The Selective Admission of Antibodies to the Foetus by the Yolk-Sac Splanchnopleur in Rabbits

The yolk-sac splanchnopleur of 24-day rabbit embryos freely admits to the foetal circulation, antibodies prepared in the rabbit whereas antibodies prepared in cattle or horses are almost, but not entirely, excluded. This selectivity appears to be independent of the molecular dimensions of the antibody particles, for both rabbit agglutinins and haemolysins are admitted equally freely, whereas both equine antitoxins and bovine agglutinins are equally retarded. Selection appears to depend on the species-specific origin of the antibody molecules themselves, being independent of the other species-specific characters of the rabbit, bovine and equine sera employed. The selectivity of the yolk-sac splanchnopleur as between rabbit and bovine agglutinins is relatively slight at 20 days post-coitum but develops progressively thereafter and is marked at 24 days post-coitum. Individual embryos, even in the same litter, vary significantly in their capacity to admit ungulate antibodies. The theoretical significance of the selectivity of the foetal splanchnopleur is discussed in relation to passive anaphylactic sensitization. The technique of direct injection of immune sera into the lumen of the pregnant uterus was employed. The antisera used throughout were the natural sera separated from the blood of immunized animals not having been subjected to any procedures for their refinement, purification or concentration.