THE DEVELOPMENT OF ORGANISMS 



763 



the large yolk-sac, which furnishes food for a month or two. They 

 cannot swim rightly because of the nutritive reserves in the yolk 

 sac, and they hide among loose stones. When they have absorbed 

 all their store, they have changed into inch-long fry which are able to 

 fend for themselves. By the end of the year those that have been 

 lucky enough and quick enough to survive have become parr, 

 perhaps four inches long, hungry for insect larvae. They show seven 

 to eleven large bluish marks, oblong or oval in shape, along each 

 side of the body, and this stage continues till the young fishes are 

 about two and a quarter years of age, and are commonly about six 

 inches long. At that time the "finger marks" are masked by a 

 silvery sea-jacket, and there is a change of constitution, expressing 



Fig. 124. 



Early Stages in the Life-History of Angler-Fish [Lophius piscatorius) . After 

 Bowman, i, the floating egg with the embryo fish developing; 2, the 

 hatched larva, with the yolk-sac uppermost in the water; 3 and 4, two 

 pelagic stages, showing the exaggerated tassel-like outgrowths, which 

 probably increase the power of flotation. In this case the larvae are 

 conspicuously unlike the subsequent adult form, or, what comes to the 

 same thing, their adult parents. 



itself in restlessness. In other words, the parr have changed into 

 smolts, which are said to "hear the call of the sea", though we do 

 not know with any precision what this means. Some internal change 

 has pulled the trigger of a hereditary impulse to make for the salt 

 water. In the sea, with a change of diet, the smolts become grilse, 

 but the intermediate stages are little known. A grilse is a young 

 salmon, about three and a half years old, that has not yet quite 

 put on the adult characters. Its scales show, for the period since it 

 entered the sea, a broad summer zone, a narrow winter band, and 

 the beginning of another summer zone. The grilse has not the fuU 

 shape of a salmon, being more slender towards the tail; and the 

 teeth of the upper jaw are different. It may re-enter 'the river 

 during the summer and may spawn in the fall of the year; but 



