90 BULLETIN OF THE BUREAU OF FISHERIES. 



caudal dark and in the trunk dark muscle. The size of the intermuscular fat droplets 

 has sharply decreased, though the number of droplets is as great or even greater than 

 in the lateral region. 



The sharpest contrast lies in the intramuscular fat. In the caudal region this fat 

 is very markedly less, especially in the size of the larger droplets. Even in the 

 fatter fish the larger droplets seem to be congregated around the superficial border of the 

 fibers. The superficial fat droplets are under the sarcolemma, though this fact is often 

 very difficult to determine. There is a large supply of the finer fat droplets and lipo- 

 somes scattered through the protoplasm of the caudal muscle substance. The contrast 

 between this and the fatter regions is not so much a matter of the number of the lipo- 

 somes as in the size and arrangement, especially of the larger droplets. The largest 

 droplets in the caudal muscle will not average more than one-half as great in diameter 

 as in the trunk muscle. 



In a salmon like no. 117, which is poor in the general amount of fat of the body, 

 the caudal dark muscle presents the sharpest contrast in comparison with the standard 

 of this station. Under the low magnification, sections of the caudal dark show that in 

 the regions bordering along the blood vessels there are areas which by contrast with 

 other portions of the section are relatively free of fat. These areas are faded. This is 

 a condition undoubtedly indicative of the removal of stored fat. The retrogressive 

 process has already gone so far that one can distinguish the regions in which the active 

 process of fat resorption is going on with most vigor. This is the first clear-cut picture 

 showing the process of fat resorption. The appearance of the section is exactly the reverse 

 of that shown for the dark muscle in the growing stage, also of that in certain pathological 

 processes wherein fat is being very rapidly laid down." In discussing later stages it is 

 argued that these contrasts are due to the irregularity of resorption of the fat from the 

 tissue. In other words, the fat is being taken up from the tissues and transported to 

 other parts of the body, to be utilized by the body in the construction of new tissues 

 (egg yolk, etc.) or in the production of energy. This movement is best facilitated in the 

 neighborhood of the small blood vessels, and is expressed microscopically by these con- 

 trasts in fat content. These facts are in further confirmation of the deduction that fish 

 no. 117 has been for some time on the migration phase of its life cycle. 



Fat in the fin muscles. — A few examinations were made of the small muscles of the 

 fins at the Ilwaco station. The samples selected were the pairs of erector and depressor 

 muscles located in a single interspace between two interhemal spines. These muscles 

 are made up of fibers rather loosely bound together. There is a small amount of inter- 

 fibrous connective tissue with a tolerably thick sheath around each muscle slip. The 

 fibers themselves are of a type somewhat like the cheek muscle of the head. 



The intermuscular fat is present in droplets of good size, but not in very large num- 

 bers. In the connective tissue sheaths around the muscles the amount of fat corre- 

 sponds more nearly to that of the myocommata of the lateral muscle. In general, the 

 amount of intermuscular fat is considerably lower than that of the pink lateral muscle 

 of the same salmon. 



a My colleague. Dr. \V. J. Calvert, tells me that in his unpublished work on the plague he often noted a striking deposit 

 of fat in the parenchyma along the immediate border of the blood vessels of the liver. This deposit in the early stages of the 

 disease extends out only a short distance into the parenchyma of the liver. The course of the smaller blood vessels is easily fol- 

 lowed througn the parenchymatous tissue by the bordering deposit of fat. This is. of course, the reverse picture of that described 

 above. la the salmon the fat is in process of removal; in the plague liver the fat is in process of rapid deposit, but in each case 

 the histological picture is that of the early, therefore difTerential, stage in the process. 



