50 MARVELS OF FISH LIFE 



this time the sparkling water, making its way in little 

 eddies among the stones, carries oxygen to the develop- 

 ing embryo. 



In six weeks two little black dots appear in the egg, 

 which are the eyes of the fish ; and in another six weeks 

 the young trout is ready to hatch. But for days before 

 this the prisoner had found his quarters too confined, 

 and had been wriggling and struggling to get free. After 

 a specially violent effort, the egg membrane splits and 

 the delicate tail appears. Next the back and the head 

 of the fish escape, and finally the encasing " shell " slips 

 off the yolk sac. Strong, healthy fish usually hatch in 

 this manner, but not infrequently the head and shoulders 

 appear first, and occasionally the yolk alone protrudes 

 as a constricted bladder. When this occurs, hatching 

 is always delayed, and not infrequently the young fish 

 dies in the egg. On one occasion, while I was watching 

 salmon hatch, I picked out an egg with just a round 

 bead of yolk sac protruding, and placed the egg in the 

 photographic tank ; presently the head and shoulders also 

 shot through, giving the hatching fish the quaint " jack- 

 in-the-box " appearance shown on the opposite plate. 



Hatching with the yolk sac nipped in, as shown, took 

 a long time, and all the while the young fish kept up a 

 continual fanning movement with his pectoral fins, the 

 object of which will be referred to later. 



The larva of the salmon family when first hatched 

 is known as an " alevin," the name being derived 

 from a French word meaning a young fish. 



