Meckel's diverticulum. II. A novel lymphoepithelial organ in the chicken.

We have studied the lymphoid development and structure of Meckel's diverticulum (MD). The lymphoid accumulation began about 2 weeks of age. Between 2 and 5 weeks of age the longitudinal folds were filled with lymphoid tissue. The intensive germinal center formation occurred between 5 and 7 weeks of age. Germinal center formation was associated with the presence of secretory cells. The absence of the secretory cells in the germinal centers was followed by germinal center inactivity which was indicated by the lack of lymphoblasts and the high number of tingible body macrophages. The lymphoid tissue of MD seemed to be fully developed by 10 weeks of age and remained lymphoid at least until 21 months of age. Meckel's diverticulum produced large numbers of plasma cells which were comparable to those of the gland of Harder. We may regard MD as the third pouch of the intestine and suggest that it may be a novel lymphoepithelial organ in the chicken.