Fish which swim in shoals are not much produced in the rivers, but are bred in the lakes, and they do as follows:--When there comes upon them the desire to breed, they swim out in shoals towards the sea; and the males lead the way shedding forth their milt as they go, while the females, coming after and swallowing it up, from it become impregnated: and when they have become full of young in the sea they swim up back again, each shoal to its own haunts. The same however no longer lead the way as before, but the lead comes now to the females, and they leading the way in shoals do just as the males did, that is to say they shed forth their eggs by a few grains at a time, and the males coming after swallow them up. Now these grains are fish, and from the grains which survive and are not swallowed, the fish grow which afterwards are bred up. Now those of the fish which are caught as they swim out towards the sea are found to be rubbed on the left side of the head, but those which are caught as they swim up again are rubbed on the right side. This happens to them because as they swim down to the sea they keep close to the land on the left side of the river, and again as they swim up they keep to the same side, approaching and touching the bank as much as they can, for fear doubtless of straying from their course by reason of the stream. When the Nile begins to swell, the hollow places of the land and the depressions by the side of the river first begin to fill, as the water soaks through from the river, and so soon as they become full of water, at once they are all filled with little fishes; and whence these are in all likelihood produced, I think that I perceive. In the preceding year, when the Nile goes down, the fish first lay eggs in the mud and then retire with the last of the retreating waters; and when the time comes round again, and the water once more comes over the land, from these eggs forthwith are produced the fishes of which I speak.