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Part III — Twenty -fourth Anmcal Report- 



March). As a rule, however, very much less spawn was collected in the 

 afternoon than in the morning, and part may have been derived from the 

 spawning during the night. 



The difference between the two females was not confined to the 

 spawning. There was a noteworthy difference in the eggs, not merely as 

 to quantity but as to size. After some experience in the enumeration, it 

 was possible to tell whether a collection was from No. 2 or No. 5, owing 

 to this difference. Those from No. 5 were smaller. 



Thus the mean number per cubic centimetre of the eggs from No. 2 

 was 250 (249*98), while the mean number from No. 5 was 245 per 

 cubic centimetre, a difference of 45 per cubic centimetre. In other 

 words, the eggs of one plaice were about 18 per cent, smaller than those 

 of the other. With one exception (23rd April) the number per cubic 

 centimetre was always greater in the second plaice, No. 5, that is to say, 

 its eggs were smaller. 



The ratio of number of eggs to 1 cubic centimetre in No. 2 was 185-35 

 at the beginning of spawning, and 394*5 at the close; in No. 5 it was 

 243-6 at the beginning, and over 300 later, the highest being 364*9 on 

 15th April. 



This difference was partly due to the fact that in No. 2 there was in 

 almost every collection a proportion of the eggs much larger than others ; in 

 some of the collections this was very noticeable. They graduated from 

 very large ones, swollen, and, it might be said, hydropic, to small ones ; 

 the range of size being considerable from the smallest to the largest. 



The eggs spawned by the other female (No. 5) were much more uniform 

 in size. Whether differences of this kind occur under natural conditions 

 I do not know, but such large hydropic eggs of the plaice are sometimes 

 taken from the spawning pond, and on one or two occasions led to the 

 suspicion that they might belong to some other species. They appear to 

 be produced by an excessive imbibition of the watery saline solution that 

 enters the egg at the last stage of maturation.* Whether they are 

 fertilised and behave like the normal egg was not determined. Now and 

 again a single egg was got stained bright yellow, no doubt from the bile ; 

 and several were blue, for what reason I did not discover. 



It may be seen from the Table, moreover, that as spawning proceeded 

 the size of the eggs tended to become smaller. 



The fact may be seen from the averages for No. 5 given in the Table 

 printed above ; and, taking the mean of the first five averages (number 

 per cubic centimetres) and the mean of the last five averages, we have 

 the following : — 



No. 2. No. 5. 



26th-30th March, 213-43 26th-30th March, 255 84 



17th-23rd April, 297 17 19th April-5th May, 320-28 



The increase in the number with the first female was thus 83-74 per 

 cubic centimetre, and in the second, 65 -04 per cubic centimetre, the 

 reduction in size amounting to 39*2 and 25*4 per cent, respectively. 



The fact is of some interest, and is in all probability due to the com- 

 parative exhaustion of the water-secreting function of the ovary, water 

 being, so far as volume is concerned, the chief product of that organ. It 

 might, however, be owing to the eggs spawned later containing less 

 yolk ; a less probable explanation, for several reasons. 



I may add that the mean of a number of fertilised plaice eggs taken 

 from the large spawning pond at the same time was 307 per cubic 

 centimetre, two samples varying from 312*27 to 301-63. 



*Vide Sixteenth Armual Report, Part III., p. 89. 



