MATERIALS AND METHODS

Male Sprague-Dawley rats approximately 90 days of age, obtained from the Charles River Breeding Laboratories, North Wilmington, Mass., were used throughout these experiments. The testes were fixed by perfusion employing a technique similar to that described by Fawcett et a!.(1969). The initial fixative used for perfusion was composed of 5% glutaraldehyde buffered with s-collidine (Bennett and Luft, 1959). Further fixation was carried out by immersion in a solution containing 2 parts of 2% osmium tetroxide and 1 part of s-collidine. In order to use lanthanum as an intercellular tracer, a modification of the procedure described by Bright-man and Reese (1969) was adopted. A 2% lanthanum nitrate solution was very slowly adjusted to pH 7.6-7.8 using 0.1 N NaOH. A barely visible precipitate is observed at this pH. The lanthanum hydroxide thus prepared was then added to an equal volume of s-collidine-buffered glutaraldehyde. The final concentration of glutaraldehyde was varied from 2 5 to 5% in different experiments. Two hundred milliliters of the fixative mixture was perfused over a period of 30 mm at a pressure of 130cm of water. This tissue was then processed in a manner similar to that employed