Hatching. 59 



seen a writhing, wriggling mass of alevins, each 

 trying with might and main to hide itself under 

 its neighbours, all heads directed towards the 

 corner, and the strongest youngsters lying in, 

 until forced out by superior numbers ; and this 

 goes on for some time, it may be a fortnight or 

 three weeks, according to the temperature of 

 the water. During this time the alevins will 

 have gained greatly in size, grace, and sym- 

 metry. They will have become wonderfully 

 active, swimming rapidly up and down the 

 tube, and their fins will be seen to have grown, 

 and the tail developed to a surprising extent. 

 The umbilical sac or vesicle, too, will have half, 

 or more than half disappeared, having been 

 gradually absorbed during this period, to yield 

 the necessary nourishment for the evolution of 

 the various organs of the fish, which were at 

 first in a rudimentary state. 



After this they gradually separate, leaving 

 the corners of the trough and covering its whole 



