1 91 7-] C. R. N. Rao : Iridocytes in Batrachian Larvae. 287 



by some organic matrix, too feeble, however, to bind them together 

 when mounted 



The argenteum is so opaque and dense that the nature of the 

 relation of the reflecting particles and histological elements of the 

 subcutaneous tissue cannot be made out. 



Time of Appearance of Coi^our EivEments. 



Such of the batrachian anuran larvae as are known to me are 

 either dark or brown at the time of hatching, and the formation of 

 coloured chromatophores is not complete till after the larvae come 

 under the influence of sunlight. The time of appearance of colour 

 varies in different families, mainly depending upon the environ- 

 mental circumstances under which development progresses. 



In Microhyla ornaia the large cephalic region remains trans- 

 parent throughout the metamorphosis, and the diamond-shaped 

 mark appears as soon as the larvae adopt habits of floating on the 

 surface of the water, when they measure about 10 to 12 mm. The 

 other characters, such as the metallic band and the reflecting sur- 

 faces, gradually emerge into view as the tadpole increases in size (16 

 to 18 mm.). It may be mentioned that at this stage the peritoneum 

 bears more numerous iridocytes than at later stages, so much so 

 that they form a continuous metallic surface over a dark back- 

 ground formed by the melanin chromatophores. Perhaps the most 

 important feature in the development of the argenteum at this stage 

 is the fact that when a piece of fresh subcutaneous tissue is exam- 

 ined under the microscope, before it has become too opaque for 

 such treatment, two kinds of metal elements can be noticed. The 

 larger crystals are fairl}^ regular in their outline, unlike those of 

 the mid-dorsal band, and the smaller ones are irregular. As no 

 broken pieces of these larger plates have yet been examined, the 

 view that they contribute towards the formation of the argenteum 

 is only tentatively put forward. When the tadpoles develop the 

 front limbs, the dorsal golden streak and the argenteum are ab- 

 sorbed and the normal colouration of the adult begins to appear. 



The Chemistry of Iridocytes and Argenteum. 



In the sixties, Barreswil' and Voit* demonstrated the pre- 

 sence of guanin in the reflecting tissues and the air-bladder of 

 fishes, and about 1845 this substance was isolated by Bodo Unger 

 from guano. Some time later, Bwald and Kriikenberg^ found 

 the occurrence of this substance in reptiles and amphibians as 

 well, and their investigations go to show that the dead chalky 

 white found on the ventral surface of the adult members of the 

 Ranid family is really caused by a lime compound of guanin, which 



' Comptes Rendus, LIII, p. 246 ; 1861 {Pliil. Trans. Rov. Soc. London, 

 CLXXXIV B, p. 781 ; 1893). 



2 Zeitscli. f. wiss. Zool., X\' {PJiil. Trans., p. 782). 



i Zeitschr. f. Biolugie, XIX, p. i ; 1883 (Phil. Trans., p. 785^. 



