The number and size of the follicular epithelium (FE) and follicles in the bursa of Fabricius.

In a four-week-old chicken .1 ml carbon solution was deposited on the vent. The carbon was sucked into the lumen of the bursa and absorbed by the follicular epithelium which appeared as black dots on the surface of the folds. The number of black dots on the surface of a fold represents the number of follicles per fold. The average number of follicles per fold was 820. Since the number of folds per bursa ranges between 10--15, we were able to calculate 8000--12000 follicles per bursa. The volumetric mathematical analysis of the follicles and folds confirmed our calculated number of follicles. On a stereomicrograph from a randomly selected area of a fold surface, a trapezoid was outlined and measured. The surface analysis of the black dots within a trapezoid area revealed that about 10% of the bursal surface was covered by follicular epithelium which is immunologically oriented.