BREEDING 417 



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 Mollusc 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 hut 

 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 

 smaU. and imperfectly developed. The mouth is usually not yet 

 formed. The median fins, which later become isolated, are 

 continuous, and the caudal fin is diphycercal, 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. 238). 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 

 ' Olt, Zeitschr. wiss. Zool. Iv. 1893, p. 643. = (y_ p, 534. 



VOL. VII 2 E 



