CHEMISTRY OF GLYCOGEN 223 



One can readily understand how a colloidal substance 

 like glycogen, distributed as it is in another colloidal sub- 

 strate, the cellular protoplasm, often comes to show an 

 atypical behavior. Thus the microchemical identification 

 of glycogen by iodine-iodide of potassium (as in a frog's 

 ovary) may be attended with difficulties even though 

 glycogen is abundantly present ; but if the combination be- 

 tween glycogen and the tissue be released by repeated freez- 

 ing and thawing of the latter, the reaction readily takes 

 place. 7 We are manifestly here dealing with a phenomenon 

 of physical chemistry ; as we have no foundation at present 

 for assuming a chemical combination of the glycogen within 

 the tissue. 8 Moreover, E. Tiirkel (in opposition to the state- 

 ments of Seegen and others) was unable to convince himself 

 that liver-extracts, freed of their glycogen, fermentescible 

 sugar and albumen, contain any appreciable quantities of a 

 substance which may be precipitated by alcohol or which 

 will yield sugar by hydrolytic cleavage. 9 



That the tremendous application of work devoted during 

 the past decades to experimental investigation of the physi- 

 ology of glycogen, the details of which obviously cannot here 

 be discussed, has not been without result, may be realized 

 when it is stated that we are able even now to indicate con- 

 cisely the conditions under which the body draws upon its 

 glycogen supply. 10 (We have more definite information 

 about this phase of the subject than in regard to the manu- 

 facture of glycogen in the body, a feature undoubtedly 

 related with the important and very imperfectly understood 

 questions of formation of sugar from proteins and fats.) 



7 M. Bleibtreu, K. Kato, Pfliiger's Arch., 127, 118, 125, 1909. 



H. Loeschke (Pfliiger's Lab.), Pfliiger's Arch., 102, 592, 1904. 



9 R. Tiirkel, Hofmeister's Beitr., 9, 89, 1906; cf. therein Literature. 



10 Literature upon Physiological and Pathological Catabolism of Glycogen 

 in the Body: O. v. Fiirth, Ergebn. d. Physiol., 2, 584-589, 1902; R. Tigerstedt, 

 Nagel's Handb. d. Physiol., 1, 495-502, 1905 ; E. Weinland, ibid., 2, 430, 1907 ; 

 A. Magnus-Levy, Handb. d. Biochem., 4' 311-316, 1909; J. Wohlgemuth, ibid., 

 3', 160-161, 168-174, 1910. 



