ae BREEDING Anz 
secure the same result." By means of its long ovipositor the 
female Fish deposits its eggs in the mantle cavity of a Unio, 
or of an Anodon. Here they are fertilised by spermatozoa 
carried in through the inhalent siphon of the Molluse with the 
inspiratory water current, and they complete their development 
in the gill-cavities (Fig. 237). 
The time which elapses between the fertilisation of the egg 
and the hatching out of the young Fish varies greatly in 
different Teleosts. The eggs of some Clupeidae hatch in a very 
short time, two to three days in the Anchovy, and three to four 
days in the Sprat. In most of the British marine food Fishes the 
period rarely exceeds twelve to fourteen days. The larger 
demersal eggs with much food-yolk are longer in hatching; in 
the Salmon the time ranging from thirty-five to one hundred 
and forty-eight days. A low temperature lengthens the time. 
The eggs of the Herring which hatched in eight to nine days at 
a temperature of 52° to 58° F. took forty-seven days in water at 
32° F. 
The extent to which the development of the embryo proceeds 
while it is still enclosed in the egg-membranes, and consequently 
the condition of the embryo when hatched, depends largely but 
not exclusively on the quantity of food-yolk which is present in 
the egg and available for the nutrition of the embryo during its 
earlier stages. Embryos hatched from pelagic ova are very 
small and imperfectly developed. The mouth is usually not yet 
formed. The median fins, which later become isolated, are 
continuous, and the caudal fin is diphycereal, although it sub- 
sequently becomes homocercal after passing through a_hetero- 
cercal stage. The blood is colourless, and even the gill-clefts 
may at first be lacking. In this condition the newly-hatched 
Fish is nourished at the expense of the residual food-yolk, which 
is enclosed in a yolk-sac projecting from the ventral surface of 
the body (Fig. 258). As the yolk is gradually used up the mouth 
is formed, and the young Fish feeds on the minute organisms of 
various kinds living in the sea, and by degrees the form, propor- 
tions, and structure of the more mature Fish are acquired. In 
the case of the larger demersal eggs the young are not only 
longer in hatching, but when hatched they are larger and more 
advanced in development. The young of many Fishes are 
1 Olt, Zeitschr. wiss. Zool. lv. 1893, p. 543. 2-Cf. p. 584. 
VOL. VII 25 
