Thymus is a complex organ, develops at least three different sources: 1; epithelial anlage of foregut endoderm; 2; the connective tissue capsule and septae are formed by the cranial neural crest of ecto-mesoderm origin, and 3; thymocytes, macrophages and dendritic cells the latter is not yet identified in vivo, are of hemopoietic origin. The cortical histologycal pattern of the thymus stained by hematoxylin-eosin and anti-vimentin recognize two compartments: cortex and medulla. However, the pancytokeratin antibody identified – in addition to the cortex two sub-compartments in the medulla. One is continouos with the cortex and other does not show epithelial cells, but it is identified with antibodies characteristic for connective tissue (collagen I, tenascin, desmin, fibronectin). Anti-laminin staining follows the surface basal lamina deep to the septae, but at the end of the septum, which is topographically close to the surface of the medulla comes to an end. At this point the space of the septae is continouos with the epithelial cell free compartment, suggesting common origin of septae and epithelial cell free compartment. In agreement with the mammalian findings the vascular bed of the thymic medulla is surrounded by mesenchyme of neural crest origin. Our observations indicate, that the medullary epitelial compartment, which shares staining with the cortex, forms much less volume than, that of the epithelial free one. The 74.3 positive dendritic cells are accumulated in the epithelial cell free compartment, therefore the selection of the thymocytes takes place in this compartment. We propose, that the epithelial compartment of the medulla serves as a supporting system, while the epithelial free one is a transit zone of thy thymus, and develops from ecto-mesoderm.