Abstract
Scanning electron micrographs are presented of the ileal epithelium of mice aged 5, 15, 20 and 25 days. During this period the villous pattern develops to full maturity. By the twentieth day of life a segmented filamentous micro-organism colonizes the ileal epithelium and is firmly attached via a small segment. During the first days of colonization the segmented filamentous micro-organisms themselves are subcolonized by small rod-shaped bacteria, presumably lactobacilli. At the age of 25 days this subcolonization was no longer observed.