MATERIALS AND METHODS

Skin from mice of the C57 Black strain was used for timed samples. For a normal young series, pieces of skin were taken from the mid-dorsum and mid-ventrum. Specimens were taken for each day of age from newborn to 19 days. In older mice, areas on the dorsum were plucked of club hairs to initiate growth of the next hair generation, then pieces of skin were taken representing daily stages from o to 19 days and at 21, 22, and 26 days. For comparison, some stages were used from albino and piebald mice.

Portions of each tissue, fixed in formol-calcium, were mounted in gelatin and were sectioned with the freezing microtome at 5 and occasionally io a. These sections were unstained. For comparison, other portions were imbedded in paraffin, and sections were stained with haematoxylin and eosin or with a modified Masson's trichrome technique. To emphasize the mitotic activity of certain stages, colchicine-treated animals were used as supplemental material. Colchicine in the concentration of or mg/3o gm body weight was injected intraperitioneally, and tissues were taken 9'hours later.