COMPARISON OF GUINEA PIG AND FETAL HOG SKIN 379 differences, however, between guinea pig and fetal hog skins (Figure 2, solid lines vs. dotted lines) the lag time for cr-naphthol penetration was shorter (0.75 rs. 1.5 hr) and the total amount found on the dermal side was greater with the fetal hog than with the guinea pig skins. There was also more animal to animal variation with fetal hog skin. Within each species the amount of cr-naphthol diffused after 24 hr was inversely related to skin thickness, expressed as mg dry weight/cm 2 (Figure 3). 2O 15 • I I 10 40 50 Fetal Hog I I 20 30 60 70 8090100 200 Dry Wt. (mg/cm2) Figure 3. Relationship of dry weight to cr-naphthol diffusion. The effects of different methods of hair removal were investigated by treating closely clipped skins with a thioglycollate containing commercial depilatory (Nair). Small increases in permeability were obtained with both species when diffusion was measured using Nair treated and untreated skins from the same animals (Figure 4). Skin from the two species responded to crosslinking in a similar direction although to different extents. Formaldehyde pretreatment reduced the total level of cr-naphthol diffusion at all sampling times. With guinea pig skin, diffusion at 24 hr was about 1% of the applied amount with fetal hog skin it dropped from 27% to about 3% (Figure 5). Formaldehyde treatment may have produced a more drastic effect on fetal hog skin than guinea pig skin because of its higher initial permeability. Formaldehyde, therefore, had the same end result in terms of ultimate level of permeability for the two species and the difference in extent of decrease was due to difference in the starting point. ETHER AND WATER EXTRACTS OF STRATUM CORNEUM AND PERIDERM Lipid content estimated by dry weight determination before and after extraction of three samples of tissue averaged 16, 11, 9 and 6% for guinea pig, human and fetal hog stratum corneum, and fetal hog periderm, respectively. Subsequent water extraction

380 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 40 -,, if---- 35- 30- 25- 20- 15 10 /• Untreated } Guinea Pig O=' •oNair © Treated •_. Untreated } Fetal Hog O-- • Nair © Treated 00 1 2 3 4 5 6 24 Time (hrs) Figure 4. Effect of commercial depilatory on diffusion of c•-naphthol through guinea pig and fetal hog skin (each point is an average of three measurements). 35 30 25- 20- 15- 10- 5 0 0 A Untreated } Guinea Pig O Formaldehyde Treated ß Untreated } ß Formaldehyde Treated Fetal Hog 1 2 3 4 5 6 24 Time (hrs) Figure 5. Effect of formaldehyde on diffusion of c•-naphthol through guinea pig and fetal hog skin (each point is an average of three measurements).