Summary. Experiments were designed to examine the relative contributions of Payer's patches and mesenteric lymph nodes to the population of circulating immunoglobulin-bearing lymphocytes in sheep. The ileum, with more than 9O' !' o of the total Peyer's patches, the mesenteric lymph nodes, or both, were removed from lambs at different stages of development and the composition of the cell populations in lymph from different sources and in the blood was examined. Lambs which had had the ileum removed before or within a few days of birth were deficient in small lymphocytes bearing membrane immunoglobulin. This deficit remained for at least the first year of the animals' lives. Neither the removal of mesenteric lymph nodes nor removal of the ileum had any statistically significant effect on the total output of cells or on the population of igA-producing cells in lymph draining from the gut.

Carbonic anhydrase cytochemistry of the ileal Peyer's patch in foetal and neonatal lambs has indicated secretion from the follicle-associated epithelium to the follicles. Reaction for carbonic anhydrase in the follicle-associated epithelium was found in the luminal plasma membrane, in cytoplasmic vesicles, and in vacuoles containing 50-nm membrane-bounded particles that seemed to be shed to the intercellular space. The lateral plasma membrane was negative for carbonic anhydrase, indicating that formation of carbonic anhydrase-positive particles was restricted to vacuoles. Administration of ferritin to ileal loops of sheep foetuses showed ferritin localized in vesicles and vacuoles of the follicle-associated epithelium followed by exocytosis, together with carbonic anhydrase-positive particles, into the indentations of the lateral cell border. The carbonic anhydrase-positive particles seemed to be transported to the centres of lymphoid follicles where many were attached to the plasma membrane of lymphocytes. Carbonic anhydrase-positive particles were also seen in vesicles and sometimes free in the cytoplasm of the lymphocytes or attached to their nuclear envelope. Light microscopically, carbonic anhydrase reactivity of the follicle-associated epithelium was associated with the early formation of the ileal Peyer's patch at about 100 days gestation. At this time the follicle-associated epithelium showed a strong luminal but at most a weak lateral staining. With further foetal development there was a progressive increase in the amount of carbonic anhydrase-positive reaction product in extracellular particles, both along the lateral cell borders of the follicle-associated epithelium and among the lymphocytes of the follicle centres.

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