THE RATE OF GROWTH OF FOOD-FISHES. 



103 



between the duration of the incubatory period and the tempera- 

 ture. Thus by hatching eggs of some of the commoner species 

 at successive constant temperatures and comparing the results, 

 he has been enabled to make curves dependent on geometrical 

 principles, which show the exact length of the period of 

 incubation at any given temperature. The subjoined table 

 is a reprint from his work\ He was led, from his observations, 

 to the following conclusions : — 



' 1. That the period 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. 



' 4. That this variation for each degree is comparatively 

 larger in a low temperature than in a high. 



' 5. That development of fish-eggs takes place also at 

 temperatures below zero (Centigrade), when the specific gravity 

 of the water is sufficiently high to prevent it from freezing.' 



Dannevig obtained almost the same results with the 

 eggs of the cod and those of the haddock, but it is well to 

 remember that although the average size of the former is 

 slightly smaller than that of the latter yet the two overlap 

 each other considerably. Thus Mr Williamson'^ recently found 

 that out of a considerable number of examples, the smallest 

 cod's egg was 1"35 mm. in diameter and the smallest haddock's 

 egg was 1"38 mm., whilst the largest haddock's was 1'458 mm. 

 and the largest cod's was 1-467 mm. in diameter. The mean 



I3th Ann. S.F.B. Report. 



