8 Fishery Board for Scotland. 



spawning season, and on any particular area there is a gradual decrease 

 in the mean size of the eggs of a species during the spawning season. 

 The mean size of the egg differs at the different developmental stages ; 

 when the embryo is almost ready to hatch there is a stretching of the 

 egg-membrane and thus an increase in size. There are personal errors 

 of measurement as well as errors of sampling. In spite of all such 

 possible variations it is believed that these do not to any appreciable 

 extent affect the identification of the plaice eggs here dealt with. 



The most general survey of the distribution of the plaice eggs from 

 November to March in the Northern North Sea is best got from the 

 combined results of all the hauls taken within the ten years' period. 

 The localities are at once divided into those in which plaice eggs were 

 found and those from which none were recorded. The positive records 

 are from the shallower areas, and the great bulk of the negative records 

 occurs over the deeper parts. As a matter of fact, all the positive records 

 are from within the fifty fathom contour line, and a closer study of the 

 distribution shows clearly that the greatest frequency is within the 

 thirty fathom contour line. It has to be remembered also that, in 

 the case of a pelagic egg, such as that of the plaice, the spawning area is 

 not necessarily coextensive with the distribution of the floating eggs. 

 The general statement may, however, be made that the distribution of 

 plaice eggs coincides very closely with the known distribution of the 

 adults. 



Spawning on the Scottish coast usually begins in the winter months 

 and continues throughout the spring, while a small number of indi- 

 viduals may extend the spawning into the early summer. According 

 to Ehrenbaum (1909) the spawning time for plaice in the North 

 Sea extends from January to the middle of June, whilst in the Baltic 

 spawning may begin as early as November and last till May. There 

 is great difficulty in fixing absolutely the first date of spawning over 

 such an extended area as the Scottish coast since visits to each locality 

 must be comparatively infrequent. Even over such a prolonged 

 period as ten years the difficulties are many, for one year may be more 

 favourable than another for an early spawning. 



The first records of the pelagic eggs may not coincide with the 

 earliest date of spawning, for unless investigations are being con- 

 tinuously carried on in a particular area the first occurrence of the eggs 

 may not be noted until some time has elapsed after their actual appear- 

 ance. In the fertilised egg when first spawned there is no trace of an 

 embryo, though very soon it assumes definite shape, and in the course of 

 time surrounds the whole yolk. It is known that the rapidity of 

 development of the embryo depends almost entirely on the temperature 

 of the water, and the degree of development of the embryo thus gives 

 some clue as to the time the egg was spawned. For the pm-pose of 

 this investigation the eggs have been separated into four groups 

 according to the degree of development of the contained embryo. It 

 is not implied that these four stages divide the hatching period into 

 equal intervals of time. 



a. — The newly-hatched egg before there is any indication of form 



in the embryo. 

 |3. — When the embryo has begun to assume a definite form but does 



not extend round half the diameter of the yolk. 



