Figure 1. -Major pattern features (terminology of Mitchell 1970) and locations from which skin samples were 

 taken: SF = spinal field (black), TP = thoracic patch (buff), FP = flank patch (gray), AF = abdominal field (white). 



(1972). The epidermis generally lacks hair (a few 

 hairs are present on the snout in early develop- 

 ment but disappear before or shortly after birth), 

 cuticular keratin, and accessory glands. Its thick- 

 ness varies, being greatest in the ventral region 

 and least on the flippers, dorsal fin, and flukes. The 

 superficial layer of the epidermis, about 10 cells 

 thick, shows considerable flattening of cells. The 

 epidermis consists almost entirely of polyhedral 

 prickle cells, many of which show clear, distended 

 infranuclear cytoplasm. This clear cytoplasm at 

 first appears as holes in the tissue when it is 

 viewed in section, but upon closer examination one 

 may identify the nucleoli inside the clear areas. 

 The epidermis interlocks with the dermis by in- 

 terdigitation of epidermal rete pegs and dermal 

 papillae (Figure 3), and the dermis grades into the 

 subcutaneous blubber layer. The dermis varies in 

 thickness and density and is composed largely of 

 collagen fibers. The dermal papillae contain blood 

 vessels, blood cells, and other loosely organized 

 connective tissue. 



Melanin pigmentation is restricted to the 

 epidermis and is consistently more concentrated 

 at the edges of the epidermis than elsewhere. It is 

 usually most concentrated around the bases of the 

 dermal papillae (at the apices of the epidermal rete 

 pegs) and extending in bands from the apices of 

 the dermal papillae. The pigment has been clas- 

 sified here into three groups for purposes of quan- 

 tification. "Diffuse" pigment appeared simply as 

 an area which stained darker green than the 

 background and in which pigment grains could not 

 be discerned even at 1,250 diameters magnifica- 

 tion. In addition to diffuse pigment, there were 

 granules ranging from less than 0.1 /xm to over 5 

 ju-m in diameter. Most were spherical or ellipsoid. 



Granules less than 5 /xm in greatest diameter were 

 termed "small grains." These seemed to be ac- 

 tually aggregations of even smaller granules. 

 Those termed "large grains" were larger than 5 

 ixm in diameter and were so dense as to appear as 

 single entities even at high power. Diffuse pig- 

 ment was characteristically situated peripheral to 

 the nuclei of the prickle cells. This was particularly 

 evident in the central areas of the epidermis. Pig- 

 ment granules were in general most concentrated 

 at the edges of the epidermis and appeared to 

 extend toward the surface in diffuse bands. In the 

 more lightly colored skin specimens these bands 

 extended only from the apices of the dermal 

 papillae, but in more densely pigmented skin they 

 were more numerous and tended to blend 

 together, resulting in uniform density of pigment 

 throughout the epidermis. 



The density of pigment was noted in three 

 regions in each sample: around the bases and 

 edges of the dermal papillae and in the bands 

 described above (Table 1). The observations were 

 each coded from to 4 as follows: for diffuse pig- 

 ment, = none, 1 = very smaU amount, 2 = small 

 amount, 3 = medium amount, 4 = large amount; 

 for small grains, = < 1 grain per mm^ 1 = 1-3 

 grains per mm^ 2 = 4-7 grains per mm^ 3 = 8-11 

 grains per mm^ 4 = > 12 grains per mm"; for 

 large grains, = grains per 4 mm^ 1=1 grain 

 per 4 mm^ 2 = 2-3 grains per 4 mm% 3 = 3-6 grains 

 per 4 mm^ 4 = > 6 grains per 4 mm". Skin 

 samples which appeared white were completely 

 unpigmented or showed very small amounts of 

 diffuse and/or small grains. The buff color 

 characteristic of the thoracic patch (terminology 

 of Mitchell 1970) was associated with either equal 

 prominence of diffuse and granular pigment or 



440 



