201 
after ripening. 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 1°2lmm. and a mean volume of 0°9276 cub. 
mm. It is heavier than sea water. The mature egg has 
a mean diameter of 1°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 107 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—tle 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 lamelle and accumulate 
* Fulton—16th An. Report Scottish Fish. Bd., Pt. iii., p. 88, 1897. 
