1917-] C. R. N. Rao : Iridocytes in Batrachian Larvae. 285 



membranes, while the latter are aggregated on the dorsal surface. 

 A third variety — the stellate t3^pe — accounts for the dark pigmen- 

 tation of the peritoneum. 



2. Cor^ouRED Pigment. 



The true chromatophores which give colour to the skin are 

 either yellow or orange and the degree of colouration depends on 

 two factors. Firstly the number of coloured chromatophores pre- 

 sent and the manner of their distribution, and secondly the extent 

 to which they are diluted by the black chromatophores. The 

 lipochrome pigment occurs in the form of scales or minute granules, 

 the latter when present give the effect of colour suffusion. Only 

 very fine granules of chromatophores produce the blue and orange, 

 such as occur in K. variegata and R. breviceps, and in the yellow 

 and scarlet red spots found in Rh. maculatus and R. tigrina res- 

 pectively only large scaly chromatophores are met with. Green 

 is simply the effect of the fusion of yellow and black, while purple 

 or brown is caused by mixing red and black in various propor- 

 tions. As will be shown in subsequent paragraphs, orange red 

 is simply intensified yellow and is not a separate pigment at 

 all. 



3. Iridocytes. 



As has been stated already, the bright metallic band in the 

 mid-dorsal line is formed by an opaque plate of iridocytes. Each 

 is scaly and is irregular in outline, and when numbers of them 

 form a thick band, they acquire strongly reflecting powers. The 

 band is sunk in a groove in the spinal region covered over by the 

 dermal tissue and bounded laterally by the nine pairs of dorsal 

 muscles. Where this band occurs, the melanin chromatophores 

 are absent. Iridocytes occur in the iris, the peritoneum, the 

 lungs and subcutaneous tissue of the tail membranes. They are 

 absent from the skin. 



In two important respects the iridocytes of the batrachian 

 larvae differ from those of fishes. In the first place they are 

 irregular in outline, and are formed of minute spherical granules. 

 The chief characters of these bodies in fishes are that they are regu- 

 lar, laminated structures with a clear nuclear spot, with divisions 

 showing common origin. In the second place the crystals are 

 most unstable in the tadpoles, falling almost to powder on removal 

 from the subcutaneous setting ; whereas in fishes they occur in 

 the majority of cases in the skin and the crystals can be easily 

 examined. 



The iridescent phenomenon caused by this dorsal band in the 

 Microhyla larva is simply the effect of light being reflected from 

 the numerous surfaces and sides of these crystalline structures. 

 But the golden colour is, however, due to a thin layer of yellow 

 lipochrome spread over this band, and the occurrence of similar 

 pigment produces the coppery hue of the tail lobes. 



