286 Records of the Indian Museum. [Vol. XIII ^ 



The Argenteum. 



The argenteum is a thick, opaque, continuous layer of reflect- 

 ing subcutaneous tissue in which the iridocytes no longer retain 

 their individual character but are broken up so minutel}^ that it 

 is by no means possible to make out any definite structure with 

 the microscope. This layer is covered over by the transparent 

 epidermis which here is singularly free from all chromatophores. 

 The silver brilliance of the argenteum is simply due to the 

 powdering of the iridocytes which are thickly impregnated into the 

 subcutaneous tissue ; the same cause accounts for all absence of 

 iridescence in this region. Thus the abdominal wall has a bright 

 silver lustre on the outer surface and a spangle-like appearance on 

 the peritoneal wall. As has been said already, the pericardium — 

 the parietal layer — is also an argenteum and the visceral layer bears 

 only chromatophores. 



The only organs that contain iridocytes and black chromato- 

 phores are the lungs; all the other organs are perfectly devoid of 

 them. The occurrence of argenteum in the air-bladder of fishes 

 has been noticed, and the homology of the lungs of air-breathing 

 Craniates with the air-bladder of fishes here receives fresh corrobora- 

 tion from the chemical side. 



Relation of Colour and Histological Elements. 



It is not possible to demonstrate the presence of connective 

 tissue corpuscles in the dermis or epidermis of grown tadpoles, 

 though gold chloride staining of the skin of very young tadpoles 

 sometimes reveals the presence of a few corpuscles. The chromato- 

 phores occur in between the epidermal cells, and their cellular 

 origin can be explained on the hypothesis that after formation in 

 the deeper tissues they migrate bodily to the surface region. In 

 sections of the skin two kinds of dots are noticeable, the smaller 

 ones belong to the granular chromatophores and the larger ones 

 represent the cut ends of the dendritic forms. 



The coloured elements are absent from these sections and 

 stained preparations, for they are most susceptible to the action 

 of even mild solvents like rectified spirit. In the fresh specimens, 

 the scale-like coloured chromatophores lie partly in the dermis and 

 partly below, only a few occurring in the epidermis. Even in 

 regard to them, their connective tissue origin can only be inferen- 

 tially gathered. 



The iridoc^^tes are ,\-, mm. in situ, while the coloured chro- 

 matophores are Vo mm. and the granules of the former are less 

 than 10 /n. The spinal groove in which the metallic band lies is 

 quite open in young specimens ; the epidermal tissue growing over 

 as metamorphosis advances. If the iridocytes from these young 

 specimens are examined, under a high power of the microscope, 

 their cellular origin can be made out. It is probable that when 

 they leave their place of origin, they become Hghtly held together 



