bursts the "shell" and makes its début in the form represented at Fig. 3. The sac or umbilical vesicle

Fig. 3.
PSM V04 D038 Fish coming out of egg.jpg
Fish coming out of Egg.

attached to the under part of the fish contains a secretion resembling albumen, which affords nourishment to the infant fish for the first six weeks or so of its existence. By that time it is quite absorbed, and for the first time we see a perfect fish, Fig. 4, with its fins, gills, and scales, which have hitherto been present, but imperceptible except under the microscope, fully formed: and now the young salmon begins to feed. His growth is not very rapid for some months, the lines a, b, c, representing the average length of a salmon at two, three, and four months old. At two years old the fish is about nine to twelve inches long.

As soon as they are large enough and strong enough, the "smolts," as they are now called, descend to the sea; here they are lost sight of until they return up the river as "grilse." The actual duration of their stay in the sea is not yet known, from one to three years being variously estimated as the probable length of time. The object of this migration to the sea is to find the food which is necessary for the secretion of the fat of the fish, who lives on the Infusoria, smaller fish and crustaceans, and the spawn of sea-fish, which abound in our seas. The length of their stay in salt-water is regulated, no doubt, by various circumstances, and is not the same in every case. When the salmon has laid up a sufficient supply of fat in its body and on its pyloric appendages, which are a wonderful provision of Nature for the secretion of an amount of fat sufficient to supply it during its so-