214 



S. FALK AND J. RHODIN 



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Fig. 1. A concentrated melanophore with nucleus (N) in 

 the center. Part of the cell membrane at arrow. The inner 

 cytoplasmic membrane (I) surrounds an inner sack. Magni- 

 fication 3800. 



Fig. 2. Part of a concentrated melanophore. Nucleus (N), 

 mitochondria (M), pigment granules (P), inner cytoplasmic 

 membrane (I), fibrillar zone (F). Magnification 29,000. 



They can be simple or double innervated. Tn the 

 former case they only possess concentrating nerve 

 fibres. In the latter case both concentrating and 

 dispersing nerve fibres are present. The principal 

 hormones acting upon the colour changes are inter- 

 medin, acetylcholine and nor-adrenaline, the former 

 two generally dispersing the pigment granules, the 

 latter concentrating them. Thus the chromatophores 

 are a type of effectors. 



In this investigation 2-3 cm long females of the com- 

 mon aquarium fish Lebistes reticulatus were used. By 

 placing the fishes on black or white backgrounds during 

 15 minutes the melanophores in the dermis on the scales 

 were fully dispersed respectively concentrated. Scales 

 from the dorsal and dorso-lateral sides were then quickly 

 removed and immediately fixed at < 4' C during 5 minutes 

 in a solution consisting of 0.289 g veronal-sodium, 0.190 g 

 sodium acetate, 0.315 g sodium chloride, 12 ml HCl 

 0.1 A', I g OsO, and distilled water to make 100 ml. This 

 solution is isotonic in relation to the blood of the fishes. 

 After fixation the scales were washed with NaCl 0.1 A" 

 and dehydrated at +4°C over 70 per cent, 95 per cent 

 and absolute alcohol, 10 minutes in each. The last change 

 of absolute alcohol was carried out at room temperature. 

 The scales were embedded in a mixture of «-butyl-metha- 

 crylate and methyl-methacrylate containing 0.3 per cent 

 benzoyl-peroxide as a catalyst. Then the mixture was 

 polymerized at 40"C. 



The sections were made with the Sjostrand Ultramicro- 

 tome. Without dissolving the embedding medium the 

 sections were examined in an RCA EMU 2c electron 

 microscope. 



The melanophore is surrounded by a thick cell 

 membrane with a structure similar to that of base- 

 ment membranes in mammalian cells. The thickness 

 of this membrane is about 1200 A. No internal 

 structure can be observed. Outside the thick cell 

 membrane a less dense zone is often present. 



An inner cytoplasmic membrane constitutes an 

 inner "sack". This sack contains the nucleus, mito- 

 chondria and the dark pigment granules. The thick- 

 ness of this inner cytoplasmic membrane is estimated 

 to about 80 A (figs. 1-2). 



The nucleus is about 6 /< in diameter. The nuclear 

 membrane consists of two opaque layers sepa- 

 rated by a less dense zone with a total thickness 

 of about 300 A. The nuclear content consists of 

 about 170 A thick granules and a nucleolus, about 1 /< 

 in diameter. 



The mitochondria are oval or rod-shaped and 

 surrounded by an external double membrane. Inside 

 this membrane is a system of internal double mem- 

 branes arranged in an irregular way. No interme- 

 diate forms between mitochondria and pigment 

 granules have been observed in contrast to du Buy 

 et al. (2). 



Within the sack there also appear special cyto- 

 plasmic structures. They consist of more or less 

 dense annular and irregularly formed single mem- 

 branes. They may represent the Golgi apparatus, but 

 they can also be special structures in the melano- 

 phore cytoplasm. 



The spherical pigment granules, containing mela- 

 nin, range from 0.25 /< to 0.55 /n in diameter, the 

 mean value being 0.40 /<. They are surrounded by a 

 membrane about 300 A thick. The granules generally 

 contain a number of dense bodies, but may some- 

 times appear homogeneous. 



Between the two cell membranes is a zone 0.5-3 ft 

 wide. This zone is transversed by fibrils, about 80 A 

 thick. In general the fibrils do not seem to be attached 

 to either of the two cell membranes. They are pre- 

 ferably arranged in a direction parallel to the cell 



