THE PARR 157 



the lack of water in a rainless season. Cold retards, 

 but does not rob the ova of their vitality after this 

 period has elapsed, so long as the eggs are moist and 

 in a moist atmosphere. From ten to fifteen weeks 

 may, however, be taken as covering the average 

 period of incubation, provided that no abnormal 

 circumstances arise to check the process. At the end 

 of this period the walls of the egg break down, probably 

 owing to the growth and strength of its inhabitant, 

 and the alevin emerges. It is supposed by some 

 that the young fish nibbles its way through the shell. 

 As, however, the mouth of the alevin is not capable 

 of any noticeably muscular action sufficiently great 

 to effect this until at least some time after leaving 

 the egg, I think this must be a purely analogous 

 supposition based on the known action of young 

 birds, etc. For a period of about five weeks the alevin 

 generally remains hidden amid the crevices of the 

 gravel and pebbles forming the redd. After it has left 

 the egg its food is provided by the yolk or umbilical 

 sac attached to its body, which not only hinders its 

 movements and compels it to remain within the 

 shelter of the redd, but provides it with sufficient food 

 to sustain life and add to its strength and growth. 



THE PARE, 



Here again a higher or lower temperature plays an 

 important part in shortening or lengthening the alevin 

 stage of the salmon fry. At the end of this stage, 



