118 THE EYE IN EVOLUTION 



MELANIN 



MELANIN (jLte'Aas-, black) is the common dark brown pigment ; it 

 is elaborated locally by the organism from a colourless precursor found 

 in the nucleus of special cells (melanoblasts). Very inert chemically, 

 it acts merely as an absorbing agent. 



Melanin is a close relative of adrenalin and was originally thought to be 

 derived from the blood (Scherl, 1893 ; Ehrmann, 1896 ; Augstein, 1912), but at 

 an early date it was shown to have nothing in common with the derivatives of 

 haemoglobin. A cellular origin therefore being necessitated, Kromayer (1893) and 

 Hertwig (1904) suggested that it was derived from the nucleus, and Meirowsky 

 (1906) narrowed this down to the nucleolus owing to the demonstration of large 

 quantities of pyronin (a nuclear constituent) in melanotic cells, a view which 

 appeared to be substantiated by the finding of this material in melanotic tumours 

 by Rossle (1904). A further advance was made by von Szily (1911) who showed 

 that the pigment was formed from a colourless precursor by the action of a 

 ferment. Masson (1913) established that the action was oxidative in nature, and 

 Bloch (1917) cleared up the matter by demonstrating that the cells of pigmented 

 regions contain a specific intracellular oxidase. Bloch then isolated from the 

 embryo of the broad bean 3-4-dihydroxyphenylalanine, a substance which he 

 conveniently called "dopa", and showed that it was readily changed 

 by this oxidase to melanin. When this svibstance is added to the epidermal 

 cells of skin in frozen forn^alin-fixed sections, granules of melanin are formed 

 (the " dopa reaction "). A large nvunber of the groupings in the protein molecule 

 form coloured products on oxidation (tyrosine, jDhenylalanine, tryptophane, 

 etc.), and it seems obvious that melanin, like adrenalin, is formed as an end- 

 product from one of these chromatogen groups. Bloch concluded that the 

 colourless " mother substance " (or melanogen) is almost certainly either 

 identical with or related to " dopa " ; this colourless substance is brovight by 

 the blood-stream to the cell ; here it meets the " dopa-oxidase " and thus is 

 turned into the coloured pigment melanin. 



THE VISUAL PIGMENTS 



Photochemical and sensitizing reactions in both plants and animals, 

 both phototactic and visual, depend almost universally upon one 

 distinctive and compact group of substances, the carotenoids — a 

 striking indication of the close evolutionary relationship between 

 phototropism and vision. These form a number of pigments varying 

 in colour from red to yellow, fat-soluble and highly unsaturated, 

 occurring alone or as the prosthetic groups of proteins ; all of them 

 seem to be related to the chromophore moiety of visual purple and 

 are identifiable by their absorption spectra, the maxima of which usually 

 lie somewhere towards the blue side of the mid-region of the visible 

 band. As we have seen in a previous chapter ^ they also have a wide 

 integumentary distribution among many species where they may 

 play a dnr atic part in the coloration of the animal. Their high 

 concentrati; 1 in the sex-glands (the interstitial cells of the gonads, the 



1 p. 87. 



