up to one-third of the longer hairs of the back. 

 Thus the fifth year is the one in which the body 

 pelage of the male tends to become grizzled; it 

 is also the one in which pigment formation in the 

 vibrissae roots is ending (Scheffer, 1962, p. 73). 



All-white hairs may be seen occasionally in 

 seals other than old males. In one 17-year 

 female, about 10 percent of the shorter hairs 

 are white. 



CHANGES IN THE DERMIS 



Change in Depth of the Dermis 



The slant depth of the dermis was measured 

 on 570 dried specimens (table 13). For com- 

 parative purposes, slant depth may serve as 

 "depth of skin." 



Mean depth of dermis for males is 5.3 mm.; 

 for females 5.1 mm. Maximum depth in the 

 male, 14 mm., is noted on a 9-year-old. 

 Maximum depth in the female, 8 mm., is noted 

 on four individuals, and 11-year-old and three 

 recorded as "age 10+." 



The depth of dermis of males and females 

 was found to increase with age (b = 0.6 mm. 

 and 0.2 mm. for males and females, respec- 

 tively (P< .01 for each)). The estimated in- 

 crease for females, 0,2 mm. per year, is 

 probably somewhat low for ages through 5 

 and probably highforolder individuals. That is, 

 a linear regression line was fitted to data which 

 probably are not linear, but because of the few 

 age groups (six) curvilinear methods would 

 do little to improve the estimate. The estimated 

 increase of 0.6 mm. per year for males seems 

 to be better, since the dermis continues to 

 increase at a fairly constant rate until age 10 

 (?) years or beyond. The dermis of molting 

 animals is thicker than that of nonmolting 

 animals (P<.01). 



Change in the Sweat Gland and Adjacent 

 Fat Cells 



Whereas hair follicles and sebaceous glands 

 are derivatives of the embryonic epidermis, 

 the sweat gland at the base of each pilose- 

 baceous unit is a derivative of the dermis. It 

 starts to develop deep in the skin and does not 



become functional for at least 4 months after 

 birth — probably much later. Its history is 

 sketched below; 



1. Newborn pup in beginning molt. The un- 

 formed sweat gland is a series of short tubes, 

 seen in cross section as circles, one-cell 

 thick. 



2. Silver pup in late molt. The first ducts 

 are visible at the surface; an appropriate sec- 

 tion may show two sebaceous ducts and a sweat 

 duct paralleling the hair shaft. 



3. Silver pup in postmolt resting stage. 

 Sweat glands are entire, though small. The 

 connective tissue sheath shows its charac- 

 teristic spiral arrangement. 



4. Late autumn yearling, after the second 

 molt. The gland is full size or nearly so. At 

 this time a few glands may show dilations or 

 reservoirs characteristic of the adult. 



The fat cells which, in the adult, lie in grape- 

 like clusters beneath and among the deeper 

 coils of the sweat gland, first appear in the 

 pelagic yearling. They start to form in spring, 

 at a maximum depth of only 2 mm., and do not 

 become conspicuous until the second year of 

 life. In old seals during molt they reach a 

 depth of 55 mm. The skin of the yearling is 

 more elastic than the skin of the adult. Perhaps 

 the scarcity of dermal fat cells is partly 

 responsible. 



In addition to histological evidence, there are 

 gross clues which point to more fat in the 

 skin in the older animal. Fifty-five envelopes 

 containing dry pelage samples from females 

 of various ages, collected in February 1961, 

 were held at room temperature for 1 month. Oil 

 stains showed on the outside of the envelopes, 

 as follows: age 3, only 1 in 14 stained; ages 

 4 and 5, 22 nearly unstained; age 6, 3 in 3 

 stained; and ages 7 and older, 12 in 16 stained. 

 Workers handling finished seal skins have re- 

 marked at the presence of a faint, dark 

 "oily" stain which extends along the mid-dorsal 

 line of the leather side of old female skins, 

 but is not seen on skins of younger animals. 



It has long been known that the blubber layer 

 tends to increase in thickness as the seal 



25 



