19 



cod and plaice do not become mature at the same time. 

 In the case of the plaice the proportion is very much 

 smaller than in the cod. As a result the spawning period 

 of each individual hsh lasts for some time. In the case of 

 the plaice it may last a fortnight at least. As successive 

 batches of eggs ripen they are expelled, until the whole 

 ovaries are spent. The exact number of ripe eggs that 

 are expelled at each successive emission is not known ; 

 but from the experience in the Piel hatchery, and our 

 observations on the steamer lead us to the conclusion 

 that it is much the same proportion in the case of trawled 

 ripe fish, we conclude the number does not go beyond a 

 feAv thousands in the case of the plaice. It is possible by 

 means of pressure on the abdomen to expel 20,000 to 

 50,000 eggs at least which are fairly transparent, but only 

 a small proportion of this number are perfectly mature 

 and can be fertilized. 



How obvious this is, is seen from a consideration of Dr. 

 Fulton's work on the maturation of pelagic Teleostean 

 eggs. Some time before the final ripening begins, the 

 ovaries contain only small opaque ova. Even then their 

 size is considerable. In plaice they extend so much into 

 the body cavity, that the volume of the latter is reduced 

 by one half, and the intestine and stomach are compressed 

 into the anterior portion of the cavity. During matura- 

 tion, as Dr. Fulton has shown, the eggs absorb water and 

 undergo a considerable increase in size. The immature 

 ovarian egg of the plaice has a volume of about 0'9276 

 cmm., whilst the ripe pelagic egg has a volume equal to 

 3'479 cmm. That is, during maturation the volume of 

 the egg is increased nearly four times. Dr. Fulton says, 

 " It is physically impossible for a female producing 

 pelagic eggs to carry all her eggs in the mature distended 

 state, because the volume of the ripe eggs may approxi- 



