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Hemen, who first lias examined the specific ' gravity of the eggs of 

 plaice, states it to be 1, 013G at 17, 3 ° C, corresponding to a salinity of 1 7S °/ . 

 (See Bericht d. Comm. wissensch. Unters. Kiel. 1884, p. 304). 



All eggs of plaice, however, have not the same specific gravity, and 

 in order to fix the limits within which it can change with eggs in our seas 

 I made a closer investigation into this matter by means of the fertilized 

 and living eggs which had been caught in the Faeno Sound, and which 

 were sent to me here in Copenhagen in a box which was isolated with re- 

 spect to heat and cold, and contained glasses with salt water in which the 

 eggs were floating. 



In a cold room I mixed liquids of different specific gravity by means 

 of distilled water and common salt, and by conveying a number of living, 

 fertilized eggs into these (by means of a glass pipette) I arrived at the 

 following results: 



1) The lowest specific gravity at which all the eggs could float was 

 1,0152 a * 9, 8 ° C. (glass areometer), corresponding to a salinity of 1, 85 %• It 

 must be observed, however, that among a great number of eggs there was 

 one, apparently living and unhurt, which would not float at this specific gravity. 



2) The highest specific gravity, at which all the eggs sank to the 

 bottom (of course I speak here only of living eggs), was 1, 0120 a * 10° C,. 

 corresponding to a salinity of l,. u %. 



3) Already at a specific gravity of l, ouo a * 9, s ° C, corresponding to a 

 salinity of 1, 70 °/o, a great number, more than half of them perhaps, sank 

 to the bottom. 



These figures agree very well with those of Hensen, especially when 

 we remember that his eggs, as far as I can see, were taken out of the 

 plaice and not fertilized. 



There was among the eggs I examined some difference in the degree 

 to which the embryos were developed, but this did not seem to influence 

 the specific gravity. — 



Owing to their specific gravity the eggs of plaice then are prevented 

 from being carried by currents into many of our little seas, viz. into those 

 whose salinity is lower than c. 1, 14 %; likewise they are excluded from the 

 Baltic Sea properly so called (perhaps with a few temporary exceptions. 

 See Petterson: Den svenske hydrografiska Expeditionen 1877, p. 116. — 

 Kgl. Sv. Vet. Acad. Handl. vol. 25, no. 1. 1892). 



We might think then that the propagation of the plaice, from this 

 reason already, was impossible in water of a lower salinity than l, u u / ; but 

 there is a possibility that the eggs can be developed on the bottom, though 



