Arr.] CHEMICAL BASIS OF THE ANIMAL BODY. 733 



mixture of achroodextrin and maltose ; the quantity of dextrose making 

 its appearance at the same time being very small. 



Opalescent solutions of glycogen usually become clear on the addition 

 of caustic alkali : Yintschgau and Dietl 1 have shewn that this is accom- 

 panied on boiling by a change which converts a portion of the glycogen 

 into a substance to which they give the name of /S.-glycogen-dextrine. 

 (Kiihne 2 had previously described a body to which he gave the name gly- 

 cogen-dextrin. That described by Yintschgau and Dietl differs slightly 

 from Kiihne's body, hence the name.) According to these authors one- 

 fifth of the glycogen is at the same time changed into some other, at pre- 

 sent undetermined, substance. Normal lead acetate gives a cloudiness, 

 the basic salt a precipitate, in solutions of glycogen. 



As tests for this body may be used the formation of a port-wine 

 colour with iodine ; this disappears on warming but returns on cooling. 

 The same colour is produced by the action of iodine on dextrin, but 

 this does not reappear on cooling after its disappearance by warming. 



Preparation of Glycogen. The following is Briicke's 3 method. The 

 filtered or simply strained decoction of perfectly fresh liver or other gly- 

 cogenic tissue is, when cold, treated alternately with dilute hydrochloric 

 acid, and a solution of the double iodide of potassium and mercury 4 , 

 as long as any precipitate occurs. In the presence of free hydrochloric 

 acid, the double iodide precipitates proteid matters so completely as to 

 render their separation by filtration easy. The proteids being thus got 

 rid of, the glycogen is precipitated from the filtrate by adding alcohol to 

 the extent of between 60 and 70 p. c. Too much alcohol is to be 

 avoided, since other substances as well as glycogen are thereby pre- 

 cipitated. The glycogen is now washed with alcohol first of 60 and 

 then of 95 per cent, afterwards with aether, and finally with absolute 

 alcohol. It is then dried over sulphuric acid. 



Tunicin. (C 6 H 10 O 5 ) U . 



This body is regarded by many observers as identical with the true 

 cellulose of plants, while others have ascribed to it properties differing 

 from those of cellulose sufficiently to justify its receiving a distinct name. 

 It appears to be more resistent to the action of chemical reagents than 

 plant-cellulose. 



It constitutes the chief part of the integument of the Ascidia or 



1 Pfluger's Arch. Bd. xvn. (1878), S. 154. 



2 Lehrb. d. physiol. Chem. (1868), S. 63. 



3 Sitzungsber. d. Wiener Akad. Bd. 63 (1871), n. Abth. 



4 This may be prepared by precipitating potassic iodide with mercuric chloride 

 and dissolving the washed precipitate in a hot solution of potassic iodide as long 

 as it continues to be taken .up. On cooling, some amount of precipitate occurs, 

 which must be filtered off; the nitrate is then ready for use. 



