14 



REPORT OF COMMISSIONER OP FISH AND FISHERIES. 



pigment cells tend to arrange themselves along the lines of the inter- 

 muscular septa, along which they are produced in slender, branching 

 fjrocesses. Pale, greenish-yellow pigment apjiears in heavy masses, of 

 which there is one on each side of the head behind the eye, another 

 behind the oil-drop, and another on each side of the tail. This latter 

 tends to be produced from the muscular trunk of the tail onto the 

 dorsal and ventral fin-fo'ds. On the third day after hatching the 

 fry measure 3.75 mm., the yolk-sac is almost entirely absorbed, the 

 mouth and the opercular slits are open and functional, the eyes are 

 fully pigmented, and the pectoral fins have assumed their vertical 

 position and adult proportions. By the fifth day after hatching the 

 yolk-sac is entirely gone, the mouth is large and conspicuous, and the 

 skeletal cartilaginous arches of the head are fully formed. 



CAUSES AND CONSEQUEiVCES OF VARIATIONS AND ALTERATIONS IN THE SPECIFIC 

 GRAVITY OF THE EGGS. 



The eggs, when received at the ship, between 2 and 3 hours after 

 mixing of the ova and sperm, were in the 2-cell to the 4-cell stage of 

 development. At this time all of those which were mature and intact 

 floated in a compact layer at the surface of water of a density which 

 varied from 1.021 to 1.0226. One entire lot of 425,000, however, of 

 what seemed to be particularly fine eggs of large size, and all fertilized, 

 sunk at once in a density of 1.0216. When the density was raised to 

 1.0225 they slowly rose and just floated at the surface. To determine 

 the variations in this respect among individual eggs, 100 living eggs 

 were taken, all in the same condition and stage of development, and 

 placed successively in water of different densities, with the following 

 result : 



A careful comparison of the lightest and the heaviest eggs in the 

 foregoing experiment showed that, excluding from consideration those 

 which were immature or otherwise imperfect, the sunken eggs were 

 more variable in size than the floating. The smallest were, in all prob- 

 ability, the not fully mature and more opaque eggs mentioned above. 

 The larger sank because the oil-drop was relatively of a smaller size 

 than in the eggs of intermediate size. 



For the first 48 hours, at a temperature of 12.9° C, at which most of 

 our observations were made, the eggs undergo no marked change in 

 specific gravity. Those which sink during this period, unless a marked 

 fall in the density of the water occurs, are either structurally imperfect 

 or have, for some reason, died during the course of development. A 

 very serious mortality occurred in many of our lots of eggs at about 18 



