Distribution of Plaice Eggs. 7 



geiieous yolk and no oil-globule. The perivitelline space is relatively 

 very small, whilst the thick capsule shows under the microscope a 

 wi'inkled appearance. The yolk becomes quite opaque when the egg 

 is preserved, and it can be distinguished from other eggs only by its 

 larger size and the corrugations on the capsule. The eggs of the plaice 

 are amongst the largest found in the plankton, and in the present col- 

 lections they form a group whose sizes are almost entirely beyond those 

 of the other gToups. As is seen in the examples already given for Feb- 

 ruary, only a very small percentage of specimens at the lower limit of 

 the range may overlap the other group. If the embryo is far advanced 

 in development a separation may at once be made ; and in addition 

 the relative thickness of the capsule, the time and place of spawning 

 are valuable aids to identification. Thus, the percentage of errors 

 of identification is negligible. 



It is unnecessary, in the present instance, to deal exhaustively with 

 the question of the distribution of sizes of plaice eggs throughout the 

 spawning period within the different Scottish areas, but one illustration 

 is given for the month of February from the Dornoch Firth. 



Variation in 6218 plaice eggs collected from the Dornoch Firth in 

 February (the material preserved in 2| per cent, formalin over one year). 

 (See Fig. 4). 



— 6217-362 6218 



90 scale divisions — 2 mm. 



Mean— 88-424089. 



Standard Deviation — 1*2759 (in units of measurement — 3-8277). 



Mode— 88-616164. 



P.— -02. 



Although this represents a curve which approaches very closely to 

 the Normal Curve, it is best described by a curve Type I. (Pearson). 

 From similar material of 6218 eggs a worse fit would occur only once 

 in fifty samples. 



Heincke and Ehrenbaum (1899) point out a nmnber of factors 

 which may influence the mean diameter of the eggs of a species. There 

 may be local races of the same species. The average diameter of the 

 eggs depends largely on the salinity of the water in which they were 

 spawned. The older and larger fish have larger eggs than the smaller 

 and younger. The eggs are relatively larger at the beginning of the 



