Ontogeny of spontaneous rosette-forming cells in the chicken. With special reference to the bursa of Fabricius.

Cells forming rosettes (RFC) with sheep erythrocytes (SRBC) and rabbit erythrocytes (RRBC) were studied during the ontogeny of the bursa of Fabricius of the chicken. The frequency of SRBC-RFC was low but significant in the bursa of 15-day-old embryos, and increased therafter in an approximately linear fashion with the age of the bursa donor until at least in 43-day-old chickens. The total number of SRBC-RFC per bursa increased throughout the ontogeny with no evidence for any abrupt increase after hatch. The mean number of SRBC bound per RFC also increased after hatch. In contrast the frequency of bursal RRBC-RFC was high in 15-day-old embryos, significantly decreased on embryonic day 18, remained low until at least 7 days after hatch and then increased significantly. The total number of RRBC-RFC per bursa in contrast remained relatively constant during the embryonic period and the first week after hatch and thereafter increased markedly. No change in the mean number of RRBC bound per RFC could be demonstrated during the ontogeny. RRBC-RFC were also frequent in the yolk sac, spleen, and thymus of 15-day-old embryos.