The Organs of Reproduction 33 
The Eggs of Fishes.—The great majority of fishes are ovipa- 
rous, 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 
Ve 
= — 
= 
Fig. 22.—White Surf-fish, viviparous, with young, Cymatogaster aggregatus 
Gibbons. San Francisco, 
the placenta of higher animals, but not of the same structure 
ororigin. Inthe 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 (Peciliide), the fin itself 
being placed in advance of its usual position. In the surf-fishes 
(Embiotocide) the structure of part of the anal fin is modified, 
although it is not used as an intromittent organ. In the Elas- 
mobranchs, as already stated, large organs of cartilage (claspers) 
are developed from the ventral fins. 
In some viviparous fishes, as in the rockfishes (Sebastodes) 
and rosefishes (Sebastes), the young aie very minute at birth. 
