202 



in the cavities of ovaries and oviducts, and are expelled at 

 intervals until tke 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 

 S2:)awning 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 

 decomposes. 



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 5"2°C. 21 days 

 were required ; at 0°, 18j days ; at 10° 12 days ; and at 

 12° 10|^ days. Among Pleuronectidse there is a general 

 correspondence between the size of the egg and the 

 developmental period. Thus the Flounder {FL fesiis) 

 with an egg of ()'95mm. in diameter hatches out in 4| 

 daj-s at 10°C. At the same temperature the Sole {Solea 

 vulgaris), which has an egg of l'4mm. in diameter, re- 

 quires 10, and the Plaice, with an egg of l'8mm., 12 days. 



The changes in the ovum immediateh' 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. 



