Fig. 2a: monocyte from the young opossum fixed at room temperature for 1 hr, incubated with cationized ferritin (CF) during the same time and observed without contrastation. The cells showed an uniform distribution of the CF particles over the whole surface and sometimes a patchy profile with several layers of CF particles could be found (arrow) (61500 X). Fig. 2b: monocytes from adult animals, fixed and incubated with CF in the same condition as the young animal were observed after stained with uranyl acetate and lead nitrate. An uniform distribution of the ferritin particles were seen in the whole surface of the mononuclear cells (arrows) (64.000 X).