201 



after lipening-. In the final stage of maturation before 

 spawning occurs these small opaque eggs acquire the 

 characters of the ripe pelagic egg* The nucleus breaks 

 down and the chromatic matter becomes rearranged, and 

 at the same time fluid of a low specific gravity, secreted by 

 the follicular epithelium enters. As a result of this im- 

 bibition of fluid the yolk becomes altered in such a way 

 as to become nearly perfectly transparent. At the same 

 time the egg becomes larger and its specific gravity 

 becomes less. The immature ovarian egg has a mean 

 diameter of l'21mm. and a mean volume of 0'92T6 cub. 

 mm. It is heavier than sea water. The mature egg has 

 a mean diameter of l*88mm. and a mean volume of 3'479 

 cub. mm. It is very slightly lighter than normal sea 

 water. The change in specific gravity during maturation 

 is from about 1'07 in the immature to about 1'025 in the 

 mature egg. Changes of this nature are general in the 

 maturation of nearly all Teleostean food fishes, and as a 

 result the eggs are pelagic — that is they float freely near 

 the surface of the sea when extruded. In the eggs belong- 

 ing to the other type — the demersal egg, of which the egg 

 of the herring is the most familiar example — the specific 

 gravity is greater than that of normal sea-water. As a 

 result of this the eggs undergo their development lying on 

 the sea bottom. 



The Plaice takes about two weeks to extrude the 

 whole contents of its ovary. Obviously in the limited 

 space at the disposal of that organ ripening of all the 

 eggs present would be impossible without injury to the 

 fish. Spawning is therefore intermittent during the 

 season of the fish, only comparatively few eggs being dis- 

 charged at one time. As the latter mature they dehisce 

 from the walls of the ovigerous lamellae and accumulate 

 * Pulton— 16th An. Eeport Scottish Fish. Bd., Pt. iii., p. 88, 1897. 



