XV 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 Anadbleps 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- 
Fic. 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 
