no BULLETIN OF THE BUREAU OF FISHERIES. 



about 0.3 to 0.5 /I. There are droplets under the sarcolemma in practically all these pink fibers. They 

 measure from 0.8 ;; up to 2 ;i in diameter. The average amount of intramuscular fat in the small fibers 

 is not more than 40 to 60 per cent of that found at Warrendale. Many of the intermediate-sized fibers 

 show only traces of fat in the center of the section. Around the circumference there is a somewhat 

 greater quantity of fat, especially just at the surface. The largest fibers in the section have irregular 

 outlines, look compressed and are very clear of fat, at least these fibers do not contain fat that stains 

 in tlie usual way. I notice an occasional small group of liposomes at some point on the surface of the 

 section, though these groups are few, often not present at all. 



An appearance that is difficult to interpret is due to the presence of very small highly' refractive 

 granules in tlie protoplasm of tlie large fibers. These granules do not take stain, at least, if they are 

 stained at all, it is very different in appearance from the normal, and they do not appear to be uniformly 

 present in all the large fibers. 



Teased muscle (M3S and 39) shows a comparatively small amoimt of fat. The smallest fibers and 

 some of the intermediate fibers have chains of liposomes. The chains in tlie small fibers are not so 

 numerous as one usually finds. The liposomes themselves are very small, and the picture is one of low 

 content of fat. In the intermediate fibers the chains of liposomes are more numerous near the surface of 

 the fiber. Just under the sarcolemma there are groups of small fat droplets rather irregular in arrange- 

 ment. In the larger fibers of these slides it is difficult to distinguish the chains except at the very 

 siu'face. 



Microscopic examination of the caudal pink muscle. — The fibers of the caudal region are closely packed 

 together and are verj' free of fat (section M46). The connective tissue septa have strands of fat droplets, 

 most of them small but some medium in size. These measure from 2 to 15 /i, chiefly the former size. 



The caudal pink muscle fibers are practically free of intracellular fat. Certain ones show traces of 

 fat at points on the surface, but these are only traces and are limited to areas bordering on the fat-bearing 

 septa. Extensive areas witli no septa between the fibers are free of fat. 



The fibers are so compact that their outlines in cross section are irregular polygons. 



The caudal pink teased muscle shows practically no fat in the fibers, traces only appear. Certain 

 of tlie fibers have a slight bluish tinge and through their substance are opaque granules which are diffi- 

 cult to identify. These granules are in the interfibrillar spaces. 



Microscopic examination of the trunk dark muscle. — The trunk dark muscle still retains a large amount 

 of fat (slide M41 under the oil immersion). Drops between the fibers measure on an average from 6 to 

 9 fi, occasionally as much as 15 /n. 



Fat is distributed throughout the substance of the fiber. The droplets vary greatly in size. The 

 largest ones run from 2 to 2.6 fi in diameter. These are more numerous aroimd the superficial portion 

 of the fiber in most of the material, though groups of fibers are found in which these large liposomes are 

 quite evenly distributed through the substance. In numerous fibers the central portion is relatively 

 free of liposomes and the fibers look lighter in color under tlie microscope. In the light areas, however, 

 there are liposomes present, though they are very small for dark fibers, 0.6 to 0,8 /(, and they are not as 

 numerous as in the superficial border. Different portions of tlie section vary greatly in the amount 

 of fat. The muscles freest of fat are those which lie along the septa which carry blood vessels. Under 

 a low magnification these areas are sharply limited. 



Teased fibers (section M44) give a good view of the amount of fat along the course of individual fibers. 

 There is much variation in different lengths of one and the same fiber. In the fatter areas numerous 

 groups of fat droplets lie over the surface of the teased fibers. These groups have a configuration such 

 as was noted in similar muscle from specimens from Warrendale. Undoubtedly the arrangement of fat 

 bears a definite relation to the arrangement of blood vessels. The chains of liposomes are continuous 

 in some areas for long distances. The individual liposomes will measure 2/1111 diameter in the larger 

 chains, but vary through a range of much smaller sizes according to the relative amount of fat present. 

 The larger chains are obviously near the surface of the fiber. Occasionally a chain is noted in which 

 the majority of the liposomes have fused, forming a fat rod such as has previously been described. 

 These rods, as observed, lie near the surface of the fiber. 



Microscopic examination of the caudal dark muscle. — The dark tail muscle has enough fat to give 

 it a relatively deep stain (M56, oil immersion), but this fat is much less in quantity than in the trunk 

 muscle of this salmon. The fat is condensed around the superficial areas of the fiber. Apparently 

 there is some intermuscular fat in droplets 3 to 6 ;i in diameter. 



