128 BULLETIN OF THE BUREAU OF FISHERIES. 



pink muscle in so far as its source of material for the production of energj' is concerned. 

 Regions of dark muscle which have reached this stage are found with the arrangement of 

 liposomes that is described as typical for the pink muscle. On the other hand, the dark 

 muscle will have the chains of relatively small liposomes rather uniformly distributed 

 throughout the muscle mass. These liposomes at this stage are relatively small, as for 

 example in the fishes described from Cazadero. Rarely will fibers be found with no 

 liposomes. It seems to me that should a certain area of dark muscle fibers through 

 excessive activity consume all of its liposomes, then fat would be thrown into those fibers 

 by the process of lipolysis and fat transference in exactly the same way that it is thrown 

 into the pink fibers. This detail is fully described in connection with the discussion of 

 the pink muscle. 



As regards the second factor mentioned above, namely, the high percentage of fat 

 still present in the dark muscle at the time of the death of the salmon, it seems to me 

 the matter is more complicated. The operation of no ordinary factor would maintain a 

 higher percentage of fat in the dark muscle at a time when the fats were almost consumed. 

 One is led to suspect that there is some special factor operative in the dark muscle. In 

 all probability this factor is the same in the late stage in the life cycle as that operating 

 in the earlier stage in the salmon development which results in the loading of fats into 

 this type of muscle. I have obser^-ed no special facts which of themselves explain this 

 situation. There are, however, certain accessory facts which permit of an explanation 

 which will be. offered as a tentative hypothesis. Of these facts the most important is the 

 fact of the loading of the dark muscle during the embryonic stage of its development. 



Undoubtedly such deposits of fat as occur in no. 97, fig. 7, represent a perfectly normal 

 process which is to be interpreted as a function of this muscle. Histologically the dark 

 muscle differs slightly in its structure from the pink muscle. The dark fibers contain 

 more sarcoplasm and somewhat larger and fewer fibrillse. At an early embryonic stage 

 this difference between the dark and pink muscle is rather more striking than it is later. 

 This suggests that the dark muscle is a less highly differentiated type of muscle than 

 the pink. One may assume, therefore, that it retains more primitive characteristics. 

 In the sections which cut the borderland between dark and pink muscle a few of the 

 pink fibers of the intermediate zone are found to be filled with liposomes. This loading 

 of liposomes is greatest in the fibers nearest the surface of the great muscle mass and is 

 totally absent in the deeper portions of the muscle. The fibers in question are of the 

 pink fiber type. Their loading of fat must therefore be due to some special factor. 

 These three facts, namely, (i) the excessive loading of fat in the growing dark muscle, 

 (2) the more generalized type of dark muscle, and (3) the tendency of the neighboring 

 zone of pink muscle to load intramuscular fat, all suggest that the dark muscle has still 

 strongly developed one of its general functions. This function is the production of 

 lipase. It may be anticipating a bit in the following discussion, but it is evident that 

 the presence of a relatively high concentration of lipase results in the seizing of the fats 

 during the growing stage and their concentration in the lipase-producing tissues. This 

 view is borne out by the deposit of large amounts of fat in the pancreas as well as in the 

 dark muscle. A relatively high concentration of lipase in the dark muscle would lead 

 to a greater concentration of lipase in the tissue lymphs in those tissues which surround 

 the dark muscle, namely, the connective tissue of the skin and the superficial layer of 



