Tht 



Organs of Reproduction 



125 



The Eggs of Fishes. — The great majority of fishes are ovipa- 

 rous, the eggs being fertihzed 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- 

 fiuid 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 



Fig. 92. — White Surf-fish, viviparous, with young, Cymatogaster aggregatus 

 Gibbons, San Francisco. 



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 sorae organ to effect 

 transfer of the sperm-cells. This is the purpose of the sword- 

 shaped anal fin in many top-minnows {Pccciliida;), the fin itself 

 being placed in advance of its usual position. In the surf-fishes 

 {EmbiotocidcB) 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 are very minute at birth. 



