of the Fishery Board for Scotland. 



143 



the specific gravity, although it is daring the final stage that the attention 

 is most attracted to this well-marked alteration. 



Kow we have seen that in both demersal and pelagic ova the specific 

 gravity is diminished while the volume is increased. And now we have 

 to find out what this is due to. In order to do so, the first thing to settle 

 is the percentage of water in the ripe and unripe eggs. 



Changes in the Percentage of Water in Different Varieties 



OF Ova. 



Is the transparency of the ripe pelagic egg simply due to an increase 

 in the Avater present, or is it due to a qualitative alteration in the nature 

 of the constituents, or is it possibly due to both these conditions 1 If 

 you place a clear ripe pelagic ovum in fresh water it becomes after some 

 time white and opaque ; the same occurs in salt water if the ova are not 

 allowed to come in contact with a pure supply of the water, as, for example, 

 if they are crowded together or if dirt of any kind has got into the 

 water. We know that the vitellins are kept in solution by weak salt 

 solutions and are precipitated by the addition of water as a whitish preci- 

 pitate. May the opacity of the ovum that occurs on prolonged contact 

 with water not be due to, first, a passage of salts from the ovum outwards, 

 and secondly, a passage in of water poorer in salts, until equilibrium with 

 regard to salt percentage is established, with a consequent precipitation of 

 the vitellin due to the substance which keeps it in solution having been partly 

 removed? Does any nitrogen-holding body pass out ? In order to put this 

 matter to the test, one might carry out the following experiment : — 



Take a known amount of ova (ripe), the water and total solids, salt and 

 N percentage of which are known, and place them in distilled water. 

 After some time has elapsed remove the ova, dry their surfaces, weigh 

 them, dry and estimate water, salts and Examine also the distilled 

 water as to reaction, salts and X. Note also the alteration in specific 

 gravity. See that the ova are not broken and yet that they come into 

 contact frequently with fresh supply of water. This I have done, and 

 shall give my results later on when I speak of the substances which diffuse 

 out from the ripe and unripe plaice ova. 



It was also important to discover whether the ripening of the demersal 

 ovum was simply that of the pelagic cut short before the final stage. 



I subjoin a Table giving the water percentage of different ova, 

 determined by the method described previously. 



Water Percentage of Pelagic ova. 





Unripe. 



Ripe. 



Plaice, 



57-50 



91-86 and 91-46 



Whiting, 



70-71 



96-41 



Cod, 



73-0 



91-14 



Haddock, 



55-00 



95-86 



Turbot, . 



69-9 





Halibut, 



63 0 





Tusk, 



77-2 





John Dory, 



73 06 





Saithe, . 





77-5 (mixture of mature 



Brill, 





and immature) 





90-00 



Flounder, 





87-20 



Ling (fully ripe). 





85-07 per cent, of water. 



Plaice ova were examined in the stage just before maturation and were 

 found to contain 65-38 per cent, water. 

 K 



