EGGS AND ALEVINS 75 



the sea-trout eggs will begin to hatch out through the imprisoned 

 embryo having become strong enough to burst its containing envelope. 

 The shorter time taken to hatch out of sea-trout and trout eggs 

 respectively may possibly be due to the higher temperature of the water 

 when the eggs are shed and during the earlier stages of incubation; at 

 any rate, in all cases, cold weather greatly retards the hatching process. 

 Most of the little fish, which when clear of the shell are known as 

 " alevins," hatch out tail first, and easily succeed in detaching them- 

 selves from the shell. Some few, however, but in what proportion is 

 not known, hatch out head first and these, unless released forthwith, 

 generally perish, but whether by strangulation or through some inherent 

 lack of vitality in the embryo I am unable to say. In Figs. 25, 26, 27 

 and 28 will be seen reproductions of several photographs of eggs and 

 alevins, in one instance (Fig. 27) of the alevins actually in the process 

 of hatching out. 



The little alevin is very unlike a fish. It is barely an inch in length, 

 and has a blunt head of which the prominent black eyes are the most 

 striking feature. The body is transparent and through it the rudimen- 

 tary organs are clearly seen. The pectoral fins are separate and keep 

 up a constant fanning movement, but the other fins yet consist of a 

 continuous fringe round the margin of the body, a fringe that will not 

 evolve into the permanent fins for several weeks. 



But the most remarkable peculiarity of the alevin is a curious 

 transparent appendage called the umbilical sac, or yolk-sac, which 

 hangs between throat and vent, at first equal in bulk to the true body of 

 the fish. This sac, in which little drops of oil may be seen, gives 

 nourishment to the alevin during such period as it is unable either to 

 swim about or to feed in a natural manner. In fact, the alevin shuns 

 the light and wriggles into the deepest crevices it can find, prompted 

 doubtless by the instinct of self-preservation. But the umbilical sac 

 becomes gradually absorbed, the fish becomes bolder, and in about 50 



