288 Records of the Indian Museum. [Vol. XIII, 



they called " Guaninkalk," About 1893 Cunningham and Mac- 

 Munn/ as a result of extensive observations on fishes, established 

 the fact that the iridescent effect produced in the skin of all fishes 

 is due to the presence of iridocytes, and the silver brilliancy is 

 caused by the reflecting tissue, — the argenteum. Guanin is the" 

 chemical substance present in both these structures. 



It may be mentioned that guanin in the tissues of the body is 

 the end product of the metabolic activity of the organism, and the 

 utilisation of this waste matter for certain physiological ends is a 

 feature of wide-spread occurrence in animals. As has been said in 

 the foregoing paragraphs, the presence of guaninkalk and its chemi- 

 cal nature have already been worked out in the skin of adult 

 batrachians, but so far the occurrence of the iridocytes and argen- 

 teum has not been determined either in the adult or the larvae. 

 On chemical analysis of these substances it is established that the}' 

 are guanin compounds identical with those worked out in the fishes, 

 and the course of the chemical enquiry adopted for such a deter- 

 mination may now be proceeded with. I must mention here that 

 in all stages of the work I have received considerable help from my 

 colleague Mr. A. Subba Rao. 



The tissues were thoroughly washed in distilled water till all 

 albuminous matter was removed and then solutions of iridocytes 

 and argenteum were obtained in nitric and hydrochloric acids. 



I. A quantit}^ of nitric acid solution was evaporated in a 

 watch glass over a hot air bath. The residue formed is a yellow 

 substance (guanin nitrate) which turned red on the addition of 

 caustic potash. This is Barreswil's reaction. 



II. A quantity of hydrochloric acid solution was evaporated 

 similarly and the residue was treated with strong nitric acid. A 

 yellow compound is obtained by reheating the solution to dryness 

 which on the addition of caustic soda turned red, and purple on 

 heating. This is Cunningham and MacMunn's test. 



III. If to the yellow compound (nitrate of guanin), obtained 

 in the first two cases, ammonia is added and heated the same 

 colour changes are noticed. This test is given in Watt's chemical 

 dictionary (p. 656). 



It may be mentioned here that there is essentially no differ- 

 ence between Barreswil, Cunningham and MacMunn and Watt's 

 reactions. In all the three cases the neutralising agent, a base, is 

 added to the nitrate of guanin which on heating becomes purple. 



IV. If silver nitrate is added to the nitrate of guanin a red- 

 dish-brown precipitate results, which on heating turns purple. 



V. Potassium chromate gives an orange red precipitate on 

 the addition of the nitrate of guanin (Watt). 



VI. Potassium ferricyanide yields a brown precipitate with 

 the same substance (Watt). 



VII. Concentrated picric acid gives a bright red solution when 

 treated with nitrate of guanin (Watt). 



i Phil. Trans. Roy. Soc. London, CLXXXIV B, p. 765 (1893). 



