Chapter X.

1. It has been already observed that fish are not always oviparous, for the selache are always viviparous. All the rest are oviparous. The selache are viviparous, having first of all produced ova internally; and these they bring up in themselves, except the batrachus. Fish have also, as I observed before, very different uteri in different kinds: for in the oviparous genera the uterus is double, and situated low down. In the selache the uterus is more like that of birds. There is this difference, however, that the ova are not placed near the diaphragm, but in an intermediate position near the spine; and when they have grown they change their place from this part. The ovum in all fish is not of two, but of one colour; and it is more white than yellow, both in its early stages, and after the formation of the embryo.

2. The development of the ovum is different in fish and in birds, in that it has not the umbilical cord which passes to the membrane of the shell; but only the passage which leads to the yolk in the eggs of birds. The rest of the development of the ovum is alike in birds and fish; for it takes place at the extremity, and the veins have their origin in a similar manner in the heart; and the head, and eyes, and upper parts of the body are larger than the rest. As the young fish increases, the ovum continues to diminish, and at last it disappears, and is absorbed, like the yolk in the eggs of birds. The umbilical cord is attached a little below the abdomen. At first the cord is long, but it becomes less as the fish grows, and at last is small, and finally absorbed, like that of birds.

3. The embryo and the ovum are enclosed in a common membrane, and beneath this there is another membrane, in which the embryo alone is enclosed. Between these membranes there is a fluid substance. The nutriment contained in the stomach of the young fish is similar to that in the young birds, partly white, and partly yellow. The form of the uterus must be learned from dissection. This organ is different in different fish, as in the galeode by themselves, and the flat fish by themselves: for in some the ova are attached near the spine to the centre of the uterus, as I observed before, as in scylia.[202] They descend when they begin to increase, when the uterus is double, and are attached to the diaphragm, as in other fish: the ova descend into each division.

4. The uterus of these fish, and of the other galeode, has a small appendage attached to the diaphragm like a white nipple, which is not present unless they are pregnant. The scylia and the batis have a shell-like substance, which contains the fluid of the ovum. In form the shell resembles the tongue of a wind instrument, and hair-like passages are attached to the shells. The young of the scylia, which some persons call nebria galei, are born when the shell falls off and bursts. The young of the batis when they are brought forth are excluded by the rupture of the shell. In the acantheas[203] galeos the ova are attached to the diaphragm above the nipples; and when the ovum descends, the young is attached to it after it is set free. The reproduction of the alopex is in the same manner.

5. Most galei which are called smooth have the ova placed between the divisions of the uterus, like those of the scylia; and as they surround it, they descend into each division of the uterus, and they are produced, attached to the uterus by an umbilical cord; so that when the ova are taken out, they appear similar to the embryo of quadrupeds. And the long umbilical cord is attached to the lower part of the uterus, each part, as it were, attached to an acetabulum; and to the middle of the embryo near the liver. And when it is dissected, the food is like an egg, though the ovum be no longer there. There is a chorion, and peculiar membranes surrounding each of the embryos, as in quadrupeds.

6. The head of the embryo when it is just produced, is upwards; but as it grows and reaches maturity, it is placed downwards. The males are placed on the left, and the females on the right, or there are males and females together on the same side. The embryo, when dissected, resembles that of quadrupeds, in having its viscera such as it has, as the liver, large, and full of blood. In all the selache the ova are placed high up, near the diaphragm; many larger, and many smaller: and the embryos are placed below, wherefore it is probable that such fish produce their young, and copulate frequently during the same month, for they do not produce all their young at once, but frequently, and for a long while; but those that are in the lower part of the uterus are matured and brought to perfection.

7. The other galei both emit and receive their young into themselves, and so do the rhine and the narca; and a large narca has been observed to contain eighty young in herself. The acanthias is the only one of the galei which does not admit its young into itself, on account of their thorns. Among the flat fish the trygon and batos do not admit their young, on account of the roughness of the tail. Neither does the batrachus admit its young, on account of the size of their heads, and their thorns; and this is the only one that is not viviparous, as I previously observed. These are their mutual differences, and the manner of the development of their ova.

8. At the season of sexual intercourse, the seminal ducts of the male are full of fluid, so that a white matter escapes when they are pressed. These passages are divided, and originate in the diaphragm and the large vein: at the same season the passages of the male are conspicuous, and may be compared with the uterus of the female. When it is not the season of sexual intercourse, they are less conspicuous, from not being in use. In some fish, and sometimes, they are not visible at all, as it was remarked of the testicles of birds. The seminal and uterine passages are different in other respects also, and because those of the male are attached to the loins, those of the female are easily moved, and enclosed in a thin membrane. The nature of the passages of the male may be seen in works on anatomy.

9. The selachea become pregnant again while with young, and the period of gestation is six months. Among the galei, the asterias produces young the oftenest; for it produces twice in a month: it begins to copulate in the month of September. All the other galei except the scylia produce twice in the year; the scylia but once. Some of them have their young in the spring. The rhine produces its first brood in the spring, and its last in the autumn, near the winter season, and the setting of the Pleiades. The second fry are the most numerous. The narca produces its young in the autumn. The selache descend from the ocean and deep water to the shore, to produce their young, both for the sake of the warmth, and care of their offspring.

10. No other fish but the rhine and the batos have ever been observed to unite with others not of their own kind, but there is a fish called the rhinobatus, which has the head and upper part of the rhine, and the lower part like the batus, as it were made up of both. The galei and the galeoeides, as the alopex, dog-fish, and the flat fish, as the narce batos, leiobatos and trygon, are in this manner ovoviparous.