186 SATISFACTORY EXPERIMENTS. 



should be expressed, they would be useless for 

 production, for their absorbing pores are still closed 

 against the interpenetration of the milt, and con- 

 sequently in this state impregnation is impossible. 

 The milt of the male, like the ova of the female 

 fish, becomes mature by degrees. When mature 

 they are very easily exuded, for even holding up 

 the fish by the heads will cause limited exudation 

 of milt and ova. The pressure, or weight of the 

 roe above on the roe beneath, is sufficient to pro- 

 duce this partial exudation. 



The facts stated in the preceding paragraph 

 are sufficient not only to account for impregnation 

 taking place at intervals, for the impossibility of 

 its being done tout d'un coup, but also for the im- 

 probability of impregnation by intermission or 

 coitus either before or at the time of the deposition 

 of ova. Mr. Young dissipated all doubts upon 

 this point by the following experiments : — He 

 took a female salmon, exuded by manipulation a 

 portion of her ova, and having simply done so, he 

 buried it beneath the gravel of one part of an 

 artificial spawning-pond. From the same salmon 

 he exuded another portion of ova, but before he co- 

 vered it over with the gravel of another portion 

 of his spawning-bed, he impregnated it by press- 

 ing milt from the male salmon, and causing it 



