122 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS enized in water. To the samples designated for analysis of total --SH, 12M urea was added in such quantity as to bring the final urea concen- tration to 3M. The concentration of tissue in the reaction vessel ranged from 2 to 25 mg/ml. Four milliliters of the --SH reagent (1.5 mg % CMPAN in amyl ace- tate) was added to each tube and the tubes were agitated for 20 minutes in a shaker. The tubes were centrifuged and the absorbancy of the organic (upper) layer was determined at 500 nm. The --SH content of the sample was calculated from the relation AXVXSX]O -a a,,• X W = •g sulfhydryl sulfur/mg tissue where A -- decrease in absorbancy of CMPAN solution V -- volume of CMPAN solution in ml S -- at. wt sulfur (32.07) W -- dry weight (105øC) of tissue sample in mg a,, -- molar absorbancy index of CMPAN solution at 500 nm -- 1.58 X 104 in the range 0-33/•g/ml (l-cm cells) Electron Microscopy Pieces of fetal skin were fixed in 1% phosphate-buffered osmic acid, washed thoroughly with distilled water, and dehydrated with a graded series of alcohols. After immersion in propylene oxide, the tissue was embedded in Epon ©* 812 contained in gelatin capsules and the Epon was allowed to polymerize at 60øC. Sections were cut on a Porter-Blum ul- tramicrotome* with a diamond knife, mounted on carbon-reinforced collodion-coated grids, and stained with 1% aqueous phosphotungstic acid. Microgzaphs were taken with a Philips EM-100B electron micro- scope,* using Kodak spectographic fihn. Light Microscopy Tissue for light •nicroscopic exa•nination was fixed in buffered forma- lin and e•nbedded in Paraplast. Sections were cut at 4 • and stained with he•natoxylin and eosin, Van Gieson's stain, Massoh's trichrome stain, and reacted for carbohydrates using the periodic acid-Schiff procedure. * Epon© Resin 812, Shell Chemical Co., New York, N.Y. ? Porter-Blum Ultramicrotome MT2, Ivan Sorvall, Inc., Norwalk, Conn. $ Philips Electronic Instruments, 7.50 So. Fulton Ave., Mt. Vernon, N.Y.

FETAL EPIDERMIS 123 RESULTS Studies in the Guinea Pig Fetus Barrier Function The water permeability of guinea pig skin was determined from the 40th day of gestation to the 4th day of postnatal life. The data on water diffusion rate are shown in Table I, presented as mean values for each litter in increasing order of body weight. Table I Water Permeability and Epidermal Sulfhydryl Content of Skin of the Fetal and Newborn Guinea Pig Body Crown- No. Weight Rump Estimated of Water Diffusion (Wet) Length Age Ani- Rate a (g) (cm) (days) mals (mg/cm2/hr) 8 12 14 25 28 32 32 32 33 41 41 43 45 48 55 57 69 74 81 87 90 98 98 102 90 95 95 96 150 5OO 52 58 63 76 80 83 85 8.5 8.4 9.2 9.6 9.3 9.2 9.4 10.5 9.9 11.0 11.4 11.0 11.3 11.3 11.2 11.3 11.8 12.7 12.1 12.1 12.1 14.7 . . . 40 Prenatal 3 43 Prenatal 3 45 Prenatal 2 50 Prenatal 3 51 Prenatal 2 52 Prenatal 3 53 Prenatal 4 53 Prenatal 3 53 Prenatal 3 55 Prenatal 2 56 Prenatal 3 55 Prenatal 3 55 Prenatal 3 56 Prenatal 4 58 Prenatal 1 57 Prenatal 3 60 Prenatal 2 61 Prenatal 3 60 Prenatal 2 61 Prenatal 2 61 Prenatal 1 60 Prenatal 2 61 Prenatal 2 62 Prenatal 3 At birth 3 At birth 2 1 2 At birth 2 4 2 Adult 5 Sulfhydryl Sulfur a "Total .... Free" (mg/g dry wt) 4 97 q- 0.27 5 12 q- 0.10 4 174-0.02 4 324-0.19 3 584-0.17 4 61 q- 0.19 3 67 q- 0.20 3.87 4- 0.32 2.31 q- 0.39 1.46 q- 0.09 1.95 q- 0.30 1.51 q- 0.20 0.60 0.88 q- 0.03 0.40 q- 0.07 0.74 q- 0.04 0.65 0.59 4- 0.01 0.32 q- 0.03 0.32 q- 0.06 0.35 q- 0.01 0.22 q- 0.00 0.16 q- 0.01 0.00 0 87 4-0.19 0 54 q- 0.05 1 184-0.36 1 31 q- 0.31 1.40 q- 0.37 2.06 q- 0.30 2.75 4- 0.15 4.71 1.48 4- 0.15 1.64 q- 0.27 0.50 q- 0.15 1.27 q- 0.24 1.17 4- 0.01 0.19 4- 0.01 0.00 0.60 4- 0.12 0.41 q- 0.08 0.84 4- 0 35 0.71 +0 13 1.29 q- 0 31 1.56 q- 0 36 2.17 q- o 12 2.38 1.16 q- 0.07 1.01 q- 0.09 0.35 q- 0.12 1.04 q- 0.22 0.88 ñ 0.05 Means and standard errors (N 2) or means and average deviations (N = 2).