BREEDING 419 



yolk present in the eggs, and partly by a nutritive secretion 

 derived from the ovarian walls or from the epithelial wall of the 

 ovisacs as the case may be. In Anableps the secretion of the 

 walls of the ovisacs is absorbed by papillae developed on 

 the surface of the yolk-sac of the embryo along the course of its 

 blood-vessels. The eggs of the Embiotocidae have little food- 

 yolk, and the embryos are mainly nourished by the secretion of 

 the ovarian walls, which is swallowed by the embryo and 

 absorbed by villi on the inner surface of the intestine. The 

 number of young produced varies considerably. In the Embi- 



e.g 



FIG. 239. Young Gymnarchus niloticus, with its large yolk-sac (y.s) and its long 

 external gills (e.g}. (From Budgett.) 



otocidae the ovarian cavity contains 40 to 50 young. The 

 viviparous Scorpaenid, Sebastes norvegicus of Northern Europe, 

 produces, it has been estimated, about 1000 young, while the 

 Blenny (Zoarces viviparus), the only other European viviparous 

 Teleost, produces from 20 to 300 or more, according to the size 

 of the female. In the Blenny the eggs are hatched in about 

 twenty days, but the young are not born until about four 

 months after fertilisation, when they are about an inch and a 

 half long, and in every outward respect similar to the adult 

 Fish. 



Besides the distinction between the sexes resulting from the 

 different nature of their gonads and sex-cells, the males and 

 females are often distinguished by secondary sexual characters 



