30 LIFE-HISTORY AND DEVELOPMENT OF 



to temperature), comes a minute and nearly transparent fish 

 (Fig. 1), which at first is often as passive in the currents as 

 the eggs themselves 1 . The tiny young in their helpless state 

 are carried along with multitudes of eggs by every tide into 

 sheltered creeks and bays, peopling waters in which, perhaps, 

 no adults are, and finding both safety and food in the shallower 

 water. These little larvae, each furnished with a yolk-sac, are 

 so fragile that they would seem to fall an easy prey to hosts 

 of swimming companions, on which in the adult state they 



FIG. 1. Larval Ling, immediately after hatching. 



would not even deign to feed. The larva soon, however, uses 

 its tail for swimming and its breast-fins for balancing. Its 

 shape is somewhat like that of a tadpole, partly from the 

 large head, but mainly from the great size of the yolk-sac, 

 which contains a store of nourishment on which the little 

 mouthless creature, about 3 mm. long, sustains itself for a 

 week or ten days. In this respect it somewhat resembles 

 the young salmon, in which a much larger collection of the 

 same food supports it about six weeks amongst the gravel in 

 the spawning-bed of the river, though a closer scrutiny reveals 

 certain essential differences. Thus the store of nourishment 

 in the yolk-sac of the salmon is taken up by the blood-vessels 

 which branch in a complex manner over the whole yolk ; 

 whereas in the young cod, though the heart is present and 

 pulsating, not a blood-vessel at first is seen, and none ever 

 enters the yolk-sac. The absorption of this nourishment there- 

 fore must take place by aid of the cells and tissues themselves, 



1 For some years the development of fishes has been studied by able 

 workers, amongst others on the Continent, by Gotte, Kupffer, Hoffman, 

 Henneguy, E. Van Beneden, Osjannikov and Raffaele; in America, by Alex. 

 Agassiz, Ryder, Whitman and Bashford Dean ; while in our own country, 

 Ransom, Klein, Cunningham, Prince, Brook and Holt have carried out similar 

 researches. 



