MATERIALS AND METHODS 



The study is based on pelage samples from 

 706 seals, of which 321 were in molt at the 

 time of collection (table 1). They were taken in 

 all months of the year on the Pribilof Islands 

 or in waters off the west coast of North 

 America, 1958-61. 



Specimens Used for Identifying Molt Stage 



From each seal, the field collector cut a 

 strip of skin about 2 by 4.5 cm. lengthwise of 

 the body from the middle of the back between 

 the fore flippers. He scraped off the fat with a 

 knife, washed the sample free of blood and dirt, 

 and placed it in 25 ml. of 10 percent neutral 

 buffered formalin. Some weeks later, each 

 sample was cut in half to provide material 

 for (a) dry slices, transversely sectioned to 

 show gross relationships of skin and pelage, 

 and (b) paraffin mounts, horizontally sectioned 

 in depth and stained to show histological detail 

 (fig. 1). 



Dry slices, transversely sectioned. — Half of 

 the sample was removed from formalin and 



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-J P 



Figure 1. --Orientation of pelage samples, diagram- 

 matic; body represented as lying on its belly with its 

 head at viewer's left. D - V= dorsal to ventral axis; 

 R - L = right to left axis; A - P = anterior to poste- 

 rior axis. (Above) dry slice, transverse. (Below) ten 

 paraffin mounts, horizontal. 



dried at room temperature under light pres- 

 sure. From it, thin slices parallel to the lay of 

 the roots were cut in a plane midway between 

 frontal and horizontal. Measurements made 

 later on the sections are referred to as 

 "slant depth." Slant depth is 142 percent of 

 vertical depth. Each slice is about 2/3 mm. 

 thick and 10 mm. wide. On a typical slice 

 about 100 pilosebaceous units can be seen. A 

 unit includes a guard hair, a bundle of under- 

 fur fibers, a skin pore (pilary funnel), a 

 sebaceous gland-complex, a sweat gland, and 

 associated minor structures. The slices were 

 degreased in methyl chloroform. 



Later, one slice was selected and perma- 

 nently mounted, anterodorsal side upward, on a 

 glass slide under a clear plastic cover slip 

 (fig. 2). Other slices were temporarily mounted 

 in various media. The refractive index (RI) of 

 the keratin of hair is about 1.548. Experimen- 

 tally, in order to see and to photograph 

 structural details, dry slices were mounted in 

 a number of liquids. Isopropanol (RI 1.378) 

 has proved to be the best nonclearing mountant 

 for rapid examination of dry slices. In it, all 

 surface structures, including pelage fibers and 

 exposed roots, standout clearly. Cedarwood oil 

 (RI 1.513) is the most useful clearing agent. 

 It clears moderately well and it clears progres- 

 sively, so that certain structures (e.g., seba- 

 ceous glands) tend to remain opaque for a few 

 minutes after others have cleared. It evapo- 

 rates rather slowly. It has the disadvantage of 

 trapping fine air bubbles which, however, dis- 

 appear in a day or two. It enables one to see 

 pigment granules and refringent root sheaths. 

 Onho-nitrotoluene (RI 1.547) clears more 

 thoroughly; its refractive index is near that of 

 keratin. However, it evaporates quickly, has 

 an offensive odor, and is toxic. 



Paraffin mounts, horizontally sectioned . — 

 The other half of the original specimen was 

 prepared as a series of horizontal sections in 

 a synthetic medium with a refractive index of 

 1.528. Each section is 15 microns thick, 

 stained with hematoxylin and eosin, represent- 

 ing levels from the surface, with lumens of the 

 pilary funnels, to the deepest parts of the sweat 

 glands and hair roots. 



