102 THE RATE OF GROWTH <>F FOOD-FISHES. 



size of some of the same age reared under natural conditions, 

 but that at 5 months the average sizes are closely similar. 

 Dr Meyer was inclined to ascribe the small size of the former 

 to the insufficient or unsuitable food. 



Detailed experiments have yet to be made with regard to 

 the direct effect of varying the supply of food upon the growth 

 of fishes of a more advanced age than these herrings. 



Temperature. One of the commonest observations in re- 

 spect to the development of fishes is the great variation in the 

 length of the period of incubation according to the temperature 

 of the surrounding medium ; indeed it is now usual for natu- 

 ralists in quoting the duration of incubation, in any given 

 instance, to mention the temperature of the water in which 

 the eggs were hatched. 



Dr Meyer, in the same series of observations as above re- 

 ferred to, experimented with the eggs of the herring, and came to 

 some important conclusions. He found that at a temperature 

 of 518F. to 536F. they hatched in 10 to 11 days, but at 

 a temperature of 356 F. the embryos emerged on the 29th 

 to 33rd day, and even later. Again at 32 F. the hatching 

 period was postponed till the 47th day, and at 3056 F. normal 

 development was no longer possible. At this low temperature 

 the yolk became opaque and burst the egg-capsule, thus 

 destroying the embryo. He also found that the influence of 

 cold in retarding growth is more marked upon the later stages 

 than upon the earlier 1 . 



It might at first be supposed that we have here a pheno- 

 menon of retardation of development or differentiation and not 

 of growth, but it must be remembered that in Vertebrate 

 embryos all the earliest stages are almost entirely taken up 

 with processes of growth or cell-division, and that differen- 

 tiation of organs only takes place later, so that it is not a great 

 assumption to suppose that we have to deal mainly with the 

 retardation of growth. 



Recently Mr Harald Dannevig' 2 has attempted with some 

 success to determine in an exact manner the connection 



1 Vide also W. C. M. Nature, Vol. 34. 



2 Superintendent of the Hatchery at Duubar. 



