ii THE SKIN AND ITS DERIVATIVES 81 



condition several distinct types may be recognized. Thus in Lepido- 

 siren the most abundant type of chromatophore is characterized by 

 the stout projections of the cell-body which carry the finer pseudo- 

 podia and by the somewhat brownish pigment granules. A less 

 abundant type has long slender, less richly branched and often vari- 

 cose pseudopodia with dense black and opaque pigment granules. 

 Still another type of chromatophore has its protoplasm charged with 

 bright yellow pigment. 



The melanin pigments are probably to be looked upon as waste 

 products of cell metabolism. They are iron -containing pigments 

 and during at least the later periods of development their production 

 is commonly associated with the breaking down of that other great 

 iron-containing pigment — haemoglobin. 



Their production is also related to the degree of activity of the 

 cell metabolism. Thus, in the male Lepidosiren, at the close of the 

 breeding season, when the moribund remains of the richly vascular 

 respiratory projections of the hind limb are being devoured by crowds 

 of voracious phagocytes, there takes place an active formation of 

 melanin. 



Again melanin apparently tends particularly to be produced when 

 the cell metabolism of comparatively unspeeialized cells is interfered 

 with by the prolonged action of light-rays. Thus as already indicated 

 the layer of protoplasm in the egg which is turned towards the light 

 frequently develops melanin granules. Again in developing eyes it 

 commonly holds that comparatively unspeeialized mesenchyme cells 

 wandering into the zone of exposure to the light deposit melanin 

 granules in their cytoplasm. 



Cells then which become chromatophores may be regarded as 

 cells which are specially sensitive to light-stimulus and whose meta- 

 bolism is liable to be so modified thereby as to produce pigment. 



Although it is reasonable to suppose that melanin-formation is 

 primarily related to the influence of light it must not be forgotten 

 that, as indicated in the preceding chapter, the actual laying down 

 of pigment in the case of species where it has become a specific 

 character may take place under circumstances in which the light- 

 stimulus is incomparably more feeble than that which probably 

 originally brought about pigmentation in the course of phylogenetic 

 evolution, as e.g. in the case of the ovarian egg of the frog. That 

 pigment - formation during individual development still remains 

 linked up with exposure to light is shown by the frequency of the 

 unpigmented condition in Anuran larvae which develop in water 

 rendered opaque by fine clay held in suspension (Wenig, 1913). 



To illustrate the dependence of pigment -formation upon light 

 during individual development the case of young flatfish (Pleuron- 

 ectidae) is sometimes quoted, where the shading of the upper and the 

 illumination of the lower surface during development brings about a 

 reversal of the ordinary colouring (Cunningham, 1893). It is possible 

 however that this reversal of colouring is due merely to the strongly 



VOL. II G 



