Immune System

Tonsil is a mass of lymphoid tissue, several of which are situated at the back of the mouth and throat in higher vertebrates. The thymus is haemopoietic as well as an endocrine gland. The thymus is the seedbed of thymic lymphocytes ( -lymphocytes). Certain stem cells, originating in yolk sac and liver in the early embryo, migrate into the thymus and proliferate to form a large number of lymphocytes.