Phylogenetic Origins of the Thymus

The lymphoid system of the amphibians is more complex than that of fishes. In addition to a well-defined thymus gland, primitive lymph node tissue is also present (Kent et al., 1964). In frogs and toads for the first time marked seasonal changes in thymic size are noted (Dustin, 1911). Reptiles have a lymphoid system that is quite similar to that seen in amphibians, including a well-developed thymus that also involutes with age (Dustin, 1911). 

The evolutionary development of the lymphoid system in birds is unique. In addition to the presence of discrete lobulated thymic tissue, there is a second discrete lymphoid organ found near the cloaca, the bursa of Fabri-cius, which functions to direct specifically the maturation of B cells and the capacity to produce humoral antibody (Glick, 1964). In contrast, the role of 

Thymic Hormonelike Activity in the Blood OF Invertebrates and Primitive Vertebrates 

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