All fishes are _dioecious_, or of distinct sex. Instances of so-called _hermaphroditism_ are, with the exception of _Serranus_, abnormal individual peculiarities, and have been observed in the Cod-fish, some Pleuronectidæ, and in the Herring. Either the generative organ of one side was found to be male, that of the other female; or the organ of one or both sides was observed to have been developed partly into an ovary partly into a testicle. In the European species of Serranus a testicle-like body is attached to the lower part of the ovary; but many specimens of this genus are undoubtedly males, having normally developed testicles only. The majority of fishes are oviparous, comparatively few viviparous; the embryos being developed either in the ovarium or in some dilated portion of the oviduct. In viviparous fishes actual copulation takes place, and the males of most of them are provided with copulatory or intromittent organs. In oviparous fishes the generative products are, during sexual excitement, discharged into the water, a very small quantity of semen being sufficient for effectual impregnation of a number of ova dispersed in a considerable quantity of water; circumstances which render _artificial impregnation_ more practicable than in any other class of animals. In _Branchiostoma_ the generative organs occupy the ventral side of the abdominal cavity, into which they discharge their contents. No ducts are developed in either sex. In the _Cyclostomes_ the generative organ is single, and fixed to or suspended from the median line of the back of the visceral cavity by a duplicature of the peritoneum (_mesoarium_); the testicle and ovary being distinguishable by their contents only. These escape by dehiscence of the cells or capsules and rupture of the peritoneal covering into the abdominal cavity, and are expelled by reciprocal pressure of the intertwined sexes through the _porus genitalis_, which is sunk between two labia of the skin in _Myxine_, and produced into a long papilla in Petromyzon. [Illustration: Fig. 69.--Ovum of Myxine glutinosa, enlarged.] The ova of the Lampreys are small, globular, like those of Teleosteans. Those of Myxine have a very peculiar shape when mature; they are of an oval form, about 15 millimetres long and 8 millimetres broad, enveloped in a horny case, which at each end is provided with a bundle of short threads, each thread ending in a triple hook. Whilst in the mesoarial fold the eggs are attached to one another by means of these hooks, and after being expelled they probably fix themselves by the same means to other objects. As in all fishes producing ova of large size, the number of ova matured in one season is but small.