76 BULLETIN OF THE BUREAU OF FISHERIES. 



Selection of tissue. — In order to follow the variations of fat in different regions of 

 the body one must, of course, select typical tissues as regards their relations to fat. 

 For this purpose I have chosen primarily the great lateral muscle, because it represents 

 by far the greatest mass of the fat-storing tissue of the salmon. This muscle is divided 

 into two types; the deep or pink muscle and the superficial or dark muscle. Each extends 

 from the head and pectoral girdle to the base of the caudal fin. There is considerable 

 variation in the amount of fat in different parts of these muscles. Two type regions 

 of the lateral muscle have, therefore, been selected for study. The first is the mid 

 region of the side of the body, in the transverse section which cuts the body just at the 

 front margin of the dorsal fin. This with the muscle anterior to the section is the fattest 

 portion of the great lateral muscle mass. Samples from this region are called trunk 

 muscle, dark or pink, respectively, as the case may be. The second region of the great 

 lateral muscle chosen is that portion at the base of the tail opposite the fifth and sixth 

 caudal vertebras, counting from the posterior end. This region is called the caudal 

 muscle, dark or pink, according to its type. The caudal muscle always has a relatively 

 less amount of fat than the trunk muscle. It is assumed that there is a more or less 

 gradual variation from the caudal region to the mid lateral or trunk region as regards 

 the percentage amount of fat. 



While the primary comparisons are with reference to the two regions of the lateral 

 muscle m-entioned, still a considerable number of samples were taken from other muscles; 

 namely, the anal fin muscles, the "belly" muscles, the intercostals, and the cheek 

 muscles. 



Fixation of tissues. — In view of Bell's" experience, it was thought better to make 

 the examination of the fat in the salmon muscle in as fresh condition as possible, and a 

 full Bardeen freezing microtome equipment was therefore taken into the collecting field. 

 Samples of the tissues from the type localities were thus cut, stained, mounted, and 

 studied with the minimum of delay. It was quickly found to be quite impossible, 

 however, to carry through all the tissues in the fresh condition. There was not time 

 enough, with a field force of three, to do the necessary work before disintegration began. 

 It was therefore found necessary to use a fixative, i. e., lo per cent formalin. 



As a matter of routine practice, tissues that were to be held in the prolonged and 

 tedious processes of working up the material were always fixed in lo per cent formalin. 

 This fixation was not found to be detrimental to the preservation of the fat, but on the 

 other hand, favorable. The frozen sections cut after two hours or more in formalin 

 were firmer, retained their shape better, and therefore were not so much torn and 

 distorted in the process of handling necessary to staining. In short, the formalin- 

 fixed sections enabled one to arrive at a better conception of the relationships 

 of the fat than is to be had from the tissue cut directly from physiological saline. 

 After a series of observations made along these lines it was decided to pass all the tissues 

 through a formalin fixation. It is not claimed that a long immersion of tissues in 

 formalin is wholly without effect on the fats, yet for salmon muscle we are convinced 

 that the change produced is very slight. In certain samples sections of fresh tissues 

 have been compared with sections made after four months' fixation in formalin. The 

 difference in quantity of fat shown is not easily determined, but there is some variation 



o Bell, E. T.: The staining of fats in epithelium and muscle fibers. Anatomical Record, vol- iv, p. 199. 1910. 



