In a very few cases both organs have been found in the same fish, as in _Serranus_, which is sometimes truly hermaphrodite. All fishes, however, seem to be normally dioecious, the two sexes in different individuals. Usually there are no external genital organs, but in some species a papilla or tube is developed at the end of the urogenital sinus. This may exist in the breeding season only, as in the fresh-water lampreys, or it may persist through life as in some gobies. In the Elasmobranchs, cartilaginous claspers, attached to the ventral fins in the male, serve as a conduit for the sperm-cells. =The Eggs of Fishes.=--The great majority of fishes are oviparous, the eggs being fertilized after deposition. The eggs are laid in gravel or sand or other places suitable for the species, and the milt containing the sperm-cells of the male is discharged over or among them in the water. A very small quantity of the sperm-fluid may impregnate a large number of eggs. But one sperm-cell can enter a particular egg. In a number of families the species are ovoviviparous, the eggs being hatched in the ovary or in a dilated part of the oviduct, the latter resembling a real uterus. In some sharks there is a structure analogous to the placenta of higher animals, but not of the same structure or origin. In the case of viviparous fishes actual copulation takes place and there is usually a modification of some organ to effect transfer of the sperm-cells. This is the purpose of the sword-shaped anal fin in many top-minnows (_Pæciliidæ_), the fin itself being placed in advance of its usual position. In the surf-fishes (_Embiotocidæ_) the structure of part of the anal fin is modified, although it is not used as an intromittent organ. In the Elasmobranchs, as already stated, large organs of cartilage (claspers) are developed from the ventral fins. [Illustration: FIG. 92.--White Surf-fish, viviparous, with young, _Cymatogaster aggregatus_ Gibbons. San Francisco.] In some viviparous fishes, as in the rockfishes (_Sebastodes_) and rosefishes (_Sebastes_), the young are very minute at birth. [Illustration: FIG. 93.--_Goodea luitpoldi_ (Steindachner). A viviparous fish from Lake Patzcuaro, Mexico. Family _Pæciliidæ_. (After Meek.)] In others, as the surf-fishes (_Embiotocidæ_), they are relatively large and few in number. In the viviparous sharks, which constitute the majority of the species of living sharks, the young are large at birth and prepared to take care of themselves. [Illustration: FIG. 94.--Egg of _Callorhynchus antarcticus_, the Bottle-nosed Chimæra. (After Parker and Haswell.)]