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. [202] Dog-fish. Squalus stellaris.