STORAGE OF FAT IN MUSCULAR TISSUE OF KING SALMON. 89 



In the fattest fish the caudal pink muscle is characterized by the smaller size of 

 the liposomes; also by a smaller number of liposomes in the center of the fiber. This 

 gives the impression of a somewhat greater quantity of fats around the superficial 

 layer of the fiber. 



In those Ilwaco fish which have less fat than the average, for example, no. 117, 

 the amount of fat in the caudal pink muscle is very strikingly less than in the middle 

 of the body. In this particular fish the fat between the fibers is very noticeably less 

 in quantity; in fact, it is practically absent except in those areas which have a relatively 

 large amount of connective tissue. The caudal intramuscular fat of no. 117 has almost 

 disappeared, or at least is present in extremely small amount. 



The caudal pink muscle also shows that fish no. 117 has already passed well into 

 the retrogressive stage which comes with the migration fast, an indication noted in 

 connection with the discussion of the trunk pink muscle. This is apparent from the 

 character and arrangement, particularly the arrangement, of the fat in the connective 

 tissue, as well as in the spaces between the muscle fibers. It is certainly true that the 

 amount of fat present sharply increases as one proceeds through successive segments 

 from the caudal peduncle to the mid-lateral region of the body. Light on the signifi- 

 cance of the above observations is had by considering the condition of the fat in the 

 dark muscle of the two regions. 



Trunk dark muscle. — The dark muscle forms a distinct type of muscle, as previously 

 announced. In this case the fat has been loaded into the muscle in enormous quan- 

 tities, both intermuscular and intramuscular. At the Ilwaco station the amount of 

 fat in the dark muscle is enormous, as illustrated by fishes no. iii, 113, 115, 116, and 

 118, in all of which the fat deposited in the dark muscle has reached its maximum. 



The intermuscular fat is relatively much less than in the pink muscle. This is due 

 among other things to a structural factor. The muscle fibers are very compactly 

 arranged, forming a much denser mass than is formed by the pink fibers. The inter- 

 stitial connective tissue is correspondingly reduced in mass, hence there is not so much 

 fat carried. On the other hand, the muscle substance has received so great a deposit 

 of intramuscular fat that one must regard this muscle as a definite fat depot. Atten- 

 tion has already been called to the fact that deposit in this muscle begins in embryonic 

 life. It increases in amount up to the time of the cessation of feeding and, we assume, 

 has not appreciably changed when the good-conditioned fish reach Ilwaco. The trunk 

 dark muscle contains so much fat in the muscle substance that one can not make 

 adequate comparisons showing slight variations. 



When this tissue is examined in teased preparations, so that a side view is had of 

 an individual fiber, it is found that the fat droplets are so large and so numerous that 

 the fibers are difficult to distinguish as individuals. This is shown in figure i , plate 

 III, where a transparency is figured of a fiber from fish no. 115. Often in the exam- 

 ination of these teased fibers one notes elongated fat drops or rods. These have formed 

 in the interfibrillar spaces owing to the fact that the droplets have increased so much 

 in size that adjacent ones have run together, thus fusing into the mass noted. 



Caudal dark muscle. — The superficial or dark muscle from the caudal region in all 

 these Ilwaco specimens has a very considerably less amount of fat than the correspond- 

 ing muscle from the lateral region. Even fish no. 113, which is as fat as any in the 

 series, presents a sharp contrast as regards the comparison of the amount of fat in the 



