58 THE SEA FISHERIES 



The egg of the plaice is buoyant, and one of the largest of the 

 Pleuronectids, being a little less than one-twelfth of an inch in 

 diameter. It is enclosed in a fairly tough capsule, the outer surface 

 being finely corrugated, and when extruded from the body of the 

 adult female is transparent. Before ripening the ovary of the 

 plaice contains opaque eggs considerably smaller than those sub- 

 sequently extruded. In the final stage of maturation before 

 spawning, these small opaque eggs acquire the character of the 

 ripe pelagic egg.^ Fluid of low specific gravity enters the egg, 

 which as a result becomes almost transparent. The egg becomes 

 larger, and its specific gravity less. The immature ovarian egg is 

 heavier than sea water, whereas the mature egg is very slightly 

 lighter than sea water. Intra-ovarian changes of this nature are 

 general in the maturation of the ovum of bony food fish, and as a 

 result the eggs are pelagic, and float near the surface of the sea 

 when extruded. The duration of the spawning period in the case 

 of a plaice is not exactly known. According to Johnstone^ the 

 plaice takes about two weeks to extrude the whole contents of its 

 ovary. Later experiments conducted by Fulton show that in the 

 case of plaice kept in confinement the extrusion of eggs was spread 

 over four weeks. Fertilisation of the eggs takes place in the sea 

 water into which the eggs are extruded. During the spawning season 

 the males and females are crowded together on the spawning 

 grounds, the extrusion of the sperms taking place at the same time 

 as that of the ova. 



It is difficult to estimate the number or proportion of eggs which 

 escape fertilisation in natural conditions in the sea ; but probably 

 the percentage is a very small one, since unfertilised eggs are 

 extremely rarely found.' 



The period of development varies with the temperature, being 

 longer at a lower temperature. Eggs fertilised in January at 

 Dunbar hatched in from i6 to i8 days, while those hatched in 

 April had a period of incubation of 8 or 9 days only. Dannevig* 

 found that at 5-2 C, 21 days were required ; at 6 C, 18J days ; at 

 10 C, 12 days ; and at 12 C, io| days. Among the Pleuronectidse 

 there is a general correspondence between the size of the egg and the 

 developmental period, other things being equal. The flounder 

 with an e^ of -095 mm, in diameter hatches out in 4I days at 10 C, 

 while at the same temperature the sole, which has an egg of 1-4 mm. 

 in diameter requires 10, and the plaice with an egg of i-8 mm. 

 12 days. For the technical details of embryonic development and 



1 Fulton, i(>th Ann. Rept. Scots. Fish. Bd., Part III, p. 88, 1897. 

 = Op. cit., p. 201. 



3 24<A Ann. Rept. Scots. Fish. Bd., Part III, p. 281, 1906. 

 ' 13th Ann. Rept. Scots. Fish. Bd., Part III, p. 147, 1894. 



