88 BULLETIN OF THE BUREAU OF FISHERIES. 



therefore the relative number of liposomes, varies in preparations from different indi- 

 vidual salmon. The amount of fat in the pink muscle fibers is measured therefore by 

 two microscopic factors; first, the number of chains in a given mass of fiber; second, 

 the size of the individual liposomes in the chains. 



The pink muscle fibers vary within a wide range of size of fiber, from 25 to 250 /i 

 in diameter. This variation is illustrated in the figure 17, plate x. In the larger fibers 

 of the Ilwaco fish (notably no. 11 1 and no. 118, the latter of which is figured in figs. 8 

 and 9, pi. VI) the chains of liposomes are quite evenly distributed throughout the 

 mass of the fiber. However, they are characterized by the relatively small number 

 of liposomes in the chains and the comparatively small size of the liposomes. In the 

 large fibers of no. 118 the largest liposomes in the centers of the chains are about 0.5 n 

 in diameter and the smallest ones which form the ends of the chains are just identifiable 

 with the oil immersion. In the smaller fibers of this same fish the liposomic chains are 

 somewhat larger, the largest liposomes in the chains about double the diameter of the 

 largest in the large fibers. The liposomes in the small fibers are more thickly dis- 

 tributed around the central core of the fiber. This variation is not noted in the rela- 

 tively large mass of the fibers whose diameters run over 200 /(. 



Fish no. 117 seems an exception to the group from the Ilwaco station. It is cer- 

 tainly verv far below the average of the other specimens as regards the amount of fat 

 revealed by the microscope. Reference to the protocol will show that this fish came 

 from a trap some little distance up Bakers Bay. The whole appearance of the salmon, 

 both its gross appearance and the microscopic appearance, suggests the type of fish 

 characterized by a certain degree of retrogression. The weight is much below the 

 standard for the length, as much below the average as certain farther advanced salmon 

 taken from stations higher up the river. These comparisons lead to the deduction 

 that salmon no. 117 has been in fresh water some time. Although it has not gone up 

 the river, the probability is that it has undergone as much migratory change in fats as 

 specimens that have gone farther up the river. The chemical quantitative determina- 

 tion of the fats abundantly confirms the above deductions. (See page 92.) 



In this salmon the amount of intermuscular fat in the trunk pink muscle is very much 

 reduced. The number of fat drops is less and the size smaller. The intramuscular fat 

 is present in all of the trunk pink, but the number of liposomic chains and the size of 

 the liposomes themselves is reduced. In the very largest fibers there is almost no 

 liposomic fat. Another point associated with the amount of fat is the decrease in the 

 intermuscular spaces, so that the fibers themselves seem more compact in arrangement. 



Caudal pink muscle. — The pink muscle from the caudal peduncle in each Ilwaco 

 specimen examined has a strikingly smaller quantity of intermuscular fat than the 

 muscle from the middle of the body of the same animal. It would seem from the 

 Ilwaco fish that the intermuscular fat is never laid down in the caudal region in as 

 great quantity as in the lateral or trunk region of the body. The fat drops are rela- 

 tively smaller and, in general, fewer in number than from the fatter region of the body. 

 The intramuscular fat of the caudal pink muscle in the specimens from the Ilwaco 

 station is less than in the trunk pink muscle. Those conditions which at the beginning 

 of the migration lead to an infiltration of fat into the muscle cells do not result in as 

 great a deposit in the caudal pink muscle as in the trunk fibers. 



