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in the cavities of ovaries and oviducts, and are expelled at 
intervals until the whole organs are exhausted. The fish 
is then said to be “ spent.” 
Fertilization takes place in the surrounding sea water 
into which the eggs are extruded. During the spawning 
season males and females are crowded together on the 
spawning grounds, and the spermatozoa of the former are 
shed simultaneously with the eggs. It is impossible to 
say what proportion of the eggs extruded escape fertiliza- 
tion. Probably it is very small, since unfertilized eggs 
are not often seen in plankton collections. Fertilized and 
unfertilized eggs would both rise towards the surface, but 
in the course of a few days the unfertilized egg becomes 
opaque, heavier, and finally sinks to the bottom and 
decom poses. 
Embryonic Development.—The period occupied by 
embryonic development varies with the temperature of the 
water in which the eggs are contained. The lower the 
temperature the longer is the period between fertilization 
and hatching. H. Dannevig* found that at 52°C. 21 days 
were required; at 6°, 18} days; at 10° 12 days; and at 
12° 104 days. Among Pleuronectide there is a general 
correspondence between the size of the egg and the 
developmental period. ‘Thus the Flounder (Pl. flesus) 
with an egg of 0'95mm. in diameter hatches out in 4} 
days at 10°C. At the same temperature the Sole (Solea 
vulgaris), which has an egg of 14mm. in diameter, re- 
quires 10, and the Plaice, with an egg of 1'8mm., 12 days. 
The changes in the ovum immediately succeeding 
fertilization have not been closely studied by any author. 
The spermatozoon enters the egg through the micropyle ; 
at this time segregation of the protoplasm takes place, 
and the latter which had previously been diffused round 
*13th An. Report Scottish Fish. Bd., Pt. iii., p. 147, 1894. 
