of the Fishery hoard for Scotland. 



151 



it will be seen that the smallest eggs have the shortest hatching-time 

 under the same circumstances, and that the curves follow one another 

 from left to right, according to the size of the eggs. The eggs of 

 the cod, however, have the same hatching time as eggs of the haddock, 

 although the latter are a little larger than the former. I am unable at 

 present to explain this apparent irregularity.* 



The hatching time for cod and haddock, at the temperature of 14°, 

 is days; at 12°, days, — a difference for 2° = 1J days, or 16 hours 

 for 1°. 



The hatching time, in water of 4°, is 20 J days ; in 3°, 23 days, a 

 difference of 2 J days, or 60 hours for 1°. The effect of 1° of difference, 

 at a temperature of 3° to 4°, is consequently nearly four times as great as 

 a change of 1° above 12°. 



If, therefore, eggs are hatched in a jar or apparatus, while the tempera- 

 ture is occasionally varying, the average hatching time that is based on 

 such observation will not be in exact correspondence with my curves ; and 

 the difference will be according to the amount of variation there has been 

 in the temperature to which the eggs have been exposed. 



In the graphic representation, I have included some observations on 

 the hatching time, based on an average temperature. 



Mr J. T. Cunningham, in his paper ' On the Relation of the Yolk to 

 ' the Gastrula in Teleosteans, and in other vertebrate types, ' states that 

 whiting hatched on the eleventh day, at an average temperature of 

 7° # 5 C. (Pi. I., 1), and that cod and haddock hatched on the twelfth day 

 at an average temperature of 7 0, 5 C. (PI. I., 2). 



This is not in correspondance with my curve No. III., which shows 

 a hatching time of about 13 days at the same temperature. But Cunningham 

 only says that ^he eggs hatched on the twelfth day, and does not mention 

 if the whole hatching period was confined to that day, or if it continued 

 longer. If his statement is meant to give che day on which emergence 

 began, his observations and mine will correspond. 



In my paper on the operations at Dunbar Marine Hatchery, during the 

 spring season 1894,f the hatching time for plaice is given as 21 days, at 

 an average temperature of 5° "24 C. (PI. I., 3), and 14 days at an average 

 temperature of 8°*86 C. (PL I., 4). 



In the same paper, the hatching time for cod is stated to be fourteen 

 days at an average temperature of 6° '52 C. (PI. I., 5) ; and that for the 

 flounder, six days at an average temperature of 6°'12 C. (PI. 1., 6). 



When compared with one another there will be found a difference in 

 the formation of the curves of incubation. 



No. 1 is nearly straight and No. 3 much curved ; No. 2 and 4 

 seem to follow one another in the high temperatures, and though both 

 sloping much to the right, No. 4 seems most inclined towards the hori- 

 zontal direction. 



The observations and experiments above detailed show: — 1. That the 

 time of incubation is various under the same circumstances for different 

 species. 2. That this difference is in relation to the size of the eggs in 

 this way, that the large eggs take a longer time than the smaller ones. 

 3. That the time of incubation for the same species varies according to 

 the temperature.J 4. That this variation for each degree is comparatively 



* The diameters of the eggs dealt with are — PI. Jlesus = 0 , 92 rain. ; G. merlangus — 

 1*25 mm. ; G. morrhua = l'3$ mm.; ceglqfiwus—1'45 mm.; Pl.platessa = 1*95 mm. 

 t Twelfth Annual Report, Fishery Board, Pt. III. p. 212. 



% In the Kiel Commission Reports for 1874-76, p. 32, Dr C. Kupffer expresses an 

 opposite opinion regarding the herring in the western Baltic. He says thai the 

 development of the eggs of the herring takes place within seven days after impregoa- 

 lion, and independently of the temperature or specific gravity of the water. 



