266 BONY FISHES ix. 12 



50,000 eggs. The large numbers laid by the pelagic species are pre- 

 sumably an insurance against failure of fertilization and especially 

 against random elimination of the eggs and young. The greater the 

 care devoted to the young by the parents the smaller the number of 

 eggs produced (see, however, p. 283). 



Demersal eggs, especially of freshwater animals, are usually laid 

 with some special sticky covering, by means of which they are 

 attached to each other and to the bottom or to stones, weeds, &c. 

 Thus the eggs of many cyprinids (carp, &c.) are attached to weeds. 

 The eggs of salmon and trout, however, though demersal, are not 

 sticky. From depositing eggs on weeds it is only a short step to the 

 building of a nest and guarding of the eggs by one or both parents. 

 Thus the sand goby (Gobius minutus) lays its eggs in some protected 

 spot, where they are guarded by the male, who aerates them by his 

 movements. Quite elaborate nests may be built, as by the sticklebacks 

 (Gasterosteus), where pairs remain together throughout the breeding- 

 season. A still further development is the retention of the young 

 within the body. In some catfishes they develop within the mouth of 

 either parent. In pipe-fishes and sea-horses the males are provided 

 with special pouches for the young. 



Although external fertilization is usual, various teleosts show 

 internal fertilization and the young then develop within the ovary 

 (Zoarces, Gambnsia, Lebistes). The mechanisms by which mating and 

 the nutrition of the embryos are assured in these cases show some 

 interesting parallels with the conditions in mammals, including the 

 formation of placentae or nutritive material. In Lebistes the female 

 adopts a special position of readiness for copulation, and this has been 

 shown to depend partly on an internal factor in the female and partly 

 on a substance secreted into the water by the male. The embryos are 

 not attached to the wall of the ovary but develop free in the sac, feed- 

 ing upon an 'embryotrophic' material, apparently produced by the 

 discharged ovarian follicles, which become highly vascular and remain 

 throughout the several months of 'pregnancy'. 



Rhodeus amarus, the bitterling (Cyprinidae), shows somewhat 

 similar conditions (Fig. 160). Association of the sexes at mating is 

 here made necessary by the fact that the eggs are laid within the 

 siphon of a swan mussel. For this purpose the cloaca of the female 

 develops into a tubular ovipositor. This development takes place 

 under the influence of a hormone produced by the ovary. Addition of 

 progesterone and related substances to the water containing the fish 

 causes growth of the ovipositor. 



