4 Fishery Board for Scotland. 



cheese-cloth material is very uniform, the average diameter of the 

 thread being about '3 mm. and on the average 90 meshes per square cm. 

 This material is of a texture sufficiently fine to capture the smallest 

 fish eggs occurring in the plankton. At most of the stations a vertical 

 haul as well as horizontal hauls at surface, mid-water, and bottom 

 layers were taken with these nets. On many occasions hauls were 

 also taken with the Petersen Young Fish Trawl, but the texture of 

 the material of this net was often too open to retain fish eggs. 



The tow-net material when collected was immediately preserved in 

 4 per cent, formalin solution in sea- water, and after a longer or shorter 

 interval the larval fishes and fish eggs were selected and transferred to 

 a 2J per cent, formalin solution. With the exception of the 1913 collec- 

 tions the eggs were not examined carefully until they had remained 

 more than a year in formalin. Such eggs have been sufficiently long 

 in formalin to reach their maximum shrinkage, and in comparing their 

 sizes with eggs from other sources this fact must not be forgotten. 



In the initial stages of development many of these pelagic eggs, 

 which become opaque when preserved in formalin, are almost character- 

 less, except as regards size. The following method has been adopted 

 in dealing with the different collections. The eggs were separated into 

 two groups according as to whether they had or had not contained oil- 

 globules. It is only in the collections made towards the end of March 

 and in the later months that relatively large eggs, with oil-globules ap- 

 pearing in the plankton, might possibly be confused with plaice eggs on 

 account of their size. But the presence or absence of an oil-globule in 

 any particular specimen may be determined by examination under the 

 microscope, and even if the oil has been extracted by the preserving 

 fluid the cavity in which the oil originally lay may still be detected. 



In the Northern North Sea during the period under observation 

 eggs without oil-globules have a wide range in size, varying in diameter 

 from "7 mm. to over 2 mm. Some, for example, those belonging to the 

 Dragonet {Callionymus sp.) and the Long Rough Dab {Drepanopsetta 

 platessoids) may be separated directly. The Long Rough Dab egg is 

 very easily identified both in the living condition and in preserved 

 material as the large peri vitelline space within the egg is characteristic. 

 In the preserved material Long Rough Dab eggs usually appear very 

 buoyant and semi-transparent owing to their relatively small amount 

 of yolk. When the large perivitelline space is indistinct a very small 

 percentage may, however, be confused with plaice eggs as the ranges in 

 size overlap, but the thin character of the capsule of the Long Rough 

 Dab egg is readily seen under the microscope. Further, eggs such as 

 those of the haddock and cod are also easily identified when the con- 

 tained embryo has reached a certain stage in development. 



All eggs in the initial stages of development have been measured 

 under the microscope with the aid of a micrometer eye-piece, ninety 

 divisions of which are equivalent to 2 mm. It is interesting to find that 

 when eggs without oil-globules from any locality for a particular period 

 of the year are arranged according to diameter they fall into natural 

 groups, and that in successive years these groups are almost identical. 

 Two illustrations are given (see Figs. 1, 2, and 3) for February 1905 and 

 February 1907. The groups are almost identical, although the relative 

 numbers within the different groups vary from year to year. 



The plaice egg is pelagic, large, and transparent, with a homo- 



