STRONG: DEVELOPMENT OF COLOR IN DEFINITIVE FEATHER. 165 



epidermis only, or (3) in both. Most writers who advocate origin from 

 the blood have described pigment as being formed in the dermis, either 

 in ordinary connective-tissue cells, or in special cells differentiated for 

 the purpose, which in the case of epidermal pigmentation wandered 

 from the dermis into the epidermis or sent amceboid processes up be- 

 tween the cells of the cylinder-cell layer. 



I have found the remiges of the tern (Sterna hirundo) especially 

 favorable material for studying the formation of epidermal pigments. 

 Their pigment cells attain a large size, are comparatively regular in 

 contour, and very abundant. 



The first signs of pigment formation appear in certain of the " inter- 

 mediate cells '.' of the fundament of the feather immediately before the 

 differentiation of the ridges. The pigment arises in the form of grayish 

 or light yellowish corpuscles, of exceedingly small size, arranged along 

 delicate protoplasmic strands, which radiate from the nucleus and 

 sometimes anastomose more or less with one another. These corpuscles 

 increase rapidly in size and are soon large enough to be recognized with 

 a -^ inch oil immersion lens as definite rod-shaped granules (Plate 6, 

 Figs. 30, 31). At the same time they become deeper in color and 

 more and more numerous until finally they form a complete ball, 

 Plate 3, Fig. 16 ; Plate 6, Fig. 35, cl. pig-), which was often taken 

 by the earlier writers to be a homogeneous mass. 



In the course of development these rods are easily seen to be radially 

 distributed about the nxacleus, an arrangement which has been described 

 for the pigment cells and chromatophores of other animals. 



The nuclei of these pigment cells are entirely destitute of the pig- 

 ment granules, a condition which Solger ('89, "90, '91) also noted in 

 the pigment cells of fishes and mammals. 



Kromayer ('97), too, observed in the developing chromatophores of 

 frog skin that the first appearance of pigment granules was along proto- 

 plasmic strands ; the granules were at first light in color, but gradually 

 grew darker. 



Post ('94, pp. 4:91, 492) found that melanin pigment granules have 

 characteristic variations in shape and size for different animals. " Die 

 Pigmenttheilchen in den Oberhautgebilden verschiedener Thierarten 

 sind ebenfalls sehr verschieden, z. B. bei der Katze lang nnd ziemlich 

 dick, beim Hunde wetzsteinforming in der Mitte verdickt, beim Meer- 

 schweinchen und Kaninchen kurz und dick, beira Rinde ziemlich lang 

 und schlank. Auch das Pijrment der Taubenfedern besteht aus Stabchen 

 von massicjer Grosse." I have also found variations in size for the birds 



