1917 Riddle; on Chinook Salmon 325 



derm covers one-third of the yolk, and two somites appear (Fig. 19). By 

 the tenth day the germ ring has passed the equator of the yolk, and thirteen 

 somites are found in the embryo (Fig. 20). The 12-day embryo has 33 

 somites (Fig. 22). The advance of the germ ring over the yolk continues 

 until the thirteenth day, when the lips of the blastopore meet and the 

 caudal end of the embryo has been formed (Fig. 23). The embryo at 

 this time is 4 mm. long and contains 38 somites. 



In the advance of the germ ring over the yolk a curious relation of it 

 to a circle of large oil drops on the surface of the yolk was noticed. In 

 the 5-day egg there was a ring of large oil drops beyond the edge of the 

 blastodisk (Fig. 14). On the sixth day the germ ring had extended to 

 the edge of this circle and a few of the globules were seen in Fig. 1 5 in the 

 margin of the disk. The oil ring was found to be covered on the following 

 day (Fig. 16), and the germ ring was directly over this ring of globules. 

 It was found on the eighth day to be just under the inner edge of the 

 germinal ring (Figs. 17, 18). Evidently the ring of oil drops remains sta- 

 tionary while the germ ring passes over it. 



The period of concrescence covers that stage in fish development 

 known to culturists as the tender or critical period, since slight handling 

 of the eggs at this time will injure or kill the developing embryo. It lasted 

 from the fifth to the fourteenth day under the conditions in which the eggs 

 used in making the observation were kept. The critical period mentioned 

 bv Rutter (43) for the Quinnat salmon in the Sacramento lasted from the 

 sixth to the sixteenth day, the difference probably being due to a difference 

 in the temperature in which the eggs were hatched. 



HATCHING AND FRY 



Th egg of the salmon is large for a teleost and its rate of development 

 is much slower than that of the smaller pelagic eggs. In the material 

 under observation, hatching began from 45 to 52 days after fertilization, ^ 

 varying with the temperature. The hatching period, the time between the 

 hatching of the first and last fish in a trough, was 19 days. 



Hatching is immediately preceded by a physiological change in the 

 zona radiata. The egg membrane, which is semi-opaque before the time 

 of hatching, becomes transparent just as the embryo is ready to emerge. 

 Also the tough membrane is tender and breaks readily at this period. Al- 

 though four fish hatched from eggs in which this change had not taken 

 place, they all died. The egg membranes from these eggs did not disin- 

 tegrate in the water, but the membranes from those eggs cast from fish 

 hatching normally all soon disappear. 



The process of hatching involves the change in the membrane men- 

 tioned above and the actual emergence of the fish from the egg. The time 



