310 E. T. Bell, 



vatioii (fig. 2). The well-known differences between the muscles 

 of summer and winter frogs are due in part to differences in the nutritive 

 condition. This point will be fully discussed later. 



As to the significance of the dark and light fibers, Knoll advanced 

 the view that the activity of the muscle is a factor of importance — the 

 most active muscles contain the largest percentage of dark fibers. Age 

 and domestication have also been regarded as factors influencing the 

 proportion of the dark fibers. 



It will be shown later on in this paper that the interstitial granules 

 in some vertebrates at least are composed essentially of fats and that 

 they are used up during inanition. They certainly represent a reserve 

 food supply. We may therefore regard the dark fibers as fibers espec- 

 ially adapted for the storage of fats. Just as one hepatic cell or one con- 

 nective tissue cell may be more adapted to store fat than another. In 

 the frog the muscles are the main place for the storage of fat, since the 

 fat body is the only representative of the adipose tissue of other animals. It 

 is easy to understand how in the course of evolution one species, e. g., 

 the cat or the frog, may have come to use its muscles more as a place for 

 the storage of fats than another species, e. g., the ox. 



The term "fat" is used in this paper in a general sense to 

 include both true fats and lipoids. 



But it is more difficult to explain why one muscle of an animal should 

 contain more fibers especially adapted for the storage of fat (dark fibers) 

 than another muscle of the same animal . Knoll's explanation does not 

 seem satisfactory since it does not explain such differences as are found 

 between the gastrocnemius and soleus of a rat, and a number of similar 

 cases might be cited. I can only suggest that these may be accidental 

 differences developed during evolution. 



There were no fibers comparable to the dark and light fibers of 

 vertebrates in the muscles of the moths and flies examined ; but I have 

 made only a few observations on these animals. 



Position of the interstitial granules in the fiber. Few observers have 

 treated this question in detail. All of course agree with Kölliker that 

 the granules lie in the sarcoplasm between the fibrils. Krause stated 



