GKAVITY OF SALMON-SPAWN. 187 



to come in contact with the last ova deposited. 

 He then covered them in beneath the gravel, and 

 in due time they produced fish. The ova he had 

 covered in without impregnation produced nothing. 

 He repeated the experiment frequently, and al- 

 ways with similar results. He has even taken 

 two female salmon in the act of spawning. The 

 ova of one he impregnated with milt from a male ; 

 the ova of the other he did not impregnate. He 

 covered in each under equal conditions, apart in 

 the same spawning-bed. The ova that he had 

 caused to be impregnated were productive ; the 

 other proved perfectly barren. This experiment 

 was repeated, and the result was ever the same. 



It may be asked, how is it that ova and milt 

 are not swept away by the action of the rapid 

 water in which they are deposited — that they are 

 not swept away in the act of deposition, and be- 

 fore they can be covered in beneath the gravel ? 

 It would appear that at the critical moment of 

 deposition the specific gravity of the water is less 

 than that of the mature spawn, for the grains of 

 ova fall to the bottom like grains of shot, and the 

 milt sinks as if it were molten lead. However, 

 we must take into account the effect of the force 

 by which ova and milt are expelled downwards 

 by the spawning fish. 



