5. The development of the ovum is alike, both in those that are internally and those that are externally oviparous. For it takes place at the extremity of the ovum, and it is enclosed in a membrane. The eyes are the first part that is conspicuous; they are large and spherical; so that it is plain that they are mistaken who say that the mode of development resembles that of vermiform creatures, for in them the order is different, and the lower parts are formed first, and afterwards the head and eyes. When the ovum is taken away, they assume a circular form, and for some time continue to grow without taking in any food, by absorbing the moisture of the ovum. They afterwards derive their nutriment, as long as they continue growing, from the water of the river. 6. When the Pontus is cleansed, something is floated out into the Hellespont which is called fucus. It is of a yellow colour. Some say that it is naturally a plant. This takes place at the beginning of summer. The oysters and small fish which live in these places feed upon this fucus; and some maritime persons say that they obtain their purple from this plant. CHAPTER XIII. 1. The pond and river fish begin to reproduce usually when five months old. They all produce their ova at the beginning of summer. Like the marine fish, the females of these kinds never emit all their ova, nor the males all their semen, at once; but both sexes are always found to contain a portion of the reproductive substance; they produce their ova at the proper season. The cyprinus five or six times a-year, and especially under the influence of the stars. The chalcis reproduces three times, all the rest but once a-year. 2. They deposit their ova in the stagnant parts of rivers and ponds among the reeds, as the phoxinus and perca. The glanis and the perca produce their ova in strings, like the frog. That which the perca produces is so involved that, on account of its breadth, the fishermen collect it together from among the reeds in ponds. The larger individuals of the glanis produce their ova in deep water, some where it is a fathom deep; but the smaller ones in shallow water, and especially at the root of the willow or some other tree, and among the reeds and mosses. 3. The fish fold themselves together, sometimes a large one with a small one, and approximate the passages, which some call their navel, from which they eject their respective seminal matter, the females their ova, and the males their spermatic fluid. Those ova with which the semen of the male has been mixed immediately or in the course of a day become whiter and larger, and in a short time the eyes of the fish make their appearance; for in all fish, as in other animals, this part is most conspicuous, and appears the largest. But, if the seminal fluid does not touch any of the ova, as in the case of sea-fish, these become useless and barren.