380 JOHN F. FULTON, JR. 



is occasioned by the chemical union of the proteid with the 

 metal. This seems extremely unlikely, however, since vanadium 

 does not exist in the blood in its elemental state — the differences 

 in colour of the corpuscles being due to vanadium in different 

 states of oxidation — and also because the vanadium probably 

 has the role of catalyst in the respiratory phenomena of 

 ascidians (Fulton, 1921 c). A more likely explanation is that 

 in the colourless antecedents of the pigment cells there exists 

 some colourless vanadium compound which, either on oxida- 

 tion or reduction, is converted into one of the coloured oxides 

 of that metal. 



7. Discussion — Pigmentation of Vertebrates. 



The results obtained in a study of the pigmentation of 

 invertebrates cannot be entirely without application to the 

 problem of coloration in vertebrates. Particularly does this 

 seem true in view of the question regarding the origin of 

 melanin. In recent years the origin of pigments in vertebrates 

 has been much discussed. One school holds that the epidermis 

 is capable of elaborating its own pigment ; -^ another maintains 

 that the pigment is carried into the integument by wandering 

 leucocytes ; ^ still another holds that melanin is derived from 

 the haemoglobin of the blood.^ For a very complete account 

 of the historical development and the present status of the 

 biological theories regarding the origin of melanin, the reader 

 is referred to Dawson's (1920) paper on the integument of 

 Necturus. But, in addition to the purely biological discussion 

 of the question, there are certain chemical investigations on 

 melanin which demand attention. 



The chemical analyses which have been made upon melanin 

 tend on the whole to support the view that the pigment is 



1 Hooker (1914) and Eycleshymer (1906) are the more important 

 advocates of this view. 



2 See the papers of Reinke (1906), Negre (1906), and Borrel (1913). 



3 Rahl (1894) maintained that the leucocytes phagocj^tized red blood- 

 cells and converted their haemoglobin into melanin. This view is also 

 supported by certain chemical investigations. 



