82 MARKETABLE BRITISH MARINE FISHES CHAP. 



trawlers in the spring of the year when the line fishermen catch 

 none but male, immature, or spent fish. 



To return now to the consideration of the size of the male 

 fish in comparison with that of the female, it cannot be said that 

 the differences are at present fully explained. The size of the 

 male which produces the milt is of no importance to fertilisation. 

 It appears however that the male fish are smaller than the 

 females in those species in which the milts are much smaller than 

 the roes, although there is by no means a close correspondence 

 between the fish and the milts in this respect. For example, 

 the males in the cod and similar fishes are usually larger than 

 the females, although the milts are smaller than the roes, and in 

 the sole and other flat-fishes the males are by no means as 

 inferior in size as are the milts in comparison with the roes. 



Owing to the fact that fertilisation of the eggs of bony fishes 

 naturally takes place in the water outside the bodies of the 

 parent fish, and in consequence of the ease with which the 

 ripe milt and eggs can be obtained from the fish by gentle 

 pressure on the abdomen, artificial fertilisation of the eggs is 

 rendered possible and in most cases easy. All that is required 

 is to squeeze the ripe eggs and a little ripe milt into a vessel of 

 clean sea-water. But in the practical application of this process 

 the questions of the rate of spawning and the size of the soft 

 roe are of considerable importance. In the case of fishes which 

 spawn rapidly for instance, the herring, cod, plaice, or flounder 

 all or a large proportion of the total number of eggs pro- 

 duced by the fish in one season can be fertilised at one time. 

 The hatching of the eggs so obtained depends on placing 

 them under conditions suitable to their healthy life. But in 

 certain cases artificial fertilisation is difficult. For instance in 

 the sole only a small number of ripe eggs can be obtained from 

 a single fish, and to fertilise these it has generally been found 

 necessary to open the male fish, take out the small milts, and 

 squeeze them up in the water containing the eggs. It has been 

 found preferable to keep the spawning fish in ponds suitably 

 arranged, and collect the fertilised eggs naturally shed. In this 

 way the risk or certainty of injuring the fish by squeezing is 

 avoided. But here in some cases another difficulty arises. The 

 plaice and cod under such conditions are found to spawn freely, 

 but until the year 1895 the sole had never spawned in the tanks 



